Chapter 17 Replication

Table of Contents

17.1 Configuring Replication
17.1.1 Binary Log File Position Based Replication Configuration Overview
17.1.2 Setting Up Binary Log File Position Based Replication
17.1.3 Replication with Global Transaction Identifiers
17.1.4 MySQL Multi-Source Replication
17.1.5 Changing Replication Modes on Online Servers
17.1.6 Replication and Binary Logging Options and Variables
17.1.7 Common Replication Administration Tasks
17.2 Replication Implementation
17.2.1 Replication Formats
17.2.2 Replication Implementation Details
17.2.3 Replication Channels
17.2.4 Replication Relay and Status Logs
17.2.5 How Servers Evaluate Replication Filtering Rules
17.3 Replication Solutions
17.3.1 Using Replication for Backups
17.3.2 Handling an Unexpected Halt of a Replication Slave
17.3.3 Monitoring Row-based Replication
17.3.4 Using Replication with Different Master and Slave Storage Engines
17.3.5 Using Replication for Scale-Out
17.3.6 Replicating Different Databases to Different Slaves
17.3.7 Improving Replication Performance
17.3.8 Switching Masters During Failover
17.3.9 Setting Up Replication to Use Encrypted Connections
17.3.10 Encrypting Binary Log Files and Relay Log Files
17.3.11 Semisynchronous Replication
17.3.12 Delayed Replication
17.4 Replication Notes and Tips
17.4.1 Replication Features and Issues
17.4.2 Replication Compatibility Between MySQL Versions
17.4.3 Upgrading a Replication Setup
17.4.4 Troubleshooting Replication
17.4.5 How to Report Replication Bugs or Problems

Replication enables data from one MySQL database server (the master) to be copied to one or more MySQL database servers (the slaves). Replication is asynchronous by default; slaves do not need to be connected permanently to receive updates from the master. Depending on the configuration, you can replicate all databases, selected databases, or even selected tables within a database.

Advantages of replication in MySQL include:

For information on how to use replication in such scenarios, see Section 17.3, “Replication Solutions”.

MySQL 8.0 supports different methods of replication. The traditional method is based on replicating events from the master's binary log, and requires the log files and positions in them to be synchronized between master and slave. The newer method based on global transaction identifiers (GTIDs) is transactional and therefore does not require working with log files or positions within these files, which greatly simplifies many common replication tasks. Replication using GTIDs guarantees consistency between master and slave as long as all transactions committed on the master have also been applied on the slave. For more information about GTIDs and GTID-based replication in MySQL, see Section 17.1.3, “Replication with Global Transaction Identifiers”. For information on using binary log file position based replication, see Section 17.1, “Configuring Replication”.

Replication in MySQL supports different types of synchronization. The original type of synchronization is one-way, asynchronous replication, in which one server acts as the master, while one or more other servers act as slaves. This is in contrast to the synchronous replication which is a characteristic of NDB Cluster (see Chapter 22, MySQL NDB Cluster 8.0). In MySQL 8.0, semisynchronous replication is supported in addition to the built-in asynchronous replication. With semisynchronous replication, a commit performed on the master blocks before returning to the session that performed the transaction until at least one slave acknowledges that it has received and logged the events for the transaction; see Section 17.3.11, “Semisynchronous Replication”. MySQL 8.0 also supports delayed replication such that a slave server deliberately lags behind the master by at least a specified amount of time; see Section 17.3.12, “Delayed Replication”. For scenarios where synchronous replication is required, use NDB Cluster (see Chapter 22, MySQL NDB Cluster 8.0).

There are a number of solutions available for setting up replication between servers, and the best method to use depends on the presence of data and the engine types you are using. For more information on the available options, see Section 17.1.2, “Setting Up Binary Log File Position Based Replication”.

There are two core types of replication format, Statement Based Replication (SBR), which replicates entire SQL statements, and Row Based Replication (RBR), which replicates only the changed rows. You can also use a third variety, Mixed Based Replication (MBR). For more information on the different replication formats, see Section 17.2.1, “Replication Formats”.

Replication is controlled through a number of different options and variables. For more information, see Section 17.1.6, “Replication and Binary Logging Options and Variables”.

You can use replication to solve a number of different problems, including performance, supporting the backup of different databases, and as part of a larger solution to alleviate system failures. For information on how to address these issues, see Section 17.3, “Replication Solutions”.

For notes and tips on how different data types and statements are treated during replication, including details of replication features, version compatibility, upgrades, and potential problems and their resolution, see Section 17.4, “Replication Notes and Tips”. For answers to some questions often asked by those who are new to MySQL Replication, see Section A.13, “MySQL 8.0 FAQ: Replication”.

For detailed information on the implementation of replication, how replication works, the process and contents of the binary log, background threads and the rules used to decide how statements are recorded and replicated, see Section 17.2, “Replication Implementation”.

17.1 Configuring Replication

This section describes how to configure the different types of replication available in MySQL and includes the setup and configuration required for a replication environment, including step-by-step instructions for creating a new replication environment. The major components of this section are:

17.1.1 Binary Log File Position Based Replication Configuration Overview

This section describes replication between MySQL servers based on the binary log file position method, where the MySQL instance operating as the master (the source of the database changes) writes updates and changes as events to the binary log. The information in the binary log is stored in different logging formats according to the database changes being recorded. Slaves are configured to read the binary log from the master and to execute the events in the binary log on the slave's local database.

Each slave receives a copy of the entire contents of the binary log. It is the responsibility of the slave to decide which statements in the binary log should be executed. Unless you specify otherwise, all events in the master binary log are executed on the slave. If required, you can configure the slave to process only events that apply to particular databases or tables.

Important

You cannot configure the master to log only certain events.

Each slave keeps a record of the binary log coordinates: the file name and position within the file that it has read and processed from the master. This means that multiple slaves can be connected to the master and executing different parts of the same binary log. Because the slaves control this process, individual slaves can be connected and disconnected from the server without affecting the master's operation. Also, because each slave records the current position within the binary log, it is possible for slaves to be disconnected, reconnect and then resume processing.

The master and each slave must be configured with a unique ID (using the server-id option). In addition, each slave must be configured with information about the master host name, log file name, and position within that file. These details can be controlled from within a MySQL session using the CHANGE MASTER TO statement on the slave. The details are stored within the slave's master info repository (see Section 17.2.4, “Replication Relay and Status Logs”).

17.1.2 Setting Up Binary Log File Position Based Replication

This section describes how to set up a MySQL server to use binary log file position based replication. There are a number of different methods for setting up replication, and the exact method to use depends on how you are setting up replication, and whether you already have data within your master database.

There are some generic tasks that are common to all setups:

Note

Certain steps within the setup process require the SUPER privilege. If you do not have this privilege, it might not be possible to enable replication.

After configuring the basic options, select your scenario:

Before administering MySQL replication servers, read this entire chapter and try all statements mentioned in Section 13.4.1, “SQL Statements for Controlling Master Servers”, and Section 13.4.2, “SQL Statements for Controlling Slave Servers”. Also familiarize yourself with the replication startup options described in Section 17.1.6, “Replication and Binary Logging Options and Variables”.

17.1.2.1 Setting the Replication Master Configuration

To configure a master to use binary log file position based replication, you must ensure that binary logging is enabled, and establish a unique server ID. If this has not already been done, a server restart is required.

Binary logging is required on the master because the binary log is the basis for replicating changes from the master to its slaves. Binary logging is enabled by default (the log_bin system variable is set to ON). The --log-bin option tells the server what base name to use for binary log files. It is recommended that you specify this option to give the binary log files a non-default base name, so that if the host name changes, you can easily continue to use the same binary log file names (see Section B.4.7, “Known Issues in MySQL”).

Each server within a replication topology must be configured with a unique server ID, which you can specify using the --server-id option. This server ID is used to identify individual servers within the replication topology, and must be a positive integer between 1 and (232)−1. If you set a server ID of 0 on a master, it refuses any connections from slaves, and if you set a server ID of 0 on a slave, it refuses to connect to a master. Other than that, how you organize and select the numbers is your choice, so long as each server ID is different from every other server ID in use by any other server in the replication topology. The server_id system variable is set to 1 by default. A server can be started with this default server ID, but an informational message is issued if you did not specify a server ID explicitly.

Note

The following options also have an impact on the replication master:

  • For the greatest possible durability and consistency in a replication setup using InnoDB with transactions, you should use innodb_flush_log_at_trx_commit=1 and sync_binlog=1 in the replication master's my.cnf file.

  • Ensure that the skip-networking option is not enabled on the replication master. If networking has been disabled, the slave cannot communicate with the master and replication fails.

17.1.2.2 Setting the Replication Slave Configuration

Each replication slave must have a unique server ID. If this has not already been done, this part of slave setup requires a server restart.

If the slave server ID is not already set, or the current value conflicts with the value that you have chosen for the master server, shut down the slave server and edit the [mysqld] section of the configuration file to specify a unique server ID. For example:

[mysqld]
server-id=2

After making the changes, restart the server.

If you are setting up multiple slaves, each one must have a unique nonzero server-id value that differs from that of the master and from any of the other slaves.

Binary logging is enabled by default on all servers. A slave is not required to have binary logging enabled for replication to take place. However, binary logging on a slave means that the slave's binary log can be used for data backups and crash recovery.

Slaves that have binary logging enabled can also be used as part of a more complex replication topology. For example, you might want to set up replication servers using this chained arrangement:

A -> B -> C

Here, A serves as the master for the slave B, and B serves as the master for the slave C. For this to work, B must be both a master and a slave. Updates received from A must be logged by B to its binary log, in order to be passed on to C. In addition to binary logging, this replication topology requires the --log-slave-updates option to be enabled. With this option, the slave writes updates that are received from a master server and performed by the slave's SQL thread to the slave's own binary log. The --log-slave-updates option is enabled by default.

If you need to disable binary logging or slave update logging on a slave server, you can do this by specifying the --skip-log-bin and --skip-log-slave-updates options for the slave.

17.1.2.3 Creating a User for Replication

Each slave connects to the master using a MySQL user name and password, so there must be a user account on the master that the slave can use to connect. The user name is specified by the MASTER_USER option on the CHANGE MASTER TO command when you set up a replication slave. Any account can be used for this operation, providing it has been granted the REPLICATION SLAVE privilege. You can choose to create a different account for each slave, or connect to the master using the same account for each slave.

Although you do not have to create an account specifically for replication, you should be aware that the replication user name and password are stored in plain text in the master info repository table mysql.slave_master_info (see Section 17.2.4.2, “Slave Status Logs”). Therefore, you may want to create a separate account that has privileges only for the replication process, to minimize the possibility of compromise to other accounts.

To create a new account, use CREATE USER. To grant this account the privileges required for replication, use the GRANT statement. If you create an account solely for the purposes of replication, that account needs only the REPLICATION SLAVE privilege. For example, to set up a new user, repl, that can connect for replication from any host within the example.com domain, issue these statements on the master:

mysql> CREATE USER 'repl'@'%.example.com' IDENTIFIED BY 'password';
mysql> GRANT REPLICATION SLAVE ON *.* TO 'repl'@'%.example.com';

See Section 13.7.1, “Account Management Statements”, for more information on statements for manipulation of user accounts.

Important

To connect to the replication master using a user account that authenticates with the caching_sha2_password plugin, you must either set up a secure connection as described in Section 17.3.9, “Setting Up Replication to Use Encrypted Connections”, or enable the unencrypted connection to support password exchange using an RSA key pair. The caching_sha2_password authentication plugin is the default for new users created from MySQL 8.0 (for details, see Section 6.4.1.3, “Caching SHA-2 Pluggable Authentication”). If the user account that you create or use for replication (as specified by the MASTER_USER option) uses this authentication plugin, and you are not using a secure connection, you must enable RSA key pair-based password exchange for a successful connection.

17.1.2.4 Obtaining the Replication Master Binary Log Coordinates

To configure the slave to start the replication process at the correct point, you need to note the master's current coordinates within its binary log.

Warning

This procedure uses FLUSH TABLES WITH READ LOCK, which blocks COMMIT operations for InnoDB tables.

If you are planning to shut down the master to create a data snapshot, you can optionally skip this procedure and instead store a copy of the binary log index file along with the data snapshot. In that situation, the master creates a new binary log file on restart. The master binary log coordinates where the slave must start the replication process are therefore the start of that new file, which is the next binary log file on the master following after the files that are listed in the copied binary log index file.

To obtain the master binary log coordinates, follow these steps:

  1. Start a session on the master by connecting to it with the command-line client, and flush all tables and block write statements by executing the FLUSH TABLES WITH READ LOCK statement:

    mysql> FLUSH TABLES WITH READ LOCK;
    
    Warning

    Leave the client from which you issued the FLUSH TABLES statement running so that the read lock remains in effect. If you exit the client, the lock is released.

  2. In a different session on the master, use the SHOW MASTER STATUS statement to determine the current binary log file name and position:

    mysql > SHOW MASTER STATUS;
    +------------------+----------+--------------+------------------+
    | File             | Position | Binlog_Do_DB | Binlog_Ignore_DB |
    +------------------+----------+--------------+------------------+
    | mysql-bin.000003 | 73       | test         | manual,mysql     |
    +------------------+----------+--------------+------------------+
    

    The File column shows the name of the log file and the Position column shows the position within the file. In this example, the binary log file is mysql-bin.000003 and the position is 73. Record these values. You need them later when you are setting up the slave. They represent the replication coordinates at which the slave should begin processing new updates from the master.

    If the master has been running previously with binary logging disabled, the log file name and position values displayed by SHOW MASTER STATUS or mysqldump --master-data will be empty. In that case, the values that you need to use later when specifying the slave's log file and position are the empty string ('') and 4.

You now have the information you need to enable the slave to start reading from the binary log in the correct place to start replication.

The next step depends on whether you have existing data on the master. Choose one of the following options:

17.1.2.5 Choosing a Method for Data Snapshots

If the master database contains existing data it is necessary to copy this data to each slave. There are different ways to dump the data from the master database. The following sections describe possible options.

To select the appropriate method of dumping the database, choose between these options:

  • Use the mysqldump tool to create a dump of all the databases you want to replicate. This is the recommended method, especially when using InnoDB.

  • If your database is stored in binary portable files, you can copy the raw data files to a slave. This can be more efficient than using mysqldump and importing the file on each slave, because it skips the overhead of updating indexes as the INSERT statements are replayed. With storage engines such as InnoDB this is not recommended.

17.1.2.5.1 Creating a Data Snapshot Using mysqldump

To create a snapshot of the data in an existing master database, use the mysqldump tool. Once the data dump has been completed, import this data into the slave before starting the replication process.

The following example dumps all databases to a file named dbdump.db, and includes the --master-data option which automatically appends the CHANGE MASTER TO statement required on the slave to start the replication process:

shell> mysqldump --all-databases --master-data > dbdump.db
Note

If you do not use --master-data, then it is necessary to lock all tables in a separate session manually. See Section 17.1.2.4, “Obtaining the Replication Master Binary Log Coordinates”.

It is possible to exclude certain databases from the dump using the mysqldump tool. If you want to choose which databases to include in the dump, do not use --all-databases. Choose one of these options:

  • Exclude all the tables in the database using --ignore-table option.

  • Name only those databases which you want dumped using the --databases option.

Note

By default, if GTIDs are in use on the master (gtid_mode=ON), mysqldump includes the GTIDs from the gtid_executed set on the master in the dump output to add them to the gtid_purged set on the slave. If you are dumping only specific databases or tables, it is important to note that the value that is included by mysqldump includes the GTIDs of all transactions in the gtid_executed set on the master, even those that changed suppressed parts of the database, or other databases on the server that were not included in the partial dump. Check the description for mysqldump's --set-gtid-purged option to find the outcome of the default behavior for the MySQL Server versions you are using, and how to change the behavior if this outcome is not suitable for your situation.

For more information, see Section 4.5.4, “mysqldump — A Database Backup Program”.

To import the data, either copy the dump file to the slave, or access the file from the master when connecting remotely to the slave.

17.1.2.5.2 Creating a Data Snapshot Using Raw Data Files

This section describes how to create a data snapshot using the raw files which make up the database. Employing this method with a table using a storage engine that has complex caching or logging algorithms requires extra steps to produce a perfect point in time snapshot: the initial copy command could leave out cache information and logging updates, even if you have acquired a global read lock. How the storage engine responds to this depends on its crash recovery abilities.

If you use InnoDB tables, you can use the mysqlbackup command from the MySQL Enterprise Backup component to produce a consistent snapshot. This command records the log name and offset corresponding to the snapshot to be used on the slave. MySQL Enterprise Backup is a commercial product that is included as part of a MySQL Enterprise subscription. See Section 30.2, “MySQL Enterprise Backup Overview” for detailed information.

This method also does not work reliably if the master and slave have different values for ft_stopword_file, ft_min_word_len, or ft_max_word_len and you are copying tables having full-text indexes.

Assuming the above exceptions do not apply to your database, use the cold backup technique to obtain a reliable binary snapshot of InnoDB tables: do a slow shutdown of the MySQL Server, then copy the data files manually.

To create a raw data snapshot of MyISAM tables when your MySQL data files exist on a single file system, you can use standard file copy tools such as cp or copy, a remote copy tool such as scp or rsync, an archiving tool such as zip or tar, or a file system snapshot tool such as dump. If you are replicating only certain databases, copy only those files that relate to those tables. For InnoDB, all tables in all databases are stored in the system tablespace files, unless you have the innodb_file_per_table option enabled.

The following files are not required for replication:

  • Files relating to the mysql database.

  • The master info repository file master.info, if used; the use of this file is now deprecated (see Section 17.2.4, “Replication Relay and Status Logs”).

  • The master's binary log files, with the exception of the binary log index file if you are going to use this to locate the master binary log coordinates for the slave.

  • Any relay log files.

Depending on whether you are using InnoDB tables or not, choose one of the following:

If you are using InnoDB tables, and also to get the most consistent results with a raw data snapshot, shut down the master server during the process, as follows:

  1. Acquire a read lock and get the master's status. See Section 17.1.2.4, “Obtaining the Replication Master Binary Log Coordinates”.

  2. In a separate session, shut down the master server:

    shell> mysqladmin shutdown
    
  3. Make a copy of the MySQL data files. The following examples show common ways to do this. You need to choose only one of them:

    shell> tar cf /tmp/db.tar ./data
    shell> zip -r /tmp/db.zip ./data
    shell> rsync --recursive ./data /tmp/dbdata
    
  4. Restart the master server.

If you are not using InnoDB tables, you can get a snapshot of the system from a master without shutting down the server as described in the following steps:

  1. Acquire a read lock and get the master's status. See Section 17.1.2.4, “Obtaining the Replication Master Binary Log Coordinates”.

  2. Make a copy of the MySQL data files. The following examples show common ways to do this. You need to choose only one of them:

    shell> tar cf /tmp/db.tar ./data
    shell> zip -r /tmp/db.zip ./data
    shell> rsync --recursive ./data /tmp/dbdata
    
  3. In the client where you acquired the read lock, release the lock:

    mysql> UNLOCK TABLES;
    

Once you have created the archive or copy of the database, copy the files to each slave before starting the slave replication process.

17.1.2.6 Setting Up Replication Slaves

The following sections describe how to set up slaves. Before you proceed, ensure that you have:

The next steps depend on whether you have existing data to import to the slave or not. See Section 17.1.2.5, “Choosing a Method for Data Snapshots” for more information. Choose one of the following:

17.1.2.6.1 Setting Up Replication with New Master and Slaves

When there is no snapshot of a previous database to import, configure the slave to start the replication from the new master.

To set up replication between a master and a new slave:

  1. Start up the MySQL slave.

  2. Execute a CHANGE MASTER TO statement to set the master replication server configuration. See Section 17.1.2.7, “Setting the Master Configuration on the Slave”.

Perform these slave setup steps on each slave.

This method can also be used if you are setting up new servers but have an existing dump of the databases from a different server that you want to load into your replication configuration. By loading the data into a new master, the data is automatically replicated to the slaves.

If you are setting up a new replication environment using the data from a different existing database server to create a new master, run the dump file generated from that server on the new master. The database updates are automatically propagated to the slaves:

shell> mysql -h master < fulldb.dump
17.1.2.6.2 Setting Up Replication with Existing Data

When setting up replication with existing data, transfer the snapshot from the master to the slave before starting replication. The process for importing data to the slave depends on how you created the snapshot of data on the master.

Choose one of the following:

If you used mysqldump:

  1. Start the slave, using the --skip-slave-start option so that replication does not start.

  2. Import the dump file:

    shell> mysql < fulldb.dump
    

If you created a snapshot using the raw data files:

  1. Extract the data files into your slave data directory. For example:

    shell> tar xvf dbdump.tar
    

    You may need to set permissions and ownership on the files so that the slave server can access and modify them.

  2. Start the slave, using the --skip-slave-start option so that replication does not start.

  3. Configure the slave with the replication coordinates from the master. This tells the slave the binary log file and position within the file where replication needs to start. Also, configure the slave with the login credentials and host name of the master. For more information on the CHANGE MASTER TO statement required, see Section 17.1.2.7, “Setting the Master Configuration on the Slave”.

  4. Start the slave threads:

    mysql> START SLAVE;
    

After you have performed this procedure, the slave connects to the master and replicates any updates that have occurred on the master since the snapshot was taken. Error messages are issued to the slave's error log if it is not able to replicate for any reason.

The slave uses information logged in its master info log and relay log info log to keep track of how much of the master's binary log it has processed. From MySQL 8.0, by default, the repositories for these slave status logs are tables named slave_master_info and slave_relay_log_info in the mysql database. The alternative settings --master-info-repository=FILE and --relay-log-info-repository=FILE, where the repositories are files named master.info and relay-log.info in the data directory, are now deprecated and will be removed in a future release.

Do not remove or edit these tables (or files, if used) unless you know exactly what you are doing and fully understand the implications. Even in that case, it is preferred that you use the CHANGE MASTER TO statement to change replication parameters. The slave uses the values specified in the statement to update the slave status logs automatically. See Section 17.2.4, “Replication Relay and Status Logs”, for more information.

Note

The contents of the master info log override some of the server options specified on the command line or in my.cnf. See Section 17.1.6, “Replication and Binary Logging Options and Variables”, for more details.

A single snapshot of the master suffices for multiple slaves. To set up additional slaves, use the same master snapshot and follow the slave portion of the procedure just described.

17.1.2.7 Setting the Master Configuration on the Slave

To set up the slave to communicate with the master for replication, configure the slave with the necessary connection information. To do this, execute the following statement on the slave, replacing the option values with the actual values relevant to your system:

mysql> CHANGE MASTER TO
    ->     MASTER_HOST='master_host_name',
    ->     MASTER_USER='replication_user_name',
    ->     MASTER_PASSWORD='replication_password',
    ->     MASTER_LOG_FILE='recorded_log_file_name',
    ->     MASTER_LOG_POS=recorded_log_position;
Note

Replication cannot use Unix socket files. You must be able to connect to the master MySQL server using TCP/IP.

The CHANGE MASTER TO statement has other options as well. For example, it is possible to set up secure replication using SSL. For a full list of options, and information about the maximum permissible length for the string-valued options, see Section 13.4.2.1, “CHANGE MASTER TO Syntax”.

Important

As noted in Section 17.1.2.3, “Creating a User for Replication”, if you are not using a secure connection and the user account named in the MASTER_USER option authenticates with the caching_sha2_password plugin (the default from MySQL 8.0), you must specify the MASTER_PUBLIC_KEY_PATH or GET_MASTER_PUBLIC_KEY option in the CHANGE MASTER TO statement to enable RSA key pair-based password exchange.

17.1.2.8 Adding Slaves to a Replication Environment

You can add another slave to an existing replication configuration without stopping the master. To do this, you can set up the new slave by copying the data directory of an existing slave, and giving the new slave a different server ID (which is user-specified) and server UUID (which is generated at startup).

To duplicate an existing slave:

  1. Stop the existing slave and record the slave status information, particularly the master binary log file and relay log file positions. You can view the slave status either in the Performance Schema replication tables (see Section 26.12.11, “Performance Schema Replication Tables”), or by issuing SHOW SLAVE STATUS as follows:

    mysql> STOP SLAVE;
    mysql> SHOW SLAVE STATUS\G
    
  2. Shut down the existing slave:

    shell> mysqladmin shutdown
    
  3. Copy the data directory from the existing slave to the new slave, including the log files and relay log files. You can do this by creating an archive using tar or WinZip, or by performing a direct copy using a tool such as cp or rsync.

    Important
    • Before copying, verify that all the files relating to the existing slave actually are stored in the data directory. For example, the InnoDB system tablespace, undo tablespace, and redo log might be stored in an alternative location. InnoDB tablespace files and file-per-table tablespaces might have been created in other directories. The binary logs and relay logs for the slave might be in their own directories outside the data directory. Check through the system variables that are set for the existing slave and look for any alternative paths that have been specified. If you find any, copy these directories over as well.

    • During copying, if files have been used for the master info and relay log info repositories (see Section 17.2.4, “Replication Relay and Status Logs”), ensure that you also copy these files from the existing slave to the new slave. If tables have been used for the repositories, which is the default from MySQL 8.0, the tables are in the data directory.

    • After copying, delete the auto.cnf file from the copy of the data directory on the new slave, so that the new slave is started with a different generated server UUID. The server UUID must be unique.

    A common problem that is encountered when adding new replication slaves is that the new slave fails with a series of warning and error messages like these:

    071118 16:44:10 [Warning] Neither --relay-log nor --relay-log-index were used; so
    replication may break when this MySQL server acts as a slave and has his hostname
    changed!! Please use '--relay-log=new_slave_hostname-relay-bin' to avoid this problem.
    071118 16:44:10 [ERROR] Failed to open the relay log './old_slave_hostname-relay-bin.003525'
    (relay_log_pos 22940879)
    071118 16:44:10 [ERROR] Could not find target log during relay log initialization
    071118 16:44:10 [ERROR] Failed to initialize the master info structure
    

    This situation can occur if the --relay-log option is not specified, as the relay log files contain the host name as part of their file names. This is also true of the relay log index file if the --relay-log-index option is not used. See Section 17.1.6, “Replication and Binary Logging Options and Variables”, for more information about these options.

    To avoid this problem, use the same value for --relay-log on the new slave that was used on the existing slave. If this option was not set explicitly on the existing slave, use existing_slave_hostname-relay-bin. If this is not possible, copy the existing slave's relay log index file to the new slave and set the --relay-log-index option on the new slave to match what was used on the existing slave. If this option was not set explicitly on the existing slave, use existing_slave_hostname-relay-bin.index. Alternatively, if you have already tried to start the new slave after following the remaining steps in this section and have encountered errors like those described previously, then perform the following steps:

    1. If you have not already done so, issue STOP SLAVE on the new slave.

      If you have already started the existing slave again, issue STOP SLAVE on the existing slave as well.

    2. Copy the contents of the existing slave's relay log index file into the new slave's relay log index file, making sure to overwrite any content already in the file.

    3. Proceed with the remaining steps in this section.

  4. When copying is complete, restart the existing slave.

  5. On the new slave, edit the configuration and give the new slave a unique server ID (using the server-id option) that is not used by the master or any of the existing slaves.

  6. Start the new slave server, specifying the --skip-slave-start option so that replication does not start yet. Use the Performance Schema replication tables or issue SHOW SLAVE STATUS to confirm that the new slave has the correct settings when compared with the existing slave. Also display the server ID and server UUID and verify that these are correct and unique for the new slave.

  7. Start the slave threads by issuing a START SLAVE statement:

    mysql> START SLAVE;

    The new slave now uses the information in its master info repository to start the replication process.

17.1.3 Replication with Global Transaction Identifiers

This section explains transaction-based replication using global transaction identifiers (GTIDs). When using GTIDs, each transaction can be identified and tracked as it is committed on the originating server and applied by any slaves; this means that it is not necessary when using GTIDs to refer to log files or positions within those files when starting a new slave or failing over to a new master, which greatly simplifies these tasks. Because GTID-based replication is completely transaction-based, it is simple to determine whether masters and slaves are consistent; as long as all transactions committed on a master are also committed on a slave, consistency between the two is guaranteed. You can use either statement-based or row-based replication with GTIDs (see Section 17.2.1, “Replication Formats”); however, for best results, we recommend that you use the row-based format.

GTIDs are always preserved between master and slave. This means that you can always determine the source for any transaction applied on any slave by examining its binary log. In addition, once a transaction with a given GTID is committed on a given server, any subsequent transaction having the same GTID is ignored by that server. Thus, a transaction committed on the master can be applied no more than once on the slave, which helps to guarantee consistency.

This section discusses the following topics:

For information about MySQL Server options and variables relating to GTID-based replication, see Section 17.1.6.5, “Global Transaction ID Options and Variables”. See also Section 12.18, “Functions Used with Global Transaction Identifiers (GTIDs)”, which describes SQL functions supported by MySQL 8.0 for use with GTIDs.

17.1.3.1 GTID Format and Storage

A global transaction identifier (GTID) is a unique identifier created and associated with each transaction committed on the server of origin (the master). This identifier is unique not only to the server on which it originated, but is unique across all servers in a given replication topology.

GTID assignment distinguishes between client transactions, which are committed on the master, and replicated transactions, which are reproduced on a slave. When a client transaction is committed on the master, it is assigned a new GTID, provided that the transaction was written to the binary log. Client transactions are guaranteed to have monotonically increasing GTIDs without gaps between the generated numbers. If a client transaction is not written to the binary log (for example, because the transaction was filtered out, or the transaction was read-only), it is not assigned a GTID on the server of origin.

Replicated transactions retain the same GTID that was assigned to the transaction on the server of origin. The GTID is present before the replicated transaction begins to execute, and is persisted even if the replicated transaction is not written to the binary log on the slave, or is filtered out on the slave. The MySQL system table mysql.gtid_executed is used to preserve the assigned GTIDs of all the transactions applied on a MySQL server, except those that are stored in a currently active binary log file.

The auto-skip function for GTIDs means that a transaction committed on the master can be applied no more than once on the slave, which helps to guarantee consistency. Once a transaction with a given GTID has been committed on a given server, any attempt to execute a subsequent transaction with the same GTID is ignored by that server. No error is raised, and no statement in the transaction is executed.

If a transaction with a given GTID has started to execute on a server, but has not yet committed or rolled back, any attempt to start a concurrent transaction on the server with the same GTID will block. The server neither begins to execute the concurrent transaction nor returns control to the client. Once the first attempt at the transaction commits or rolls back, concurrent sessions that were blocking on the same GTID may proceed. If the first attempt rolled back, one concurrent session proceeds to attempt the transaction, and any other concurrent sessions that were blocking on the same GTID remain blocked. If the first attempt committed, all the concurrent sessions stop being blocked, and auto-skip all the statements of the transaction.

A GTID is represented as a pair of coordinates, separated by a colon character (:), as shown here:

GTID = source_id:transaction_id

The source_id identifies the originating server. Normally, the master's server_uuid is used for this purpose. The transaction_id is a sequence number determined by the order in which the transaction was committed on the master. For example, the first transaction to be committed has 1 as its transaction_id, and the tenth transaction to be committed on the same originating server is assigned a transaction_id of 10. It is not possible for a transaction to have 0 as a sequence number in a GTID. For example, the twenty-third transaction to be committed originally on the server with the UUID 3E11FA47-71CA-11E1-9E33-C80AA9429562 has this GTID:

3E11FA47-71CA-11E1-9E33-C80AA9429562:23

The GTID for a transaction is shown in the output from mysqlbinlog, and it is used to identify an individual transaction in the Performance Schema replication status tables, for example, replication_applier_status_by_worker. The value stored by the gtid_next system variable (@@GLOBAL.gtid_next) is a single GTID.

GTID Sets

A GTID set is a set comprising one or more single GTIDs or ranges of GTIDs. GTID sets are used in a MySQL server in several ways. For example, the values stored by the gtid_executed and gtid_purged system variables are GTID sets. The START SLAVE clauses UNTIL SQL_BEFORE_GTIDS and UNTIL SQL_AFTER_GTIDS can be used to make a slave process transactions only up to the first GTID in a GTID set, or stop after the last GTID in a GTID set. The built-in functions GTID_SUBSET() and GTID_SUBTRACT() require GTID sets as input.

A range of GTIDs originating from the same server can be collapsed into a single expression, as shown here:

3E11FA47-71CA-11E1-9E33-C80AA9429562:1-5

The above example represents the first through fifth transactions originating on the MySQL server whose server_uuid is 3E11FA47-71CA-11E1-9E33-C80AA9429562. Multiple single GTIDs or ranges of GTIDs originating from the same server can also be included in a single expression, with the GTIDs or ranges separated by colons, as in the following example:

3E11FA47-71CA-11E1-9E33-C80AA9429562:1-3:11:47-49

A GTID set can include any combination of single GTIDs and ranges of GTIDs, and it can include GTIDs originating from different servers. This example shows the GTID set stored in the gtid_executed system variable (@@GLOBAL.gtid_executed) of a slave that has applied transactions from more than one master:

2174B383-5441-11E8-B90A-C80AA9429562:1-3, 24DA167-0C0C-11E8-8442-00059A3C7B00:1-19

When GTID sets are returned from server variables, UUIDs are in alphabetical order, and numeric intervals are merged and in ascending order.

The syntax for a GTID set is as follows:

gtid_set:
    uuid_set [, uuid_set] ...
    | ''

uuid_set:
    uuid:interval[:interval]...

uuid:
    hhhhhhhh-hhhh-hhhh-hhhh-hhhhhhhhhhhh

h:
    [0-9|A-F]

interval:
    n[-n]

    (n >= 1)
mysql.gtid_executed Table

GTIDs are stored in a table named gtid_executed, in the mysql database. A row in this table contains, for each GTID or set of GTIDs that it represents, the UUID of the originating server, and the starting and ending transaction IDs of the set; for a row referencing only a single GTID, these last two values are the same.

The mysql.gtid_executed table is created (if it does not already exist) when MySQL Server is installed or upgraded, using a CREATE TABLE statement similar to that shown here:

CREATE TABLE gtid_executed (
    source_uuid CHAR(36) NOT NULL,
    interval_start BIGINT(20) NOT NULL,
    interval_end BIGINT(20) NOT NULL,                                                                                                                                                                                  
    PRIMARY KEY (source_uuid, interval_start)
)      
Warning

As with other MySQL system tables, do not attempt to create or modify this table yourself.

The mysql.gtid_executed table is provided for internal use by the MySQL server. It enables a slave to use GTIDs when binary logging is disabled on the slave, and it enables retention of the GTID state when the binary logs have been lost. Note that the mysql.gtid_executed table is cleared if you issue RESET MASTER.

GTIDs are stored in the mysql.gtid_executed table only when gtid_mode is ON or ON_PERMISSIVE. The point at which GTIDs are stored depends on whether binary logging is enabled or disabled:

  • If binary logging is disabled (log_bin is OFF), or if log_slave_updates is disabled, the server stores the GTID belonging to each transaction together with the transaction in the table. In addition, the table is compressed periodically at a user-configurable rate; see mysql.gtid_executed Table Compression, for more information. This situation can only apply on a replication slave where binary logging or slave update logging is disabled. It does not apply on a replication master, because on a master, binary logging must be enabled for replication to take place.

  • If binary logging is enabled (log_bin is ON), whenever the binary log is rotated or the server is shut down, the server writes GTIDs for all transactions that were written into the previous binary log into the mysql.gtid_executed table. This situation applies on a replication master, or a replication slave where binary logging is enabled.

    In the event of the server stopping unexpectedly, the set of GTIDs from the current binary log file is not saved in the mysql.gtid_executed table. These GTIDs are added to the table from the binary log file during recovery. The exception to this is if you disable binary logging when the server is restarted (using --skip-log-bin or --disable-log-bin). In this situation, the server cannot access the binary log file to recover the GTIDs, so replication cannot be started.

    When binary logging is enabled, the mysql.gtid_executed table does not hold a complete record of the GTIDs for all executed transactions. That information is provided by the global value of the gtid_executed system variable. Always use @@GLOBAL.gtid_executed, which is updated after every commit, to represent the GTID state for the MySQL server, and do not query the mysql.gtid_executed table.

The MySQL server can write to the mysql.gtid_executed table even when the server is in read only or super read only mode, so that the binary log file can still be rotated in these modes. If the mysql.gtid_executed table cannot be accessed for writes, and the binary log file is rotated for any reason other than reaching the maximum file size (max_binlog_size), the current binary log file continues to be used. An error message is returned to the client that requested the rotation, and a warning is logged on the server. If the mysql.gtid_executed table cannot be accessed for writes and max_binlog_size is reached, the server responds according to its binlog_error_action setting. If IGNORE_ERROR is set, an error is logged on the server and binary logging is halted, or if ABORT_SERVER is set, the server shuts down.

mysql.gtid_executed Table Compression

Over the course of time, the mysql.gtid_executed table can become filled with many rows referring to individual GTIDs that originate on the same server, and whose transaction IDs make up a range, similar to what is shown here:

+--------------------------------------+----------------+--------------+
| source_uuid                          | interval_start | interval_end |
|--------------------------------------+----------------+--------------|
| 3E11FA47-71CA-11E1-9E33-C80AA9429562 | 37             | 37           |
| 3E11FA47-71CA-11E1-9E33-C80AA9429562 | 38             | 38           |
| 3E11FA47-71CA-11E1-9E33-C80AA9429562 | 39             | 39           |
| 3E11FA47-71CA-11E1-9E33-C80AA9429562 | 40             | 40           |
| 3E11FA47-71CA-11E1-9E33-C80AA9429562 | 41             | 41           |
| 3E11FA47-71CA-11E1-9E33-C80AA9429562 | 42             | 42           |
| 3E11FA47-71CA-11E1-9E33-C80AA9429562 | 43             | 43           |
...

To save space, the MySQL server compresses the mysql.gtid_executed table periodically by replacing each such set of rows with a single row that spans the entire interval of transaction identifiers, like this:

+--------------------------------------+----------------+--------------+
| source_uuid                          | interval_start | interval_end |
|--------------------------------------+----------------+--------------|
| 3E11FA47-71CA-11E1-9E33-C80AA9429562 | 37             | 43           |
...

You can control the number of transactions that are allowed to elapse before the table is compressed, and thus the compression rate, by setting the gtid_executed_compression_period system variable. This variable's default value is 1000, meaning that by default, compression of the table is performed after each 1000 transactions. Setting gtid_executed_compression_period to 0 prevents the compression from being performed at all, and you should be prepared for a potentially large increase in the amount of disk space that may be required by the gtid_executed table if you do this.

Note

When binary logging is enabled, the value of gtid_executed_compression_period is not used and the mysql.gtid_executed table is compressed on each binary log rotation.

Compression of the mysql.gtid_executed table is performed by a dedicated foreground thread named thread/sql/compress_gtid_table. This thread is not listed in the output of SHOW PROCESSLIST, but it can be viewed as a row in the threads table, as shown here:

mysql> SELECT * FROM performance_schema.threads WHERE NAME LIKE '%gtid%'\G
*************************** 1. row ***************************
          THREAD_ID: 26
               NAME: thread/sql/compress_gtid_table
               TYPE: FOREGROUND
     PROCESSLIST_ID: 1
   PROCESSLIST_USER: NULL
   PROCESSLIST_HOST: NULL
     PROCESSLIST_DB: NULL
PROCESSLIST_COMMAND: Daemon
   PROCESSLIST_TIME: 1509
  PROCESSLIST_STATE: Suspending
   PROCESSLIST_INFO: NULL
   PARENT_THREAD_ID: 1
               ROLE: NULL
       INSTRUMENTED: YES
            HISTORY: YES
    CONNECTION_TYPE: NULL
       THREAD_OS_ID: 18677

The thread/sql/compress_gtid_table thread normally sleeps until gtid_executed_compression_period transactions have been executed, then wakes up to perform compression of the mysql.gtid_executed table as described previously. It then sleeps until another gtid_executed_compression_period transactions have taken place, then wakes up to perform the compression again, repeating this loop indefinitely. Setting this value to 0 when binary logging is disabled means that the thread always sleeps and never wakes up.

17.1.3.2 GTID Life Cycle

The life cycle of a GTID consists of the following steps:

  1. A transaction is executed and committed on the master. This client transaction is assigned a GTID composed of the master's UUID and the smallest nonzero transaction sequence number not yet used on this server. The GTID is written to the master's binary log (immediately preceding the transaction itself in the log). If a client transaction is not written to the binary log (for example, because the transaction was filtered out, or the transaction was read-only), it is not assigned a GTID.

  2. If a GTID was assigned for the transaction, the GTID is persisted atomically at commit time by writing it to the binary log at the beginning of the transaction (as a Gtid_log_event). Whenever the binary log is rotated or the server is shut down, the server writes GTIDs for all transactions that were written into the previous binary log file into the mysql.gtid_executed table.

  3. If a GTID was assigned for the transaction, the GTID is externalized non-atomically (very shortly after the transaction is committed) by adding it to the set of GTIDs in the gtid_executed system variable (@@GLOBAL.gtid_executed). This GTID set contains a representation of the set of all committed GTID transactions, and it is used in replication as a token that represents the server state. With binary logging enabled (as required for the master), the set of GTIDs in the gtid_executed system variable is a complete record of the transactions applied, but the mysql.gtid_executed table is not, because the most recent history is still in the current binary log file.

  4. After the binary log data is transmitted to the slave and stored in the slave's relay log (using established mechanisms for this process, see Section 17.2, “Replication Implementation”, for details), the slave reads the GTID and sets the value of its gtid_next system variable as this GTID. This tells the slave that the next transaction must be logged using this GTID. It is important to note that the slave sets gtid_next in a session context.

  5. The slave verifies that no thread has yet taken ownership of the GTID in gtid_next in order to process the transaction. By reading and checking the replicated transaction's GTID first, before processing the transaction itself, the slave guarantees not only that no previous transaction having this GTID has been applied on the slave, but also that no other session has already read this GTID but has not yet committed the associated transaction. So if multiple clients attempt to apply the same transaction concurrently, the server resolves this by letting only one of them execute. The gtid_owned system variable (@@GLOBAL.gtid_owned) for the slave shows each GTID that is currently in use and the ID of the thread that owns it. If the GTID has already been used, no error is raised, and the auto-skip function is used to ignore the transaction.

  6. If the GTID has not been used, the slave applies the replicated transaction. Because gtid_next is set to the GTID already assigned by the master, the slave does not attempt to generate a new GTID for this transaction, but instead uses the GTID stored in gtid_next.

  7. If binary logging is enabled on the slave, the GTID is persisted atomically at commit time by writing it to the binary log at the beginning of the transaction (as a Gtid_log_event). Whenever the binary log is rotated or the server is shut down, the server writes GTIDs for all transactions that were written into the previous binary log file into the mysql.gtid_executed table.

  8. If binary logging is disabled on the slave, the GTID is persisted atomically by writing it directly into the mysql.gtid_executed table. MySQL appends a statement to the transaction to insert the GTID into the table. From MySQL 8.0, this operation is atomic for DDL statements as well as for DML statements. In this situation, the mysql.gtid_executed table is a complete record of the transactions applied on the slave.

  9. Very shortly after the replicated transaction is committed on the slave, the GTID is externalized non-atomically by adding it to the set of GTIDs in the gtid_executed system variable (@@GLOBAL.gtid_executed) for the slave. As for the master, this GTID set contains a representation of the set of all committed GTID transactions. If binary logging is disabled on the slave, the mysql.gtid_executed table is also a complete record of the transactions applied on the slave. If binary logging is enabled on the slave, meaning that some GTIDs are only recorded in the binary log, the set of GTIDs in the gtid_executed system variable is the only complete record.

Client transactions that are completely filtered out on the master are not assigned a GTID, therefore they are not added to the set of transactions in the gtid_executed system variable, or added to the mysql.gtid_executed table. However, the GTIDs of replicated transactions that are completely filtered out on the slave are persisted. If binary logging is enabled on the slave, the filtered-out transaction is written to the binary log as a Gtid_log_event followed by an empty transaction containing only BEGIN and COMMIT statements. If binary logging is disabled, the GTID of the filtered-out transaction is written to the mysql.gtid_executed table. Preserving the GTIDs for filtered-out transactions ensures that the mysql.gtid_executed table and the set of GTIDs in the gtid_executed system variable can be compressed. It also ensures that the filtered-out transactions are not retrieved again if the slave reconnects to the master, as explained in Section 17.1.3.3, “GTID Auto-Positioning”.

On a multithreaded replication slave (with slave_parallel_workers > 0 ), transactions can be applied in parallel, so replicated transactions can commit out of order (unless slave_preserve_commit_order=1 is set). When that happens, the set of GTIDs in the gtid_executed system variable will contain multiple GTID ranges with gaps between them. (On a master or a single-threaded replication slave, there will be monotonically increasing GTIDs without gaps between the numbers.) Gaps on multithreaded replication slaves only occur among the most recently applied transactions, and are filled in as replication progresses. When replication threads are stopped cleanly using the STOP SLAVE statement, ongoing transactions are applied so that the gaps are filled in. In the event of a shutdown such as a server failure or the use of the KILL statement to stop replication threads, the gaps might remain.

What changes are assigned a GTID?

The typical scenario is that the server generates a new GTID for a committed transaction. However, GTIDs can also be assigned to other changes besides transactions, and in some cases a single transaction can be assigned multiple GTIDs.

Every database change (DDL or DML) that is written to the binary log is assigned a GTID. This includes changes that are autocommitted, and changes that are committed using BEGIN and COMMIT or START TRANSACTION statements. A GTID is also assigned to the creation, alteration, or deletion of a database, and of a non-table database object such as a procedure, function, trigger, event, view, user, role, or grant.

Non-transactional updates as well as transactional updates are assigned GTIDs. In addition, for a non-transactional update, if a disk write failure occurs while attempting to write to the binary log cache and a gap is therefore created in the binary log, the resulting incident log event is assigned a GTID.

When a table is automatically dropped by a generated statement in the binary log, a GTID is assigned to the statement. Temporary tables are dropped automatically when a replication slave begins to apply events from a master that has just been started, and when statement-based replication is in use (binlog_format=STATEMENT) and a user session that has open temporary tables disconnects. Tables that use the MEMORY storage engine are deleted automatically the first time they are accessed after the server is started, because rows might have been lost during the shutdown.

When a transaction is not written to the binary log on the server of origin, the server does not assign a GTID to it. This includes transactions that are rolled back and transactions that are executed while binary logging is disabled on the server of origin, either globally (with --skip-log-bin specified in the server's configuration) or for the session (SET @@SESSION.sql_log_bin = 0). This also includes no-op transactions when row-based replication is in use (binlog_format=ROW).

XA transactions are assigned separate GTIDs for the XA PREPARE phase of the transaction and the XA COMMIT or XA ROLLBACK phase of the transaction. XA transactions are persistently prepared so that users can commit them or roll them back in the case of a failure (which in a replication topology might include a failover to another server). The two parts of the transaction are therefore replicated separately, so they must have their own GTIDs, even though a non-XA transaction that is rolled back would not have a GTID.

In the following special cases, a single statement can generate multiple transactions, and therefore be assigned multiple GTIDs:

  • A stored procedure is invoked that commits multiple transactions. One GTID is generated for each transaction that the procedure commits.

  • A multi-table DROP TABLE statement drops tables of different types. Multiple GTIDs can be generated if any of the tables use storage engines that do not support atomic DDL, or if any of the tables are temporary tables.

  • A CREATE TABLE ... SELECT statement is issued when row-based replication is in use (binlog_format=ROW). One GTID is generated for the CREATE TABLE action and one GTID is generated for the row-insert actions.

The gtid_next System Variable

By default, for new transactions committed in user sessions, the server automatically generates and assigns a new GTID. When the transaction is applied on a replication slave, the GTID from the server of origin is preserved. You can change this behavior by setting the session value of the gtid_next system variable:

  • When gtid_next is set to AUTOMATIC, which is the default, and a transaction is committed and written to the binary log, the server automatically generates and assigns a new GTID. If a transaction is rolled back or not written to the binary log for another reason, the server does not generate and assign a GTID.

  • If you set gtid_next to a valid GTID (consisting of a UUID and a transaction sequence number, separated by a colon), the server assigns that GTID to your transaction. This GTID is assigned and added to gtid_executed even when the transaction is not written to the binary log, or when the transaction is empty.

Note that after you set gtid_next to a specific GTID, and the transaction has been committed or rolled back, an explicit SET @@SESSION.gtid_next statement must be issued before any other statement. You can use this to set the GTID value back to AUTOMATIC if you do not want to assign any more GTIDs explicitly.

When replication applier threads apply replicated transactions, they use this technique, setting @@SESSION.gtid_next explicitly to the GTID of the replicated transaction as assigned on the server of origin. This means the GTID from the server of origin is retained, rather than a new GTID being generated and assigned by the replication slave. It also means the GTID is added to gtid_executed on the replication slave even when binary logging or slave update logging is disabled on the slave, or when the transaction is a no-op or is filtered out on the slave.

It is possible for a client to simulate a replicated transaction by setting @@SESSION.gtid_next to a specific GTID before executing the transaction. This technique is used by mysqlbinlog to generate a dump of the binary log that the client can replay to preserve GTIDs. A simulated replicated transaction committed through a client is completely equivalent to a replicated transaction committed through a replication applier thread, and they cannot be distinguished after the fact.

The gtid_purged System Variable

The set of GTIDs in the gtid_purged system variable (@@GLOBAL.gtid_purged) contains the GTIDs of all the transactions that have been committed on the server, but do not exist in any binary log file on the server. gtid_purged is a subset of gtid_executed. The following categories of GTIDs are in gtid_purged:

  • GTIDs of replicated transactions that were committed with binary logging disabled on the slave.

  • GTIDs of transactions that were written to a binary log file that has now been purged.

  • GTIDs that were added explicitly to the set by the statement SET @@GLOBAL.gtid_purged.

You can change the value of gtid_purged in order to record on the server that the transactions in a certain GTID set have been applied, although they do not exist in any binary log on the server. When you add GTIDs to gtid_purged, they are also added to gtid_executed. An example use case for this action is when you are restoring a backup of one or more databases on a server, but you do not have the relevant binary logs containing the transactions on the server. Before MySQL 8.0, you could only change the value of gtid_purged when gtid_executed (and therefore gtid_purged) was empty. From MySQL 8.0, this restriction does not apply, and you can also choose whether to replace the whole GTID set in gtid_purged with a specified GTID set, or to add a specified GTID set to the GTIDs already in gtid_purged. For details of how to do this, see the description for gtid_purged.

The sets of GTIDs in the gtid_executed and gtid_purged system variables are initialized when the server starts. Every binary log file begins with the event Previous_gtids_log_event, which contains the set of GTIDs in all previous binary log files (composed from the GTIDs in the preceding file's Previous_gtids_log_event, and the GTIDs of every Gtid_log_event in the preceding file itself). The contents of Previous_gtids_log_event in the oldest and most recent binary log files are used to compute the gtid_executed and gtid_purged sets at server startup:

  • gtid_executed is computed as the union of the GTIDs in Previous_gtids_log_event in the most recent binary log file, the GTIDs of transactions in that binary log file, and the GTIDs stored in the mysql.gtid_executed table. This GTID set contains all the GTIDs that have been used (or added explicitly to gtid_purged) on the server, whether or not they are currently in a binary log file on the server. It does not include the GTIDs for transactions that are currently being processed on the server (@@GLOBAL.gtid_owned).

  • gtid_purged is computed by first adding the GTIDs in Previous_gtids_log_event in the most recent binary log file and the GTIDs of transactions in that binary log file. This step gives the set of GTIDs that are currently, or were once, recorded in a binary log on the server (gtids_in_binlog). Next, the GTIDs in Previous_gtids_log_event in the oldest binary log file are subtracted from gtids_in_binlog. This step gives the set of GTIDs that are currently recorded in a binary log on the server (gtids_in_binlog_not_purged). Finally, gtids_in_binlog_not_purged is subtracted from gtid_executed. The result is the set of GTIDs that have been used on the server, but are not currently recorded in a binary log file on the server, and this result is used to initialize gtid_purged.

If binary logs from MySQL 5.7.7 or older are involved in these computations, it is possible for incorrect GTID sets to be computed for gtid_executed and gtid_purged, and they remain incorrect even if the server is later restarted. For details, see the description for the binlog_gtid_simple_recovery system variable, which controls how the binary logs are iterated to compute the GTID sets. If one of the situations described there applies on a server, set binlog_gtid_simple_recovery=FALSE in the server's configuration file before starting it. That setting makes the server iterate all the binary log files (not just the newest and oldest) to find where GTID events start to appear. This process could take a long time if the server has a large number of binary log files without GTID events.

Resetting the GTID Execution History

If you need to reset the GTID execution history on a server, use the RESET MASTER statement. For example, you might need to do this after carrying out test queries to verify a replication setup on new GTID-enabled servers, or when you want to join a new server to a replication group but it contains some unwanted local transactions that are not accepted by Group Replication.

Warning

Use RESET MASTER with caution to avoid losing any wanted GTID execution history and binary log files.

Before issuing the statement, ensure that you have backups of the server's binary log files and binary log index file, if any, and obtain and save the GTID set held in the global value of the gtid_executed system variable (for example, by issuing a SELECT @@GLOBAL.gtid_executed statement and saving the results). If you are removing unwanted transactions from that GTID set, use mysqlbinlog to examine the contents of the transactions to ensure that they have no value, contain no data that must be saved or replicated, and did not result in data changes on the server.

When you issue RESET MASTER, the following reset operations are carried out:

  • The value of the gtid_purged system variable is set to an empty string ('').

  • The global value (but not the session value) of the gtid_executed system variable is set to an empty string.

  • The mysql.gtid_executed table is cleared (see mysql.gtid_executed Table).

  • If the server has binary logging enabled, the existing binary log files are deleted and the binary log index file is cleared.

Note that RESET MASTER is the method to reset the GTID execution history even if the server is a replication slave where binary logging is disabled. RESET SLAVE has no effect on the GTID execution history.

17.1.3.3 GTID Auto-Positioning

GTIDs replace the file-offset pairs previously required to determine points for starting, stopping, or resuming the flow of data between master and slave. When GTIDs are in use, all the information that the slave needs for synchronizing with the master is obtained directly from the replication data stream.

To start a slave using GTID-based replication, you do not include MASTER_LOG_FILE or MASTER_LOG_POS options in the CHANGE MASTER TO statement used to direct the slave to replicate from a given master. These options specify the name of the log file and the starting position within the file, but with GTIDs the slave does not need this nonlocal data. Instead, you need to enable the MASTER_AUTO_POSITION option. For full instructions to configure and start masters and slaves using GTID-based replication, see Section 17.1.3.4, “Setting Up Replication Using GTIDs”.

The MASTER_AUTO_POSITION option is disabled by default. If multi-source replication is enabled on the slave, you need to set the option for each applicable replication channel. Disabling the MASTER_AUTO_POSITION option again makes the slave revert to file-based replication, in which case you must also specify one or both of the MASTER_LOG_FILE or MASTER_LOG_POS options.

When a replication slave has GTIDs enabled (GTID_MODE=ON, ON_PERMISSIVE, or OFF_PERMISSIVE ) and the MASTER_AUTO_POSITION option enabled, auto-positioning is activated for connection to the master. The master must have GTID_MODE=ON set in order for the connection to succeed. In the initial handshake, the slave sends a GTID set containing the transactions that it has already received, committed, or both. This GTID set is equal to the union of the set of GTIDs in the gtid_executed system variable (@@GLOBAL.gtid_executed), and the set of GTIDs recorded in the Performance Schema replication_connection_status table as received transactions (the result of the statement SELECT RECEIVED_TRANSACTION_SET FROM PERFORMANCE_SCHEMA.replication_connection_status).

The master responds by sending all transactions recorded in its binary log whose GTID is not included in the GTID set sent by the slave. This exchange ensures that the master only sends the transactions with a GTID that the slave has not already received or committed. If the slave receives transactions from more than one master, as in the case of a diamond topology, the auto-skip function ensures that the transactions are not applied twice.

If any of the transactions that should be sent by the master have been purged from the master's binary log, or added to the set of GTIDs in the gtid_purged system variable by another method, the master sends the error ER_MASTER_HAS_PURGED_REQUIRED_GTIDS to the slave, and replication does not start. The GTIDs of the missing purged transactions are identified and listed in the master's error log in the warning message ER_FOUND_MISSING_GTIDS. The slave cannot recover automatically from this error because parts of the transaction history that are needed to catch up with the master have been purged. Attempting to reconnect without the MASTER_AUTO_POSITION option enabled only results in the loss of the purged transactions on the slave. The correct approach to recover from this situation is for the slave to replicate the missing transactions listed in the ER_FOUND_MISSING_GTIDS message from another source, or for the slave to be replaced by a new slave created from a more recent backup. Consider revising the binary log expiration period (binlog_expire_logs_seconds) on the master to ensure that the situation does not occur again.

If during the exchange of transactions it is found that the slave has received or committed transactions with the master's UUID in the GTID, but the master itself does not have a record of them, the master sends the error ER_SLAVE_HAS_MORE_GTIDS_THAN_MASTER to the slave and replication does not start. This situation can occur if a master that does not have sync_binlog=1 set experiences a power failure or operating system crash, and loses committed transactions that have not yet been synchronized to the binary log file, but have been received by the slave. The master and slave can diverge if any clients commit transactions on the master after it is restarted, which can lead to the situation where the master and slave are using the same GTID for different transactions. The correct approach to recover from this situation is to check manually whether the master and slave have diverged. If the same GTID is now in use for different transactions, you either need to perform manual conflict resolution for individual transactions as required, or remove either the master or the slave from the replication topology. If the issue is only missing transactions on the master, you can make the master into a slave instead, allow it to catch up with the other servers in the replication topology, and then make it a master again if needed.

17.1.3.4 Setting Up Replication Using GTIDs

This section describes a process for configuring and starting GTID-based replication in MySQL 8.0. This is a cold start procedure that assumes either that you are starting the replication master for the first time, or that it is possible to stop it; for information about provisioning replication slaves using GTIDs from a running master, see Section 17.1.3.5, “Using GTIDs for Failover and Scaleout”. For information about changing GTID mode on servers online, see Section 17.1.5, “Changing Replication Modes on Online Servers”.

The key steps in this startup process for the simplest possible GTID replication topology, consisting of one master and one slave, are as follows:

  1. If replication is already running, synchronize both servers by making them read-only.

  2. Stop both servers.

  3. Restart both servers with GTIDs enabled and the correct options configured.

    The mysqld options necessary to start the servers as described are discussed in the example that follows later in this section.

  4. Instruct the slave to use the master as the replication data source and to use auto-positioning. The SQL statements needed to accomplish this step are described in the example that follows later in this section.

  5. Take a new backup. Binary logs containing transactions without GTIDs cannot be used on servers where GTIDs are enabled, so backups taken before this point cannot be used with your new configuration.

  6. Start the slave, then disable read-only mode on both servers, so that they can accept updates.

In the following example, two servers are already running as master and slave, using MySQL's binary log position-based replication protocol. If you are starting with new servers, see Section 17.1.2.3, “Creating a User for Replication” for information about adding a specific user for replication connections and Section 17.1.2.1, “Setting the Replication Master Configuration” for information about setting the server_id variable. The following examples show how to store mysqld startup options in server's option file, see Section 4.2.2.2, “Using Option Files” for more information. Alternatively you can use startup options when running mysqld.

Most of the steps that follow require the use of the MySQL root account or another MySQL user account that has the SUPER privilege. mysqladmin shutdown requires either the SUPER privilege or the SHUTDOWN privilege.

Step 1: Synchronize the servers.  This step is only required when working with servers which are already replicating without using GTIDs. For new servers proceed to Step 3. Make the servers read-only by setting the read_only system variable to ON on each server by issuing the following:

mysql> SET @@GLOBAL.read_only = ON;

Wait for all ongoing transactions to commit or roll back. Then, allow the slave to catch up with the master. It is extremely important that you make sure the slave has processed all updates before continuing.

If you use binary logs for anything other than replication, for example to do point in time backup and restore, wait until you do not need the old binary logs containing transactions without GTIDs. Ideally, wait for the server to purge all binary logs, and wait for any existing backup to expire.

Important

It is important to understand that logs containing transactions without GTIDs cannot be used on servers where GTIDs are enabled. Before proceeding, you must be sure that transactions without GTIDs do not exist anywhere in the topology.

Step 2: Stop both servers.  Stop each server using mysqladmin as shown here, where username is the user name for a MySQL user having sufficient privileges to shut down the server:

shell> mysqladmin -uusername -p shutdown

Then supply this user's password at the prompt.

Step 3: Start both servers with GTIDs enabled.  To enable GTID-based replication, each server must be started with GTID mode enabled by setting the gtid_mode variable to ON, and with the enforce_gtid_consistency variable enabled to ensure that only statements which are safe for GTID-based replication are logged. For example:

gtid_mode=ON
enforce-gtid-consistency=true

In addition, you should start slaves with the --skip-slave-start option before configuring the slave settings. For more information on GTID related options and variables, see Section 17.1.6.5, “Global Transaction ID Options and Variables”.

It is not mandatory to have binary logging enabled in order to use GTIDs when using the mysql.gtid_executed Table. Masters must always have binary logging enabled in order to be able to replicate. However, slave servers can use GTIDs but without binary logging. If you need to disable binary logging on a slave server, you can do this by specifying the --skip-log-bin and --skip-log-slave-updates options for the slave.

Step 4: Configure the slave to use GTID-based auto-positioning.  Tell the slave to use the master with GTID based transactions as the replication data source, and to use GTID-based auto-positioning rather than file-based positioning. Issue a CHANGE MASTER TO statement on the slave, including the MASTER_AUTO_POSITION option in the statement to tell the slave that the master's transactions are identified by GTIDs.

You may also need to supply appropriate values for the master's host name and port number as well as the user name and password for a replication user account which can be used by the slave to connect to the master; if these have already been set prior to Step 1 and no further changes need to be made, the corresponding options can safely be omitted from the statement shown here.

mysql> CHANGE MASTER TO
     >     MASTER_HOST = host,
     >     MASTER_PORT = port,
     >     MASTER_USER = user,
     >     MASTER_PASSWORD = password,
     >     MASTER_AUTO_POSITION = 1;

Neither the MASTER_LOG_FILE option nor the MASTER_LOG_POS option may be used with MASTER_AUTO_POSITION set equal to 1. Attempting to do so causes the CHANGE MASTER TO statement to fail with an error.

Step 5: Take a new backup.  Existing backups that were made before you enabled GTIDs can no longer be used on these servers now that you have enabled GTIDs. Take a new backup at this point, so that you are not left without a usable backup.

For instance, you can execute FLUSH LOGS on the server where you are taking backups. Then either explicitly take a backup or wait for the next iteration of any periodic backup routine you may have set up.

Step 6: Start the slave and disable read-only mode.  Start the slave like this:

mysql> START SLAVE;

The following step is only necessary if you configured a server to be read-only in Step 1. To allow the server to begin accepting updates again, issue the following statement:

mysql> SET @@GLOBAL.read_only = OFF;

GTID-based replication should now be running, and you can begin (or resume) activity on the master as before. Section 17.1.3.5, “Using GTIDs for Failover and Scaleout”, discusses creation of new slaves when using GTIDs.

17.1.3.5 Using GTIDs for Failover and Scaleout

There are a number of techniques when using MySQL Replication with Global Transaction Identifiers (GTIDs) for provisioning a new slave which can then be used for scaleout, being promoted to master as necessary for failover. This section describes the following techniques:

Global transaction identifiers were added to MySQL Replication for the purpose of simplifying in general management of the replication data flow and of failover activities in particular. Each identifier uniquely identifies a set of binary log events that together make up a transaction. GTIDs play a key role in applying changes to the database: the server automatically skips any transaction having an identifier which the server recognizes as one that it has processed before. This behavior is critical for automatic replication positioning and correct failover.

The mapping between identifiers and sets of events comprising a given transaction is captured in the binary log. This poses some challenges when provisioning a new server with data from another existing server. To reproduce the identifier set on the new server, it is necessary to copy the identifiers from the old server to the new one, and to preserve the relationship between the identifiers and the actual events. This is neccessary for restoring a slave that is immediately available as a candidate to become a new master on failover or switchover.

Simple replication.  The easiest way to reproduce all identifiers and transactions on a new server is to make the new server into the slave of a master that has the entire execution history, and enable global transaction identifiers on both servers. See Section 17.1.3.4, “Setting Up Replication Using GTIDs”, for more information.

Once replication is started, the new server copies the entire binary log from the master and thus obtains all information about all GTIDs.

This method is simple and effective, but requires the slave to read the binary log from the master; it can sometimes take a comparatively long time for the new slave to catch up with the master, so this method is not suitable for fast failover or restoring from backup. This section explains how to avoid fetching all of the execution history from the master by copying binary log files to the new server.

Copying data and transactions to the slave.  Executing the entire transaction history can be time-consuming when the source server has processed a large number of transactions previously, and this can represent a major bottleneck when setting up a new replication slave. To eliminate this requirement, a snapshot of the data set, the binary logs and the global transaction information the source server contains can be imported to the new slave. The source server can be either the master or the slave, but you must ensure that the source has processed all required transactions before copying the data.

There are several variants of this method, the difference being in the manner in which data dumps and transactions from binary logs are transfered to the slave, as outlined here:

Data Set
  1. Create a dump file using mysqldump on the source server. Set the mysqldump option --master-data (with the default value of 1) to include a CHANGE MASTER TO statement with binary logging information. Set the --set-gtid-purged option to AUTO (the default) or ON, to include information about executed transactions in the dump. Then use the mysql client to import the dump file on the target server.

  2. Alternatively, create a data snapshot of the source server using raw data files, then copy these files to the target server, following the instructions in Section 17.1.2.5, “Choosing a Method for Data Snapshots”. If you use InnoDB tables, you can use the mysqlbackup command from the MySQL Enterprise Backup component to produce a consistent snapshot. This command records the log name and offset corresponding to the snapshot to be used on the slave. MySQL Enterprise Backup is a commercial product that is included as part of a MySQL Enterprise subscription. See Section 30.2, “MySQL Enterprise Backup Overview” for detailed information.

  3. Alternatively, stop both the source and target servers, copy the contents of the source's data directory to the new slave's data directory, then restart the slave. If you use this method, the slave must be configured for GTID-based replication, in other words with gtid_mode=ON. For instructions and important information for this method, see Section 17.1.2.8, “Adding Slaves to a Replication Environment”.

Transaction History

If the source server has a complete transaction history in its binary logs (that is, the GTID set @@GLOBAL.gtid_purged is empty), you can use these methods.

  1. Import the binary logs from the source server to the new slave using mysqlbinlog, with the --read-from-remote-server and --read-from-remote-master options.

  2. Alternatively, copy the source server's binary log files to the slave. You can make copies from the slave using mysqlbinlog with the --read-from-remote-server and --raw options. These can be read into the slave by using mysqlbinlog > file (without the --raw option) to export the binary log files to SQL files, then passing these files to the mysql client for processing. Ensure that all of the binary log files are processed using a single mysql process, rather than multiple connections. For example:

    shell> mysqlbinlog copied-binlog.000001 copied-binlog.000002 | mysql -u root -p
    

    For more information, see Section 4.6.8.3, “Using mysqlbinlog to Back Up Binary Log Files”.

This method has the advantage that a new server is available almost immediately; only those transactions that were committed while the snapshot or dump file was being replayed still need to be obtained from the existing master. This means that the slave's availability is not instantanteous, but only a relatively short amount of time should be required for the slave to catch up with these few remaining transactions.

Copying over binary logs to the target server in advance is usually faster than reading the entire transaction execution history from the master in real time. However, it may not always be feasible to move these files to the target when required, due to size or other considerations. The two remaining methods for provisioning a new slave discussed in this section use other means to transfer information about transactions to the new slave.

Injecting empty transactions.  The master's global gtid_executed variable contains the set of all transactions executed on the master. Rather than copy the binary logs when taking a snapshot to provision a new server, you can instead note the content of gtid_executed on the server from which the snapshot was taken. Before adding the new server to the replication chain, simply commit an empty transaction on the new server for each transaction identifier contained in the master's gtid_executed, like this:

SET GTID_NEXT='aaa-bbb-ccc-ddd:N';

BEGIN;
COMMIT;

SET GTID_NEXT='AUTOMATIC';

Once all transaction identifiers have been reinstated in this way using empty transactions, you must flush and purge the slave's binary logs, as shown here, where N is the nonzero suffix of the current binary log file name:

FLUSH LOGS;
PURGE BINARY LOGS TO 'master-bin.00000N';

You should do this to prevent this server from flooding the replication stream with false transactions in the event that it is later promoted to master. (The FLUSH LOGS statement forces the creation of a new binary log file; PURGE BINARY LOGS purges the empty transactions, but retains their identifiers.)

This method creates a server that is essentially a snapshot, but in time is able to become a master as its binary log history converges with that of the replication stream (that is, as it catches up with the master or masters). This outcome is similar in effect to that obtained using the remaining provisioning method, which we discuss in the next few paragraphs.

Excluding transactions with gtid_purged.  The master's global gtid_purged variable contains the set of all transactions that have been purged from the master's binary log. As with the method discussed previously (see Injecting empty transactions), you can record the value of gtid_executed on the server from which the snapshot was taken (in place of copying the binary logs to the new server). Unlike the previous method, there is no need to commit empty transactions (or to issue PURGE BINARY LOGS); instead, you can set gtid_purged on the slave directly, based on the value of gtid_executed on the server from which the backup or snapshot was taken.

As with the method using empty transactions, this method creates a server that is functionally a snapshot, but in time is able to become a master as its binary log history converges with that of the replication master or group.

Restoring GTID mode slaves.  When restoring a slave in a GTID based replication setup that has encountered an error, injecting an empty transaction may not solve the problem because an event does not have a GTID.

Use mysqlbinlog to find the next transaction, which is probably the first transaction in the next log file after the event. Copy everything up to the COMMIT for that transaction, being sure to include the SET @@SESSION.gtid_next. Even if you are not using row-based replication, you can still run binary log row events in the command line client.

Stop the slave and run the transaction you copied. The mysqlbinlog output sets the delimiter to /*!*/;, so set it back:

mysql> DELIMITER ;

Restart replication from the correct position automatically:

mysql> SET GTID_NEXT=automatic;
mysql> RESET SLAVE;
mysql> START SLAVE;

17.1.3.6 Restrictions on Replication with GTIDs

Because GTID-based replication is dependent on transactions, some features otherwise available in MySQL are not supported when using it. This section provides information about restrictions on and limitations of replication with GTIDs.

Updates involving nontransactional storage engines.  When using GTIDs, updates to tables using nontransactional storage engines such as MyISAM cannot be made in the same statement or transaction as updates to tables using transactional storage engines such as InnoDB.

This restriction is due to the fact that updates to tables that use a nontransactional storage engine mixed with updates to tables that use a transactional storage engine within the same transaction can result in multiple GTIDs being assigned to the same transaction.

Such problems can also occur when the master and the slave use different storage engines for their respective versions of the same table, where one storage engine is transactional and the other is not. Also be aware that triggers that are defined to operate on nontransactional tables can be the cause of these problems.

In any of the cases just mentioned, the one-to-one correspondence between transactions and GTIDs is broken, with the result that GTID-based replication cannot function correctly.

CREATE TABLE ... SELECT statements.  CREATE TABLE ... SELECT statements are not allowed when using GTID-based replication. When binlog_format is set to STATEMENT, a CREATE TABLE ... SELECT statement is recorded in the binary log as one transaction with one GTID, but if ROW format is used, the statement is recorded as two transactions with two GTIDs. If a master used STATEMENT format and a slave used ROW format, the slave would be unable to handle the transaction correctly, therefore the CREATE TABLE ... SELECT statement is disallowed with GTIDs to prevent this scenario.

Temporary tables.  When binlog_format is set to STATEMENT, CREATE TEMPORARY TABLE and DROP TEMPORARY TABLE statements cannot be used inside transactions, procedures, functions, and triggers when GTIDs are in use on the server (that is, when the enforce_gtid_consistency system variable is set to ON). They can be used outside these contexts when GTIDs are in use, provided that autocommit=1 is set. From MySQL 8.0.13, when binlog_format is set to ROW or MIXED, CREATE TEMPORARY TABLE and DROP TEMPORARY TABLE statements are allowed inside a transaction, procedure, function, or trigger when GTIDs are in use. The statements are not written to the binary log and are therefore not replicated to slaves. The use of row-based replication means that the slaves remain in sync without the need to replicate temporary tables. If the removal of these statements from a transaction results in an empty transaction, the transaction is not written to the binary log.

Preventing execution of unsupported statements.  To prevent execution of statements that would cause GTID-based replication to fail, all servers must be started with the --enforce-gtid-consistency option when enabling GTIDs. This causes statements of any of the types discussed previously in this section to fail with an error.

Note that --enforce-gtid-consistency only takes effect if binary logging takes place for a statement. If binary logging is disabled on the server, or if statements are not written to the binary log because they are removed by a filter, GTID consistency is not checked or enforced for the statements that are not logged.

For information about other required startup options when enabling GTIDs, see Section 17.1.3.4, “Setting Up Replication Using GTIDs”.

Skipping transactions.  sql_slave_skip_counter is not supported when using GTIDs. If you need to skip transactions, use the value of the master's gtid_executed variable instead; see Injecting empty transactions, for more information.

Ignoring servers.  The IGNORE_SERVER_IDS option of the CHANGE MASTER TO statement is deprecated when using GTIDs, because transactions that have already been applied are automatically ignored. Before starting GTID-based replication, check for and clear all ignored server ID lists that have previously been set on the servers involved. The SHOW SLAVE STATUS statement, which can be issued for individual channels, displays the list of ignored server IDs if there is one. If there is no list, the Replicate_Ignore_Server_Ids field is blank.

GTID mode and mysqldump.  It is possible to import a dump made using mysqldump into a MySQL server running with GTID mode enabled, provided that there are no GTIDs in the target server's binary log.

GTID mode and mysql_upgrade.  Prior to MySQL 8.0.16, when the server is running with global transaction identifiers (GTIDs) enabled (gtid_mode=ON), do not enable binary logging by mysql_upgrade (the --write-binlog option). As of MySQL 8.0.16, the server performs the entire MySQL upgrade procedure, but disables binary logging during the upgrade, so there is no issue.

17.1.3.7 Stored Function Examples to Manipulate GTIDs

MySQL includes some built-in (native) functions for use with GTID-based replication. These functions are as follows:

GTID_SUBSET(set1,set2)

Given two sets of global transaction identifiers set1 and set2, returns true if all GTIDs in set1 are also in set2. Returns false otherwise.

GTID_SUBTRACT(set1,set2)

Given two sets of global transaction identifiers set1 and set2, returns only those GTIDs from set1 that are not in set2.

WAIT_FOR_EXECUTED_GTID_SET(gtid_set[, timeout])

Wait until the server has applied all of the transactions whose global transaction identifiers are contained in gtid_set. The optional timeout stops the function from waiting after the specified number of seconds have elapsed.

WAIT_UNTIL_SQL_THREAD_AFTER_GTIDS(gtid_set[, timeout][,channel])

Like WAIT_FOR_EXECUTED_GTID_SET(), but for a single started replication channel. Use WAIT_FOR_EXECUTED_GTID_SET() instead to ensure all channels are covered in all states.

For details of these functions, see Section 12.18, “Functions Used with Global Transaction Identifiers (GTIDs)”.

You can define your own stored functions to work with GTIDs. For information on defining stored functions, see Chapter 24, Stored Objects. The following examples show some useful stored functions that can be created based on the built-in GTID_SUBSET() and GTID_SUBTRACT() functions.

Note that in these stored functions, the delimiter command has been used to change the MySQL statement delimiter to a vertical bar, as follows:

mysql> delimiter |

All of these functions take string representations of GTID sets as arguments, so GTID sets must always be quoted when used with them.

This function returns nonzero (true) if two GTID sets are the same set, even if they are not formatted in the same way.

CREATE FUNCTION GTID_IS_EQUAL(gtid_set_1 LONGTEXT, gtid_set_2 LONGTEXT)
RETURNS INT
  RETURN GTID_SUBSET(gtid_set_1, gtid_set_2) AND GTID_SUBSET(gtid_set_2, gtid_set_1)|

This function returns nonzero (true) if two GTID sets are disjoint.

CREATE FUNCTION GTID_IS_DISJOINT(gtid_set_1 LONGTEXT, gtid_set_2 LONGTEXT)
RETURNS INT
  RETURN GTID_SUBSET(gtid_set_1, GTID_SUBTRACT(gtid_set_1, gtid_set_2))|

This function returns nonzero (true) if two GTID sets are disjoint, and sum is the union of the two sets.

CREATE FUNCTION GTID_IS_DISJOINT_UNION(gtid_set_1 LONGTEXT, gtid_set_2 LONGTEXT, sum LONGTEXT)
RETURNS INT
  RETURN GTID_IS_EQUAL(GTID_SUBTRACT(sum, gtid_set_1), gtid_set_2) AND
         GTID_IS_EQUAL(GTID_SUBTRACT(sum, gtid_set_2), gtid_set_1)|

This function returns a normalized form of the GTID set, in all uppercase, with no whitespace and no duplicates. The UUIDs are arranged in alphabetic order and intervals are arranged in numeric order.

CREATE FUNCTION GTID_NORMALIZE(g LONGTEXT)
RETURNS LONGTEXT
RETURN GTID_SUBTRACT(g, '')|

This function returns the union of two GTID sets.

CREATE FUNCTION GTID_UNION(gtid_set_1 LONGTEXT, gtid_set_2 LONGTEXT)
RETURNS LONGTEXT
  RETURN GTID_NORMALIZE(CONCAT(gtid_set_1, ',', gtid_set_2))|

This function returns the intersection of two GTID sets.

CREATE FUNCTION GTID_INTERSECTION(gtid_set_1 LONGTEXT, gtid_set_2 LONGTEXT)
RETURNS LONGTEXT
  RETURN GTID_SUBTRACT(gtid_set_1, GTID_SUBTRACT(gtid_set_1, gtid_set_2))|

This function returns the symmetric difference between two GTID sets, that is, the GTIDs that exist in gtid_set_1 but not in gtid_set_2, and also the GTIDs that exist in gtid_set_2 but not in gtid_set_1.

CREATE FUNCTION GTID_SYMMETRIC_DIFFERENCE(gtid_set_1 LONGTEXT, gtid_set_2 LONGTEXT)
RETURNS LONGTEXT
  RETURN GTID_SUBTRACT(CONCAT(gtid_set_1, ',', gtid_set_2), GTID_INTERSECTION(gtid_set_1, gtid_set_2))|

This function removes from a GTID set all the GTIDs from a specified origin, and returns the remaining GTIDs, if any. The UUID is the identifier used by the server where the transaction originated, which is normally the server_uuid value.

CREATE FUNCTION GTID_SUBTRACT_UUID(gtid_set LONGTEXT, uuid TEXT)
RETURNS LONGTEXT
  RETURN GTID_SUBTRACT(gtid_set, CONCAT(UUID, ':1-', (1 << 63) - 2))|

This function reverses the previously listed function to return only those GTIDs from the GTID set that originate from the server with the specified identifier (UUID).

CREATE FUNCTION GTID_INTERSECTION_WITH_UUID(gtid_set LONGTEXT, uuid TEXT)
RETURNS LONGTEXT
  RETURN GTID_SUBTRACT(gtid_set, GTID_SUBTRACT_UUID(gtid_set, uuid))|

Example 17.1 Verifying that a replication slave is up to date

The built-in functions GTID_SUBSET and GTID_SUBTRACT can be used to check that a replication slave has applied at least every transaction that a master has applied.

To perform this check with GTID_SUBSET, execute the following statement on the slave:

SELECT GTID_SUBSET(master_gtid_executed, slave_gtid_executed)

If this returns 0 (false), some GTIDs in master_gtid_executed are not present in slave_gtid_executed, so the master has applied some transactions that the slave has not applied, and the slave is therefore not up to date.

To perform the check with GTID_SUBTRACT, execute the following statement on the slave:

SELECT GTID_SUBTRACT(master_gtid_executed, slave_gtid_executed)

This statement returns any GTIDs that are in master_gtid_executed but not in slave_gtid_executed. If any GTIDs are returned, the master has applied some transactions that the slave has not applied, and the slave is therefore not up to date.


Example 17.2 Backup and restore scenario

The stored functions GTID_IS_EQUAL, GTID_IS_DISJOINT, and GTID_IS_DISJOINT_UNION could be used to verify backup and restore operations involving multiple databases and servers. In this example scenario, server1 contains database db1, and server2 contains database db2. The goal is to copy database db2 to server1, and the result on server1 should be the union of the two databases. The procedure used is to back up server2 using mysqlpump or mysqldump, then restore this backup on server1.

Provided the backup program's option --set-gtid-purged was set to ON or the default of AUTO, the program's output contains a SET @@GLOBAL.gtid_purged statement that will add the gtid_executed set from server2 to the gtid_purged set on server1. The gtid_purged set contains the GTIDs of all the transactions that have been committed on a server but do not exist in any binary log file on the server. When database db2 is copied to server1, the GTIDs of the transactions committed on server2, which are not in the binary log files on server1, must be added to server1's gtid_purged set to make the set complete.

The stored functions can be used to assist with the following steps in this scenario:

  • Use GTID_IS_EQUAL to verify that the backup operation computed the correct GTID set for the SET @@GLOBAL.gtid_purged statement. On server2, extract that statement from the mysqlpump or mysqldump output, and store the GTID set into a local variable, such as $gtid_purged_set. Then execute the following statement:

    server2> SELECT GTID_IS_EQUAL($gtid_purged_set, @@GLOBAL.gtid_executed); 

    If the result is 1, the two GTID sets are equal, and the set has been computed correctly.

  • Use GTID_IS_DISJOINT to verify that the GTID set in the mysqlpump or mysqldump output does not overlap with the gtid_executed set on server1. If there is any overlap, with identical GTIDs present on both servers for some reason, you will see errors when copying database db2 to server1. To check, on server1, extract and store the gtid_purged set from the output into a local variable as above, then execute the following statement:

    server1> SELECT GTID_IS_DISJOINT($gtid_purged_set, @@GLOBAL.gtid_executed); 

    If the result is 1, there is no overlap between the two GTID sets, so no duplicate GTIDs are present.

  • Use GTID_IS_DISJOINT_UNION to verify that the restore operation resulted in the correct GTID state on server1. Before restoring the backup, on server1, obtain the existing gtid_executed set by executing the following statement:

    server1> SELECT @@GLOBAL.gtid_executed;

    Store the result in a local variable $original_gtid_executed. Also store the gtid_purged set in a local variable as described above. When the backup from server2 has been restored onto server1, execute the following statement to verify the GTID state:

    server1> SELECT GTID_IS_DISJOINT_UNION($original_gtid_executed, 
                                           $gtid_purged_set, 
                                           @@GLOBAL.gtid_executed); 

    If the result is 1, the stored function has verified that the original gtid_executed set from server1 ($original_gtid_executed) and the gtid_purged set that was added from server2 ($gtid_purged_set) have no overlap, and also that the updated gtid_executed set on server1 now consists of the previous gtid_executed set from server1 plus the gtid_purged set from server2, which is the desired result. Ensure that this check is carried out before any further transactions take place on server1, otherwise the new transactions in the gtid_executed set will cause it to fail.


Example 17.3 Selecting the most up-to-date slave for manual failover

The stored function GTID_UNION could be used to identify the most up-to-date replication slave from a set of slaves, in order to perform a manual failover operation after a replication master has stopped unexpectedly. If some of the slaves are experiencing replication lag, this stored function can be used to compute the most up-to-date slave without waiting for all the slaves to apply their existing relay logs, and therefore to minimize the failover time. The function can return the union of the gtid_executed set on each slave with the set of transactions received by the slave, which is recorded in the Performance Schema table replication_connection_status. You can compare these results to find which slave's record of transactions is the most up-to-date, even if not all of the transactions have been committed yet.

On each replication slave, compute the complete record of transactions by issuing the following statement:

SELECT GTID_UNION(RECEIVED_TRANSACTION_SET, @@GLOBAL.gtid_executed) 
    FROM performance_schema.replication_connection_status 
    WHERE channel_name = 'name';

You can then compare the results from each slave to see which one has the most up-to-date record of transactions, and use this slave as the new replication master.


Example 17.4 Checking for extraneous transactions on a replication slave

The stored function GTID_SUBTRACT_UUID could be used to check whether a replication slave has received transactions that did not originate from its designated master or masters. If it has, there might be an issue with your replication setup, or with a proxy, router, or load balancer. This function works by removing from a GTID set all the GTIDs from a specified originating server, and returning the remaining GTIDs, if any.

For a replication slave with a single master, issue the following statement, giving the identifier of the originating replication master, which is normally the server_uuid value:

SELECT GTID_SUBTRACT_UUID(@@GLOBAL.gtid_executed, server_uuid_of_master);

  If the result is not empty, the transactions returned are extra transactions that did not originate from the designated master.

For a slave in a multi-master replication topology, repeat the function, for example:

SELECT GTID_SUBTRACT_UUID(GTID_SUBTRACT_UUID(@@GLOBAL.gtid_executed,
                                             server_uuid_of_master_1),
                                             server_uuid_of_master_2);

If the result is not empty, the transactions returned are extra transactions that did not originate from any of the designated masters.


Example 17.5 Verifying that a server in a replication topology is read-only

The stored function GTID_INTERSECTION_WITH_UUID could be used to verify that a server has not originated any GTIDs and is in a read-only state. The function returns only those GTIDs from the GTID set that originate from the server with the specified identifier. If any of the transactions in the server's gtid_executed set have the server's own identifier, the server itself originated those transactions. You can issue the following statement on the server to check:

SELECT GTID_INTERSECTION_WITH_UUID(@@GLOBAL.gtid_executed, my_server_uuid);


Example 17.6 Validating an additional slave in a multi-master replication setup

The stored function GTID_INTERSECTION_WITH_UUID could be used to find out if a slave attached to a multi-master replication setup has applied all the transactions originating from one particular master. In this scenario, master1 and master2 are both masters and slaves and replicate to each other. master2 also has its own replication slave. The replication slave will also receive and apply master1's transactions if master2 is configured with log_slave_updates=ON, but it will not do so if master2 uses log_slave_updates=OFF. Whatever the case, we currently only want to find out if the replication slave is up to date with master2. In this situation, the stored function GTID_INTERSECTION_WITH_UUID can be used to identify the transactions that master2 originated, discarding the transactions that master2 has replicated from master1. The built-in function GTID_SUBSET can then be used to compare the result to the gtid_executed set on the slave. If the slave is up to date with master2, the gtid_executed set on the slave contains all the transactions in the intersection set (the transactions that originated from master2).

To carry out this check, store master2's gtid_executed set, master2's server UUID, and the slave's gtid_executed set, into client-side variables as follows:

    $master2_gtid_executed :=
      master2> SELECT @@GLOBAL.gtid_executed;
    $master2_server_uuid :=
      master2> SELECT @@GLOBAL.server_uuid;
    $slave_gtid_executed :=
      slave> SELECT @@GLOBAL.gtid_executed;

Then use GTID_INTERSECTION_WITH_UUID and GTID_SUBSET with these variables as input, as follows:

SELECT GTID_SUBSET(GTID_INTERSECTION_WITH_UUID($master2_gtid_executed,
                                               $master2_server_uuid),
                                               $slave_gtid_executed);

The server identifier from master2 ($master2_server_uuid) is used with GTID_INTERSECTION_WITH_UUID to identify and return only those GTIDs from master2's gtid_executed set that originated on master2, omitting those that originated on master1. The resulting GTID set is then compared with the set of all executed GTIDs on the slave, using GTID_SUBSET. If this statement returns nonzero (true), all the identified GTIDs from master2 (the first set input) are also in the slave's gtid_executed set (the second set input), meaning that the slave has replicated all the transactions that originated from master2.


17.1.4 MySQL Multi-Source Replication

This section describes MySQL Multi-Source Replication, which enables you to replicate from multiple immediate masters in parallel. This section describes multi-source replication, and how to configure, monitor and troubleshoot it.

17.1.4.1 MySQL Multi-Source Replication Overview

MySQL Multi-Source Replication enables a replication slave to receive transactions from multiple sources simultaneously. Multi-source replication can be used to back up multiple servers to a single server, to merge table shards, and consolidate data from multiple servers to a single server. Multi-source replication does not implement any conflict detection or resolution when applying the transactions, and those tasks are left to the application if required. In a multi-source replication topology, a slave creates a replication channel for each master that it should receive transactions from. See Section 17.2.3, “Replication Channels”. The following sections describe how to set up multi-source replication.

17.1.4.2 Multi-Source Replication Tutorials

This section provides tutorials on how to configure masters and slaves for multi-source replication, and how to start, stop and reset multi-source slaves.

17.1.4.2.1 Configuring Multi-Source Replication

This section explains how to configure a multi-source replication topology, and provides details about configuring masters and slaves. Such a topology requires at least two masters and one slave configured.

Masters in a multi-source replication topology can be configured to use either global transaction identifier (GTID) based replication, or binary log position-based replication. See Section 17.1.3.4, “Setting Up Replication Using GTIDs” for how to configure a master using GTID based replication. See Section 17.1.2.1, “Setting the Replication Master Configuration” for how to configure a master using file position based replication.

Slaves in a multi-source replication topology require TABLE repositories for the master info log and relay log info log, which are the default in MySQL 8.0. Multi-source replication is not compatible with FILE based repositories, and the FILE setting for the --master-info-repository and --relay-log-info-repository options is now deprecated.

To modify an existing replication slave that is using a FILE repository for the slave status logs to use TABLE repositories, convert the existing replication repositories dynamically by running the following commands:

STOP SLAVE;
SET GLOBAL master_info_repository = 'TABLE';
SET GLOBAL relay_log_info_repository = 'TABLE';
17.1.4.2.2 Adding a GTID Based Master to a Multi-Source Replication Slave

This section assumes you have enabled GTID based transactions on the master using gtid_mode=ON, enabled a replication user, and ensured that the slave is using TABLE based replication repositories. Use the CHANGE MASTER TO statement to add a new master to a channel by using a FOR CHANNEL channel clause. For more information on replication channels, see Section 17.2.3, “Replication Channels”

For example, to add a new master with the host name master1 using port 3451 to a channel called master-1:


CHANGE MASTER TO MASTER_HOST='master1', MASTER_USER='rpl', MASTER_PORT=3451, MASTER_PASSWORD='', \
MASTER_AUTO_POSITION = 1 FOR CHANNEL 'master-1';

Multi-source replication is compatible with auto-positioning. See Section 13.4.2.1, “CHANGE MASTER TO Syntax” for more information.

Repeat this process for each extra master that you want to add to a channel, changing the host name, port and channel as appropriate.

17.1.4.2.3 Adding a Binary Log Based Master to a Multi-Source Replication Slave

This section assumes that binary logging is enabled on the master (which is the default), the slave is using TABLE based replication repositories (which is the default in MySQL 8.0), and that you have enabled a replication user and noted the current binary log position. You need to know the current MASTER_LOG_FILE and MASTER_LOG_POSITION. Use the CHANGE MASTER TO statement to add a new master to a channel by specifying a FOR CHANNEL channel clause. For example, to add a new master with the host name master1 using port 3451 to a channel called master-1:


CHANGE MASTER TO MASTER_HOST='master1', MASTER_USER='rpl', MASTER_PORT=3451, MASTER_PASSWORD='' \
MASTER_LOG_FILE='master1-bin.000006', MASTER_LOG_POS=628 FOR CHANNEL 'master-1';

Repeat this process for each extra master that you want to add to a channel, changing the host name, port and channel as appropriate.

17.1.4.2.4 Starting Multi-Source Replication Slaves

Once you have added all of the channels you want to use as replication masters, use a START SLAVE thread_types statement to start replication. When you have enabled multiple channels on a slave, you can choose to either start all channels, or select a specific channel to start.

  • To start all currently configured replication channels:

     
    START SLAVE thread_types;
  • To start only a named channel, use a FOR CHANNEL channel clause:

     
    START SLAVE thread_types FOR CHANNEL channel;
    

Use the thread_types option to choose specific threads you want the above statements to start on the slave. See Section 13.4.2.6, “START SLAVE Syntax” for more information.

17.1.4.2.5 Stopping Multi-Source Replication Slaves

The STOP SLAVE statement can be used to stop a multi-source replication slave. By default, if you use the STOP SLAVE statement on a multi-source replication slave all channels are stopped. Optionally, use the FOR CHANNEL channel clause to stop only a specific channel.

  • To stop all currently configured replication channels:

    
    STOP SLAVE thread_types;
  • To stop only a named channel, use a FOR CHANNEL channel clause:

     
    STOP SLAVE thread_types FOR CHANNEL channel;
    

Use the thread_types option to choose specific threads you want the above statements to stop on the slave. See Section 13.4.2.7, “STOP SLAVE Syntax” for more information.

17.1.4.2.6 Resetting Multi-Source Replication Slaves

The RESET SLAVE statement can be used to reset a multi-source replication slave. By default, if you use the RESET SLAVE statement on a multi-source replication slave all channels are reset. Optionally, use the FOR CHANNEL channel clause to reset only a specific channel.

  • To reset all currently configured replication channels:

     
    RESET SLAVE;
  • To reset only a named channel, use a FOR CHANNEL channel clause:

     
    RESET SLAVE FOR CHANNEL channel;
    

See Section 13.4.2.4, “RESET SLAVE Syntax” for more information.

17.1.4.3 Multi-Source Replication Monitoring

To monitor the status of replication channels the following options exist:

  • Using the replication Performance Schema tables. The first column of these tables is Channel_Name. This enables you to write complex queries based on Channel_Name as a key. See Section 26.12.11, “Performance Schema Replication Tables”.

  • Using SHOW SLAVE STATUS FOR CHANNEL channel. By default, if the FOR CHANNEL channel clause is not used, this statement shows the slave status for all channels with one row per channel. The identifier Channel_name is added as a column in the result set. If a FOR CHANNEL channel clause is provided, the results show the status of only the named replication channel.

Note

The SHOW VARIABLES statement does not work with multiple replication channels. The information that was available through these variables has been migrated to the replication performance tables. Using a SHOW VARIABLES statement in a topology with multiple channels shows the status of only the default channel.

17.1.4.3.1 Monitoring Channels Using Performance Schema Tables

This section explains how to use the replication Performance Schema tables to monitor channels. You can choose to monitor all channels, or a subset of the existing channels.

To monitor the connection status of all channels:

mysql> SELECT * FROM replication_connection_status\G;
*************************** 1. row ***************************
CHANNEL_NAME: master1
GROUP_NAME:
SOURCE_UUID: 046e41f8-a223-11e4-a975-0811960cc264
THREAD_ID: 24
SERVICE_STATE: ON
COUNT_RECEIVED_HEARTBEATS: 0
LAST_HEARTBEAT_TIMESTAMP: 0000-00-00 00:00:00
RECEIVED_TRANSACTION_SET: 046e41f8-a223-11e4-a975-0811960cc264:4-37
LAST_ERROR_NUMBER: 0
LAST_ERROR_MESSAGE:
LAST_ERROR_TIMESTAMP: 0000-00-00 00:00:00
*************************** 2. row ***************************
CHANNEL_NAME: master2
GROUP_NAME:
SOURCE_UUID: 7475e474-a223-11e4-a978-0811960cc264
THREAD_ID: 26
SERVICE_STATE: ON
COUNT_RECEIVED_HEARTBEATS: 0
LAST_HEARTBEAT_TIMESTAMP: 0000-00-00 00:00:00
RECEIVED_TRANSACTION_SET: 7475e474-a223-11e4-a978-0811960cc264:4-6
LAST_ERROR_NUMBER: 0
LAST_ERROR_MESSAGE:
LAST_ERROR_TIMESTAMP: 0000-00-00 00:00:00
2 rows in set (0.00 sec)
	    

In the above output there are two channels enabled, and as shown by the CHANNEL_NAME field they are called master1 and master2.

The addition of the CHANNEL_NAME field enables you to query the Performance Schema tables for a specific channel. To monitor the connection status of a named channel, use a WHERE CHANNEL_NAME=channel clause:

mysql> SELECT * FROM replication_connection_status WHERE CHANNEL_NAME='master1'\G
*************************** 1. row ***************************
CHANNEL_NAME: master1
GROUP_NAME:
SOURCE_UUID: 046e41f8-a223-11e4-a975-0811960cc264
THREAD_ID: 24
SERVICE_STATE: ON
COUNT_RECEIVED_HEARTBEATS: 0
LAST_HEARTBEAT_TIMESTAMP: 0000-00-00 00:00:00
RECEIVED_TRANSACTION_SET: 046e41f8-a223-11e4-a975-0811960cc264:4-37
LAST_ERROR_NUMBER: 0
LAST_ERROR_MESSAGE:
LAST_ERROR_TIMESTAMP: 0000-00-00 00:00:00
1 row in set (0.00 sec)

Similarly, the WHERE CHANNEL_NAME=channel clause can be used to monitor the other replication Performance Schema tables for a specific channel. For more information, see Section 26.12.11, “Performance Schema Replication Tables”.

17.1.4.4 Multi-Source Replication Error Messages

Error codes and messages provide information about errors encountered in a multi-source replication topology. These error codes and messages are only emitted when multi-source replication is enabled, and provide information related to the channel which generated the error. For example:

Slave is already running and Slave is already stopped have been replaced with Replication thread(s) for channel channel_name are already running and Replication threads(s) for channel channel_name are already stopped respectively.

The server log messages have also been changed to indicate which channel the log messages relate to. This makes debugging and tracing easier.

17.1.5 Changing Replication Modes on Online Servers

This section describes how to change the mode of replication being used without having to take the server offline.

17.1.5.1 Replication Mode Concepts

To be able to safely configure the replication mode of an online server it is important to understand some key concepts of replication. This section explains these concepts and is essential reading before attempting to modify the replication mode of an online server.

The modes of replication available in MySQL rely on different techniques for identifying transactions which are logged. The types of transactions used by replication are as follows:

  • GTID transactions are identified by a global transaction identifier (GTID) in the form UUID:NUMBER. Every GTID transaction in a log is always preceded by a Gtid_log_event. GTID transactions can be addressed using either the GTID or using the file name and position.

  • Anonymous transactions do not have a GTID assigned, and MySQL ensures that every anonymous transaction in a log is preceded by an Anonymous_gtid_log_event. In previous versions, anonymous transactions were not preceded by any particular event. Anonymous transactions can only be addressed using file name and position.

When using GTIDs you can take advantage of auto-positioning and automatic fail-over, as well as use WAIT_FOR_EXECUTED_GTID_SET(), session_track_gtids, and monitor replicated transactions using Performance Schema tables. With GTIDs enabled you cannot use sql_slave_skip_counter, instead use empty transactions.

Transactions in a relay log that was received from a master running a previous version of MySQL may not be preceded by any particular event at all, but after being replayed and logged in the slave's binary log, they are preceded with an Anonymous_gtid_log_event.

The ability to configure the replication mode online means that the gtid_mode and enforce_gtid_consistency variables are now both dynamic and can be set from a top-level statement by an account that has privileges sufficient to set global system variables. See Section 5.1.9.1, “System Variable Privileges”. In MySQL 5.6 and earlier, both of these variables could only be configured using the appropriate option at server start, meaning that changes to the replication mode required a server restart. In all versions gtid_mode could be set to ON or OFF, which corresponded to whether GTIDs were used to identify transactions or not. When gtid_mode=ON it is not possible to replicate anonymous transactions, and when gtid_mode=OFF only anonymous transactions can be replicated. When gtid_mode=OFF_PERMISSIVE then new transactions are anonymous while permitting replicated transactions to be either GTID or anonymous transactions. When gtid_mode=ON_PERMISSIVE then new transactions use GTIDs while permitting replicated transactions to be either GTID or anonymous transactions. This means it is possible to have a replication topology that has servers using both anonymous and GTID transactions. For example a master with gtid_mode=ON could be replicating to a slave with gtid_mode=ON_PERMISSIVE. The valid values for gtid_mode are as follows and in this order:

  • OFF

  • OFF_PERMISSIVE

  • ON_PERMISSIVE

  • ON

It is important to note that the state of gtid_mode can only be changed by one step at a time based on the above order. For example, if gtid_mode is currently set to OFF_PERMISSIVE, it is possible to change to OFF or ON_PERMISSIVE but not to ON. This is to ensure that the process of changing from anonymous transactions to GTID transactions online is correctly handled by the server. When you switch between gtid_mode=ON and gtid_mode=OFF, the GTID state (in other words the value of gtid_executed) is persistent. This ensures that the GTID set that has been applied by the server is always retained, regardless of changes between types of gtid_mode.

The fields related to GTIDs display the correct information regardless of the currently selected gtid_mode. This means that fields which display GTID sets, such as gtid_executed, gtid_purged, RECEIVED_TRANSACTION_SET in the replication_connection_status Performance Schema table, and the GTID related results of SHOW SLAVE STATUS, now return the empty string when there are no GTIDs present. Fields that display a single GTID, such as CURRENT_TRANSACTION in the Performance Schema replication_applier_status_by_worker table, now display ANONYMOUS when GTID transactions are not being used.

Replication from a master using gtid_mode=ON provides the ability to use auto-positioning, configured using the CHANGE MASTER TO MASTER_AUTO_POSITION = 1; statement. The replication topology being used impacts on whether it is possible to enable auto-positioning or not, as this feature relies on GTIDs and is not compatible with anonymous transactions. An error is generated if auto-positioning is enabled and an anonymous transaction is encountered. It is strongly recommended to ensure there are no anonymous transactions remaining in the topology before enabling auto-positioning, see Section 17.1.5.2, “Enabling GTID Transactions Online”.

The valid combinations of gtid_mode and auto-positioning on master and slave are shown in the following table, where the master's gtid_mode is shown on the horizontal and the slave's gtid_mode is on the vertical. The meaning of each entry is as follows:

  • Y: the gtid_mode of master and slave is compatible

  • N: the gtid_mode of master and slave is not compatible

  • *: auto-positioning can be used with this combination

Table 17.1 Valid Combinations of Master and Slave gtid_mode

gtid_mode

Master OFF

Master OFF_PERMISSIVE

Master ON_PERMISSIVE

Master ON

Slave OFF

Y

Y

N

N

Slave OFF_PERMISSIVE

Y

Y

Y

Y*

Slave ON_PERMISSIVE

Y

Y

Y

Y*

Slave ON

N

N

Y

Y*


The currently selected gtid_mode also impacts on the gtid_next variable. The following table shows the behavior of the server for the different values of gtid_mode and gtid_next. The meaning of each entry is as follows:

  • ANONYMOUS: generate an anonymous transaction.

  • Error: generate an error and fail to execute SET GTID_NEXT.

  • UUID:NUMBER: generate a GTID with the specified UUID:NUMBER.

  • New GTID: generate a GTID with an automatically generated number.

Table 17.2 Valid Combinations of gtid_mode and gtid_next

gtid_next AUTOMATIC

binary log on

gtid_next AUTOMATIC

binary log off

gtid_next ANONYMOUS

gtid_next UUID:NUMBER

gtid_mode OFF

ANONYMOUS

ANONYMOUS

ANONYMOUS

Error

gtid_mode OFF_PERMISSIVE

ANONYMOUS

ANONYMOUS

ANONYMOUS

UUID:NUMBER

gtid_mode ON_PERMISSIVE

New GTID

ANONYMOUS

ANONYMOUS

UUID:NUMBER

gtid_mode ON

New GTID

ANONYMOUS

Error

UUID:NUMBER


When the binary log is off and gtid_next is set to AUTOMATIC, then no GTID is generated. This is consistent with the behavior of previous versions.

17.1.5.2 Enabling GTID Transactions Online

This section describes how to enable GTID transactions, and optionally auto-positioning, on servers that are already online and using anonymous transactions. This procedure does not require taking the server offline and is suited to use in production. However, if you have the possibility to take the servers offline when enabling GTID transactions that process is easier.

Before you start, ensure that the servers meet the following pre-conditions:

  • All servers in your topology must use MySQL 5.7.6 or later. You cannot enable GTID transactions online on any single server unless all servers which are in the topology are using this version.

  • All servers have gtid_mode set to the default value OFF.

The following procedure can be paused at any time and later resumed where it was, or reversed by jumping to the corresponding step of Section 17.1.5.3, “Disabling GTID Transactions Online”, the online procedure to disable GTIDs. This makes the procedure fault-tolerant because any unrelated issues that may appear in the middle of the procedure can be handled as usual, and then the procedure continued where it was left off.

Note

It is crucial that you complete every step before continuing to the next step.

To enable GTID transactions:

  1. On each server, execute:

    SET @@GLOBAL.ENFORCE_GTID_CONSISTENCY = WARN;

    Let the server run for a while with your normal workload and monitor the logs. If this step causes any warnings in the log, adjust your application so that it only uses GTID-compatible features and does not generate any warnings.

    Important

    This is the first important step. You must ensure that no warnings are being generated in the error logs before going to the next step.

  2. On each server, execute:

    SET @@GLOBAL.ENFORCE_GTID_CONSISTENCY = ON;
  3. On each server, execute:

    SET @@GLOBAL.GTID_MODE = OFF_PERMISSIVE;

    It does not matter which server executes this statement first, but it is important that all servers complete this step before any server begins the next step.

  4. On each server, execute:

    SET @@GLOBAL.GTID_MODE = ON_PERMISSIVE;

    It does not matter which server executes this statement first.

  5. On each server, wait until the status variable ONGOING_ANONYMOUS_TRANSACTION_COUNT is zero. This can be checked using:

    SHOW STATUS LIKE 'ONGOING_ANONYMOUS_TRANSACTION_COUNT';
    Note

    On a replication slave, it is theoretically possible that this shows zero and then nonzero again. This is not a problem, it suffices that it shows zero once.

  6. Wait for all transactions generated up to step 5 to replicate to all servers. You can do this without stopping updates: the only important thing is that all anonymous transactions get replicated.

    See Section 17.1.5.4, “Verifying Replication of Anonymous Transactions” for one method of checking that all anonymous transactions have replicated to all servers.

  7. If you use binary logs for anything other than replication, for example point in time backup and restore, wait until you do not need the old binary logs having transactions without GTIDs.

    For instance, after step 6 has completed, you can execute FLUSH LOGS on the server where you are taking backups. Then either explicitly take a backup or wait for the next iteration of any periodic backup routine you may have set up.

    Ideally, wait for the server to purge all binary logs that existed when step 6 was completed. Also wait for any backup taken before step 6 to expire.

    Important

    This is the second important point. It is vital to understand that binary logs containing anonymous transactions, without GTIDs cannot be used after the next step. After this step, you must be sure that transactions without GTIDs do not exist anywhere in the topology.

  8. On each server, execute:

    SET @@GLOBAL.GTID_MODE = ON;
  9. On each server, add gtid_mode=ON and enforce_gtid_consistency=ON to my.cnf.

    You are now guaranteed that all transactions have a GTID (except transactions generated in step 5 or earlier, which have already been processed). To start using the GTID protocol so that you can later perform automatic fail-over, execute the following on each slave. Optionally, if you use multi-source replication, do this for each channel and include the FOR CHANNEL channel clause:

    STOP SLAVE [FOR CHANNEL 'channel'];
    CHANGE MASTER TO MASTER_AUTO_POSITION = 1 [FOR CHANNEL 'channel'];
    START SLAVE [FOR CHANNEL 'channel'];

17.1.5.3 Disabling GTID Transactions Online

This section describes how to disable GTID transactions on servers that are already online. This procedure does not require taking the server offline and is suited to use in production. However, if you have the possibility to take the servers offline when disabling GTIDs mode that process is easier.

The process is similar to enabling GTID transactions while the server is online, but reversing the steps. The only thing that differs is the point at which you wait for logged transactions to replicate.

Before you start, ensure that the servers meet the following pre-conditions:

  • All servers in your topology must use MySQL 5.7.6 or later. You cannot disable GTID transactions online on any single server unless all servers which are in the topology are using this version.

  • All servers have gtid_mode set to ON.

  • The --replicate-same-server-id option is not set on any server. You cannot disable GTID transactions if this option is set together with the --log-slave-updates option (which is the default) and binary logging is enabled (which is also the default). Without GTIDs, this combination of options causes infinite loops in circular replication.

  1. Execute the following on each slave, and if you using multi-source replication, do it for each channel and include the FOR CHANNEL channel clause:

    STOP SLAVE [FOR CHANNEL 'channel'];
    CHANGE MASTER TO MASTER_AUTO_POSITION = 0, MASTER_LOG_FILE = file, \
    MASTER_LOG_POS = position [FOR CHANNEL 'channel'];
    START SLAVE [FOR CHANNEL 'channel'];
     
  2. On each server, execute:

    SET @@GLOBAL.GTID_MODE = ON_PERMISSIVE;
  3. On each server, execute:

    SET @@GLOBAL.GTID_MODE = OFF_PERMISSIVE;
  4. On each server, wait until the variable @@GLOBAL.GTID_OWNED is equal to the empty string. This can be checked using:

    SELECT @@GLOBAL.GTID_OWNED;

    On a replication slave, it is theoretically possible that this is empty and then nonempty again. This is not a problem, it suffices that it is empty once.

  5. Wait for all transactions that currently exist in any binary log to replicate to all slaves. See Section 17.1.5.4, “Verifying Replication of Anonymous Transactions” for one method of checking that all anonymous transactions have replicated to all servers.

  6. If you use binary logs for anything else than replication, for example to do point in time backup or restore: wait until you do not need the old binary logs having GTID transactions.

    For instance, after step 5 has completed, you can execute FLUSH LOGS on the server where you are taking the backup. Then either explicitly take a backup or wait for the next iteration of any periodic backup routine you may have set up.

    Ideally, wait for the server to purge all binary logs that existed when step 5 was completed. Also wait for any backup taken before step 5 to expire.

    Important

    This is the one important point during this procedure. It is important to understand that logs containing GTID transactions cannot be used after the next step. Before proceeding you must be sure that GTID transactions do not exist anywhere in the topology.

  7. On each server, execute:

    SET @@GLOBAL.GTID_MODE = OFF;
  8. On each server, set gtid_mode=OFF in my.cnf.

    If you want to set enforce_gtid_consistency=OFF, you can do so now. After setting it, you should add enforce_gtid_consistency=OFF to your configuration file.

If you want to downgrade to an earlier version of MySQL, you can do so now, using the normal downgrade procedure.

17.1.5.4 Verifying Replication of Anonymous Transactions

This section explains how to monitor a replication topology and verify that all anonymous transactions have been replicated. This is helpful when changing the replication mode online as you can verify that it is safe to change to GTID transactions.

There are several possible ways to wait for transactions to replicate:

The simplest method, which works regardless of your topology but relies on timing is as follows: if you are sure that the slave never lags more than N seconds, just wait for a bit more than N seconds. Or wait for a day, or whatever time period you consider safe for your deployment.

A safer method in the sense that it does not depend on timing: if you only have a master with one or more slaves, do the following:

  1. On the master, execute:

    SHOW MASTER STATUS;

    Note down the values in the File and Position column.

  2. On every slave, use the file and position information from the master to execute:

    SELECT MASTER_POS_WAIT(file, position);

If you have a master and multiple levels of slaves, or in other words you have slaves of slaves, repeat step 2 on each level, starting from the master, then all the direct slaves, then all the slaves of slaves, and so on.

If you use a circular replication topology where multiple servers may have write clients, perform step 2 for each master-slave connection, until you have completed the full circle. Repeat the whole process so that you do the full circle twice.

For example, suppose you have three servers A, B, and C, replicating in a circle so that A -> B -> C -> A. The procedure is then:

  • Do step 1 on A and step 2 on B.

  • Do step 1 on B and step 2 on C.

  • Do step 1 on C and step 2 on A.

  • Do step 1 on A and step 2 on B.

  • Do step 1 on B and step 2 on C.

  • Do step 1 on C and step 2 on A.

17.1.6 Replication and Binary Logging Options and Variables

The following sections contain information about mysqld options and server variables that are used in replication and for controlling the binary log. Options and variables for use on replication masters and replication slaves are covered separately, as are options and variables relating to binary logging and global transaction identifiers (GTIDs). A set of quick-reference tables providing basic information about these options and variables is also included.

Of particular importance is the --server-id option.

Property Value
Command-Line Format --server-id=#
System Variable server_id
Scope Global
Dynamic Yes
SET_VAR Hint Applies No
Type Integer
Default Value (>= 8.0.3) 1
Default Value (<= 8.0.2) 0
Minimum Value 0
Maximum Value 4294967295

Specifies the server ID. The server_id system variable is set to 1 by default. The server can be started with this default ID, but when binary logging is enabled, an informational message is issued if you did not specify a server ID explicitly using the --server-id option.

For servers that are used in a replication topology, you must specify a unique server ID for each replication server, in the range from 1 to 232 − 1. Unique means that each ID must be different from every other ID in use by any other replication master or slave. For additional information, see Section 17.1.6.2, “Replication Master Options and Variables”, and Section 17.1.6.3, “Replication Slave Options and Variables”.

If the server ID is set to 0, binary logging takes place, but a master with a server ID of 0 refuses any connections from slaves, and a slave with a server ID of 0 refuses to connect to a master. Note that although you can change the server ID dynamically to a nonzero value, doing so does not enable replication to start immediately. You must change the server ID and then restart the server to initialize the replication slave.

For more information, see Section 17.1.2.2, “Setting the Replication Slave Configuration”.

server_uuid

The MySQL server generates a true UUID in addition to the default or user-supplied server ID set in the server_id system variable. This is available as the global, read-only variable server_uuid.

Note

The presence of the server_uuid system variable does not change the requirement for setting a unique --server-id for each MySQL server as part of preparing and running MySQL replication, as described earlier in this section.

Property Value
System Variable server_uuid
Scope Global
Dynamic No
SET_VAR Hint Applies No
Type String

When starting, the MySQL server automatically obtains a UUID as follows:

  1. Attempt to read and use the UUID written in the file data_dir/auto.cnf (where data_dir is the server's data directory).

  2. If data_dir/auto.cnf is not found, generate a new UUID and save it to this file, creating the file if necessary.

The auto.cnf file has a format similar to that used for my.cnf or my.ini files. auto.cnf has only a single [auto] section containing a single server_uuid setting and value; the file's contents appear similar to what is shown here:

[auto]
server_uuid=8a94f357-aab4-11df-86ab-c80aa9429562
Important

The auto.cnf file is automatically generated; do not attempt to write or modify this file.

When using MySQL replication, masters and slaves know each other's UUIDs. The value of a slave's UUID can be seen in the output of SHOW SLAVE HOSTS. Once START SLAVE has been executed, the value of the master's UUID is available on the slave in the output of SHOW SLAVE STATUS.

Note

Issuing a STOP SLAVE or RESET SLAVE statement does not reset the master's UUID as used on the slave.

A server's server_uuid is also used in GTIDs for transactions originating on that server. For more information, see Section 17.1.3, “Replication with Global Transaction Identifiers”.

When starting, the slave I/O thread generates an error and aborts if its master's UUID is equal to its own unless the --replicate-same-server-id option has been set. In addition, the slave I/O thread generates a warning if either of the following is true:

17.1.6.1 Replication and Binary Logging Option and Variable Reference

The following two lists provide basic information about the MySQL command-line options and system variables applicable to replication and the binary log.

The command-line options and system variables in the following list relate to replication masters and replication slaves. Section 17.1.6.2, “Replication Master Options and Variables”, provides more detailed information about options and variables relating to replication master servers. For more information about options and variables relating to replication slaves, see Section 17.1.6.3, “Replication Slave Options and Variables”.

The command-line options and system variables in the following list relate to the binary log. Section 17.1.6.4, “Binary Logging Options and Variables”, provides more detailed information about options and variables relating to binary logging. For additional general information about the binary log, see Section 5.4.4, “The Binary Log”.

For a listing of all command-line options, system and status variables used with mysqld, see Section 5.1.4, “Server Option, System Variable, and Status Variable Reference”.

17.1.6.2 Replication Master Options and Variables

This section describes the server options and system variables that you can use on replication master servers. You can specify the options either on the command line or in an option file. You can specify system variable values using SET.

On the master and each slave, you must use the server-id option to establish a unique replication ID. For each server, you should pick a unique positive integer in the range from 1 to 232 − 1, and each ID must be different from every other ID in use by any other replication master or slave. Example: server-id=3.

For options used on the master for controlling binary logging, see Section 17.1.6.4, “Binary Logging Options and Variables”.

Startup Options for Replication Masters

The following list describes startup options for controlling replication master servers. Replication-related system variables are discussed later in this section.

System Variables Used on Replication Masters

The following system variables are used for or by replication masters:

  • auto_increment_increment

    Property Value
    Command-Line Format --auto-increment-increment=#
    System Variable auto_increment_increment
    Scope Global, Session
    Dynamic Yes
    SET_VAR Hint Applies Yes
    Type Integer
    Default Value 1
    Minimum Value 1
    Maximum Value 65535

    auto_increment_increment and auto_increment_offset are intended for use with master-to-master replication, and can be used to control the operation of AUTO_INCREMENT columns. Both variables have global and session values, and each can assume an integer value between 1 and 65,535 inclusive. Setting the value of either of these two variables to 0 causes its value to be set to 1 instead. Attempting to set the value of either of these two variables to an integer greater than 65,535 or less than 0 causes its value to be set to 65,535 instead. Attempting to set the value of auto_increment_increment or auto_increment_offset to a noninteger value produces an error, and the actual value of the variable remains unchanged.

    As of MySQL 8.0.14, setting the session value of this system variable is a restricted operation. The session user must have privileges sufficient to set restricted session variables. See Section 5.1.9.1, “System Variable Privileges”.

    Note

    auto_increment_increment is also supported for use with NDB tables.

    When Group Replication is started on a server, the value of auto_increment_increment is changed to the value of group_replication_auto_increment_increment, which defaults to 7, and the value of auto_increment_offset is changed to the server ID. The changes are reverted when Group Replication is stopped. These changes are only made and reverted if auto_increment_increment and auto_increment_offset each have their default value of 1. If their values have already been modified from the default, Group Replication does not alter them. From MySQL 8.0, the system variables are also not modified when Group Replication is in single-primary mode, where only one server writes.

    auto_increment_increment and auto_increment_offset affect AUTO_INCREMENT column behavior as follows:

    • auto_increment_increment controls the interval between successive column values. For example:

      mysql> SHOW VARIABLES LIKE 'auto_inc%';
      +--------------------------+-------+
      | Variable_name            | Value |
      +--------------------------+-------+
      | auto_increment_increment | 1     |
      | auto_increment_offset    | 1     |
      +--------------------------+-------+
      2 rows in set (0.00 sec)
      
      mysql> CREATE TABLE autoinc1
          -> (col INT NOT NULL AUTO_INCREMENT PRIMARY KEY);
        Query OK, 0 rows affected (0.04 sec)
      
      mysql> SET @@auto_increment_increment=10;
      Query OK, 0 rows affected (0.00 sec)
      
      mysql> SHOW VARIABLES LIKE 'auto_inc%';
      +--------------------------+-------+
      | Variable_name            | Value |
      +--------------------------+-------+
      | auto_increment_increment | 10    |
      | auto_increment_offset    | 1     |
      +--------------------------+-------+
      2 rows in set (0.01 sec)
      
      mysql> INSERT INTO autoinc1 VALUES (NULL), (NULL), (NULL), (NULL);
      Query OK, 4 rows affected (0.00 sec)
      Records: 4  Duplicates: 0  Warnings: 0
      
      mysql> SELECT col FROM autoinc1;
      +-----+
      | col |
      +-----+
      |   1 |
      |  11 |
      |  21 |
      |  31 |
      +-----+
      4 rows in set (0.00 sec)
      
    • auto_increment_offset determines the starting point for the AUTO_INCREMENT column value. Consider the following, assuming that these statements are executed during the same session as the example given in the description for auto_increment_increment:

      mysql> SET @@auto_increment_offset=5;
      Query OK, 0 rows affected (0.00 sec)
      
      mysql> SHOW VARIABLES LIKE 'auto_inc%';
      +--------------------------+-------+
      | Variable_name            | Value |
      +--------------------------+-------+
      | auto_increment_increment | 10    |
      | auto_increment_offset    | 5     |
      +--------------------------+-------+
      2 rows in set (0.00 sec)
      
      mysql> CREATE TABLE autoinc2
          -> (col INT NOT NULL AUTO_INCREMENT PRIMARY KEY);
      Query OK, 0 rows affected (0.06 sec)
      
      mysql> INSERT INTO autoinc2 VALUES (NULL), (NULL), (NULL), (NULL);
      Query OK, 4 rows affected (0.00 sec)
      Records: 4  Duplicates: 0  Warnings: 0
      
      mysql> SELECT col FROM autoinc2;
      +-----+
      | col |
      +-----+
      |   5 |
      |  15 |
      |  25 |
      |  35 |
      +-----+
      4 rows in set (0.02 sec)
      

      When the value of auto_increment_offset is greater than that of auto_increment_increment, the value of auto_increment_offset is ignored.

    If either of these variables is changed, and then new rows inserted into a table containing an AUTO_INCREMENT column, the results may seem counterintuitive because the series of AUTO_INCREMENT values is calculated without regard to any values already present in the column, and the next value inserted is the least value in the series that is greater than the maximum existing value in the AUTO_INCREMENT column. The series is calculated like this:

    auto_increment_offset + N × auto_increment_increment

    where N is a positive integer value in the series [1, 2, 3, ...]. For example:

    mysql> SHOW VARIABLES LIKE 'auto_inc%';
    +--------------------------+-------+
    | Variable_name            | Value |
    +--------------------------+-------+
    | auto_increment_increment | 10    |
    | auto_increment_offset    | 5     |
    +--------------------------+-------+
    2 rows in set (0.00 sec)
    
    mysql> SELECT col FROM autoinc1;
    +-----+
    | col |
    +-----+
    |   1 |
    |  11 |
    |  21 |
    |  31 |
    +-----+
    4 rows in set (0.00 sec)
    
    mysql> INSERT INTO autoinc1 VALUES (NULL), (NULL), (NULL), (NULL);
    Query OK, 4 rows affected (0.00 sec)
    Records: 4  Duplicates: 0  Warnings: 0
    
    mysql> SELECT col FROM autoinc1;
    +-----+
    | col |
    +-----+
    |   1 |
    |  11 |
    |  21 |
    |  31 |
    |  35 |
    |  45 |
    |  55 |
    |  65 |
    +-----+
    8 rows in set (0.00 sec)
    

    The values shown for auto_increment_increment and auto_increment_offset generate the series 5 + N × 10, that is, [5, 15, 25, 35, 45, ...]. The highest value present in the col column prior to the INSERT is 31, and the next available value in the AUTO_INCREMENT series is 35, so the inserted values for col begin at that point and the results are as shown for the SELECT query.

    It is not possible to restrict the effects of these two variables to a single table; these variables control the behavior of all AUTO_INCREMENT columns in all tables on the MySQL server. If the global value of either variable is set, its effects persist until the global value is changed or overridden by setting the session value, or until mysqld is restarted. If the local value is set, the new value affects AUTO_INCREMENT columns for all tables into which new rows are inserted by the current user for the duration of the session, unless the values are changed during that session.

    The default value of auto_increment_increment is 1. See Section 17.4.1.1, “Replication and AUTO_INCREMENT”.

  • auto_increment_offset

    Property Value
    Command-Line Format --auto-increment-offset=#
    System Variable auto_increment_offset
    Scope Global, Session
    Dynamic Yes
    SET_VAR Hint Applies Yes
    Type Integer
    Default Value 1
    Minimum Value 1
    Maximum Value 65535

    This variable has a default value of 1. If it is left with its default value, and Group Replication is started on the server in multi-primary mode, it is changed to the server ID. For more information, see the description for auto_increment_increment.

    As of MySQL 8.0.14, setting the session value of this system variable is a restricted operation. The session user must have privileges sufficient to set restricted session variables. See Section 5.1.9.1, “System Variable Privileges”.

    Note

    auto_increment_offset is also supported for use with NDB tables.

  • immediate_server_version

    Property Value
    Introduced 8.0.14
    System Variable immediate_server_version
    Scope Session
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Integer

    For internal use by replication. This session system variable holds the MySQL Server release number of the server that is the immediate master in a replication topology (for example, 80014 for a MySQL 8.0.14 server instance). If this immediate server is at a release that does not support the session system variable, the value of the variable is set to 0 (UNKNOWN_SERVER_VERSION).

    The value of the variable is replicated from a master to a slave. With this information the slave can correctly process data originating from a master at an older release, by recognizing where syntax changes or semantic changes have occurred between the releases involved and handling these appropriately. The information can also be used in a Group Replication environment where one or more members of the replication group is at a newer release than the others. The value of the variable can be viewed in the binary log for each transaction (as part of the Gtid_log_event, or Anonymous_gtid_log_event if GTIDs are not in use on the server), and could be helpful in debugging cross-version replication issues.

    Setting the session value of this system variable is a restricted operation. See Section 5.1.9.1, “System Variable Privileges”. However, note that the variable is not intended for users to set; it is set automatically by the replication infrastructure.

  • original_server_version

    Property Value
    Introduced 8.0.14
    System Variable original_server_version
    Scope Session
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Integer

    For internal use by replication. This session system variable holds the MySQL Server release number of the server where a transaction was originally committed (for example, 80014 for a MySQL 8.0.14 server instance). If this original server is at a release that does not support the session system variable, the value of the variable is set to 0 (UNKNOWN_SERVER_VERSION). Note that when a release number is set by the original server, the value of the variable is reset to 0 if the immediate server or any other intervening server in the replication topology does not support the session system variable, and so does not replicate its value.

    The value of the variable is set and used in the same ways as for the immediate_server_version system variable. If the value of the variable is the same as that for the immediate_server_version system variable, only the latter is recorded in the binary log, with an indicator that the original server version is the same.

    In a Group Replication environment, view change log events, which are special transactions queued by each group member when a new member joins the group, are tagged with the server version of the group member queuing the transaction. This ensures that the server version of the original donor is known to the joining member. Because the view change log events queued for a particular view change have the same GTID on all members, for this case only, instances of the same GTID might have a different original server version.

    Setting the session value of this system variable is a restricted operation. See Section 5.1.9.1, “System Variable Privileges”. However, note that the variable is not intended for users to set; it is set automatically by the replication infrastructure.

  • rpl_semi_sync_master_enabled

    Property Value
    Command-Line Format --rpl-semi-sync-master-enabled[={OFF|ON}]
    System Variable rpl_semi_sync_master_enabled
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Boolean
    Default Value OFF

    Controls whether semisynchronous replication is enabled on the master. To enable or disable the plugin, set this variable to ON or OFF (or 1 or 0), respectively. The default is OFF.

    This variable is available only if the master-side semisynchronous replication plugin is installed.

  • rpl_semi_sync_master_timeout

    Property Value
    Command-Line Format --rpl-semi-sync-master-timeout=#
    System Variable rpl_semi_sync_master_timeout
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Integer
    Default Value 10000

    A value in milliseconds that controls how long the master waits on a commit for acknowledgment from a slave before timing out and reverting to asynchronous replication. The default value is 10000 (10 seconds).

    This variable is available only if the master-side semisynchronous replication plugin is installed.

  • rpl_semi_sync_master_trace_level

    Property Value
    Command-Line Format --rpl-semi-sync-master-trace-level=#
    System Variable rpl_semi_sync_master_trace_level
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Integer
    Default Value 32

    The semisynchronous replication debug trace level on the master. Four levels are defined:

    • 1 = general level (for example, time function failures)

    • 16 = detail level (more verbose information)

    • 32 = net wait level (more information about network waits)

    • 64 = function level (information about function entry and exit)

    This variable is available only if the master-side semisynchronous replication plugin is installed.

  • rpl_semi_sync_master_wait_for_slave_count

    Property Value
    Command-Line Format --rpl-semi-sync-master-wait-for-slave-count=#
    System Variable rpl_semi_sync_master_wait_for_slave_count
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Integer
    Default Value 1
    Minimum Value 1
    Maximum Value 65535

    The number of slave acknowledgments the master must receive per transaction before proceeding. By default rpl_semi_sync_master_wait_for_slave_count is 1, meaning that semisynchronous replication proceeds after receiving a single slave acknowledgment. Performance is best for small values of this variable.

    For example, if rpl_semi_sync_master_wait_for_slave_count is 2, then 2 slaves must acknowledge receipt of the transaction before the timeout period configured by rpl_semi_sync_master_timeout for semisynchronous replication to proceed. If less slaves acknowledge receipt of the transaction during the timeout period, the master reverts to normal replication.

    Note

    This behavior also depends on rpl_semi_sync_master_wait_no_slave

    This variable is available only if the master-side semisynchronous replication plugin is installed.

  • rpl_semi_sync_master_wait_no_slave

    Property Value
    Command-Line Format --rpl-semi-sync-master-wait-no-slave[={OFF|ON}]
    System Variable rpl_semi_sync_master_wait_no_slave
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Boolean
    Default Value ON

    Controls whether the master waits for the timeout period configured by rpl_semi_sync_master_timeout to expire, even if the slave count drops to less than the number of slaves configured by rpl_semi_sync_master_wait_for_slave_count during the timeout period.

    When the value of rpl_semi_sync_master_wait_no_slave is ON (the default), it is permissible for the slave count to drop to less than rpl_semi_sync_master_wait_for_slave_count during the timeout period. As long as enough slaves acknowledge the transaction before the timeout period expires, semisynchronous replication continues.

    When the value of rpl_semi_sync_master_wait_no_slave is OFF, if the slave count drops to less than the number configured in rpl_semi_sync_master_wait_for_slave_count at any time during the timeout period configured by rpl_semi_sync_master_timeout, the master reverts to normal replication.

    This variable is available only if the master-side semisynchronous replication plugin is installed.

  • rpl_semi_sync_master_wait_point

    Property Value
    Command-Line Format --rpl-semi-sync-master-wait-point=value
    System Variable rpl_semi_sync_master_wait_point
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Enumeration
    Default Value AFTER_SYNC
    Valid Values

    AFTER_SYNC

    AFTER_COMMIT

    This variable controls the point at which a semisynchronous replication master waits for slave acknowledgment of transaction receipt before returning a status to the client that committed the transaction. These values are permitted:

    • AFTER_SYNC (the default): The master writes each transaction to its binary log and the slave, and syncs the binary log to disk. The master waits for slave acknowledgment of transaction receipt after the sync. Upon receiving acknowledgment, the master commits the transaction to the storage engine and returns a result to the client, which then can proceed.

    • AFTER_COMMIT: The master writes each transaction to its binary log and the slave, syncs the binary log, and commits the transaction to the storage engine. The master waits for slave acknowledgment of transaction receipt after the commit. Upon receiving acknowledgment, the master returns a result to the client, which then can proceed.

    The replication characteristics of these settings differ as follows:

    • With AFTER_SYNC, all clients see the committed transaction at the same time: After it has been acknowledged by the slave and committed to the storage engine on the master. Thus, all clients see the same data on the master.

      In the event of master failure, all transactions committed on the master have been replicated to the slave (saved to its relay log). A crash of the master and failover to the slave is lossless because the slave is up to date. Note, however, that the master cannot be restarted in this scenario and must be discarded, because its binary log might contain uncommitted transactions that would cause a conflict with the slave when externalized after binary log recovery.

    • With AFTER_COMMIT, the client issuing the transaction gets a return status only after the server commits to the storage engine and receives slave acknowledgment. After the commit and before slave acknowledgment, other clients can see the committed transaction before the committing client.

      If something goes wrong such that the slave does not process the transaction, then in the event of a master crash and failover to the slave, it is possible that such clients will see a loss of data relative to what they saw on the master.

    This variable is available only if the master-side semisynchronous replication plugin is installed.

    With the addition of rpl_semi_sync_master_wait_point in MySQL 5.7, a version compatibility constraint was created because it increments the semisynchronous interface version: Servers for MySQL 5.7 and higher do not work with semisynchronous replication plugins from older versions, nor do servers from older versions work with semisynchronous replication plugins for MySQL 5.7 and higher.

17.1.6.3 Replication Slave Options and Variables

This section explains the server options and system variables that apply to slave replication servers and contains the following:

Specify the options either on the command line or in an option file. Many of the options can be set while the server is running by using the CHANGE MASTER TO statement. Specify system variable values using SET.

Server ID.  On the master and each slave, you must use the server-id option to establish a unique replication ID in the range from 1 to 232 − 1. Unique means that each ID must be different from every other ID in use by any other replication master or slave. Example my.cnf file:

[mysqld]
server-id=3
Startup Options for Replication Slaves

This section explains startup options for controlling replication slave servers. Many of these options can be set while the server is running by using the CHANGE MASTER TO statement. Others, such as the --replicate-* options, can be set only when the slave server starts. Replication-related system variables are discussed later in this section.

  • --log-slave-updates

    Property Value
    Command-Line Format --log-slave-updates[={OFF|ON}]
    System Variable log_slave_updates
    Scope Global
    Dynamic No
    SET_VAR Hint Applies No
    Type Boolean
    Default Value (>= 8.0.3) ON
    Default Value (<= 8.0.2) OFF

    This option makes a slave write updates that are received from a master server and performed by the slave's SQL thread to the slave's own binary log. Binary logging, which is controlled by the --log-bin option and is enabled by default, must also be enabled on the slave for updates to be logged. --log-slave-updates is enabled by default, unless you specify --skip-log-bin to disable binary logging, in which case MySQL also disables slave update logging by default. If you need to disable slave update logging when binary logging is enabled, specify --skip-log-slave-updates.

    --log-slave-updates enables replication servers to be chained. For example, you might want to set up replication servers using this arrangement:

    A -> B -> C
    

    Here, A serves as the master for the slave B, and B serves as the master for the slave C. For this to work, B must be both a master and a slave. With binary logging and the --log-slave-updates option enabled, which are the default settings, updates received from A are logged by B to its binary log, and can therefore be passed on to C.

  • --master-info-file=file_name

    Property Value
    Command-Line Format --master-info-file=file_name
    Type File name
    Default Value master.info

    The name for the master info log, if --master-info-repository=FILE is set. The default name is master.info in the data directory. --master-info-repository=FILE is now deprecated. For information about the master info log, see Section 17.2.4.2, “Slave Status Logs”.

  • --master-retry-count=count

    Property Value
    Command-Line Format --master-retry-count=#
    Deprecated Yes
    Type Integer
    Default Value 86400
    Minimum Value 0
    Maximum Value (64-bit platforms) 18446744073709551615
    Maximum Value (32-bit platforms) 4294967295

    The number of times that the slave tries to reconnect to the master before giving up. The default value is 86400 times. A value of 0 means infinite, and the slave attempts to connect forever. Reconnection attempts are triggered when the slave reaches its connection timeout (specified by the --slave-net-timeout option) without receiving data or a heartbeat signal from the master. Reconnection is attempted at intervals set by the MASTER_CONNECT_RETRY option of the CHANGE MASTER TO statement (which defaults to every 60 seconds).

    This option is deprecated and will be removed in a future MySQL release. Use the MASTER_RETRY_COUNT option of the CHANGE MASTER TO statement instead.

  • --max-relay-log-size=size

    Property Value
    Command-Line Format --max-relay-log-size=#
    System Variable max_relay_log_size
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Integer
    Default Value 0
    Minimum Value 0
    Maximum Value 1073741824

    The size at which the server rotates relay log files automatically. If this value is nonzero, the relay log is rotated automatically when its size exceeds this value. If this value is zero (the default), the size at which relay log rotation occurs is determined by the value of max_binlog_size. For more information, see Section 17.2.4.1, “The Slave Relay Log”.

  • --relay-log=file_name

    Property Value
    Command-Line Format --relay-log=file_name
    System Variable relay_log
    Scope Global
    Dynamic No
    SET_VAR Hint Applies No
    Type File name

    The base name for the relay log. The server creates relay log files in sequence by adding a numeric suffix to the base name.

    For the default replication channel, the default base name for relay logs is host_name-relay-bin, using the name of the host machine. For non-default replication channels, the default base name for relay logs is host_name-relay-bin-channel, where channel is the name of the replication channel recorded in this relay log.

    The default location for relay log files is the data directory. You can use the --relay-log option to specify an alternative location, by adding a leading absolute path name to the base name to specify a different directory.

    The relay log and relay log index on a replication server cannot be given the same names as the binary log and binary log index, whose names are specified by the --log-bin and --log-bin-index options. The server issues an error message and does not start if the binary log and relay log file base names would be the same.

    Due to the manner in which MySQL parses server options, if you specify this option, you must supply a value; the default base name is used only if the option is not actually specified. If you use the --relay-log option without specifying a value, unexpected behavior is likely to result; this behavior depends on the other options used, the order in which they are specified, and whether they are specified on the command line or in an option file. For more information about how MySQL handles server options, see Section 4.2.2, “Specifying Program Options”.

    If you specify this option, the value specified is also used as the base name for the relay log index file. You can override this behavior by specifying a different relay log index file base name using the --relay-log-index option.

    When the server reads an entry from the index file, it checks whether the entry contains a relative path. If it does, the relative part of the path is replaced with the absolute path set using the --relay-log option. An absolute path remains unchanged; in such a case, the index must be edited manually to enable the new path or paths to be used. Previously, manual intervention was required whenever relocating the binary log or relay log files. (Bug #11745230, Bug #12133)

    You may find the --relay-log option useful in performing the following tasks:

    • Creating relay logs whose names are independent of host names.

    • If you need to put the relay logs in some area other than the data directory because your relay logs tend to be very large and you do not want to decrease max_relay_log_size.

    • To increase speed by using load-balancing between disks.

    You can obtain the relay log file name (and path) from the relay_log_basename system variable.

  • --relay-log-index=file_name

    Property Value
    Command-Line Format --relay-log-index=file_name
    System Variable relay_log_index
    Scope Global
    Dynamic No
    SET_VAR Hint Applies No
    Type File name

    The name for the relay log index file. If you do not specify the --relay-log-index option, but the --relay-log option is specified, its value is used as the default base name for the relay log index file. If the --relay-log option is also not specified, then for the default replication channel, the default name is host_name-relay-bin.index, using the name of the host machine. For non-default replication channels, the default name is host_name-relay-bin-channel.index, where channel is the name of the replication channel recorded in this relay log index.

    The default location for relay log files is the data directory, or any other location that was specified using the --relay-log option. You can use the --relay-log-index option to specify an alternative location, by adding a leading absolute path name to the base name to specify a different directory.

    The relay log and relay log index on a replication server cannot be given the same names as the binary log and binary log index, whose names are specified by the --log-bin and --log-bin-index options. The server issues an error message and does not start if the binary log and relay log file base names would be the same.

    Due to the manner in which MySQL parses server options, if you specify this option, you must supply a value; the default base name is used only if the option is not actually specified. If you use the --relay-log-index option without specifying a value, unexpected behavior is likely to result; this behavior depends on the other options used, the order in which they are specified, and whether they are specified on the command line or in an option file. For more information about how MySQL handles server options, see Section 4.2.2, “Specifying Program Options”.

  • --relay-log-info-file=file_name

    Property Value
    Command-Line Format --relay-log-info-file=file_name
    Type File name
    Default Value relay-log.info

    The name for the relay log info file, if --relay-log-info-repository is set to FILE. The default name is relay-log.info in the data directory. --relay-log-info-repository=FILE is now deprecated. For information about the relay log info log, see Section 17.2.4.2, “Slave Status Logs”.

  • --relay-log-purge={0|1}

    Property Value
    Command-Line Format --relay-log-purge[={OFF|ON}]
    System Variable relay_log_purge
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Boolean
    Default Value ON

    Disable or enable automatic purging of relay logs as soon as they are no longer needed. The default value is 1 (enabled). This is a global variable that can be changed dynamically with SET GLOBAL relay_log_purge = N. Disabling purging of relay logs when using the --relay-log-recovery option risks data consistency and is therefore not crash-safe.

  • --relay-log-recovery={0|1}

    Property Value
    Command-Line Format --relay-log-recovery[={OFF|ON}]
    Type Boolean
    Default Value OFF

    Enables automatic relay log recovery immediately following server startup. The recovery process creates a new relay log file, initializes the SQL thread position to this new relay log, and initializes the I/O thread to the SQL thread position. Reading of the relay log from the master then continues. This should be used following a crash on the replication slave to ensure that no possibly corrupted relay logs are processed. The default value is 0 (disabled).

    To provide a crash-safe slave, this option must be enabled (set to 1), --relay-log-info-repository must be set to TABLE, and relay-log-purge must be enabled. Enabling the --relay-log-recovery option when relay-log-purge is disabled risks reading the relay log from files that were not purged, leading to data inconsistency, and is therefore not crash-safe. See Making replication resilient to unexpected halts, for more information.

    When using a multithreaded slave (in other words slave_parallel_workers is greater than 0), inconsistencies such as gaps can occur in the sequence of transactions that have been executed from the relay log. Enabling the --relay-log-recovery option when there are inconsistencies causes an error and the option has no effect. The solution in this situation is to issue START SLAVE UNTIL SQL_AFTER_MTS_GAPS, which brings the server to a more consistent state, then issue RESET SLAVE to remove the relay logs. See Section 17.4.1.33, “Replication and Transaction Inconsistencies” for more information.

    Note

    This option does not affect the following Group Replication channels:

    • group_replication_applier

    • group_replication_recovery

    Any other channels running on a group are affected, example a channel which is replicating from an outside master or another group.

  • --relay-log-space-limit=size

    Property Value
    Command-Line Format --relay-log-space-limit=#
    System Variable relay_log_space_limit
    Scope Global
    Dynamic No
    SET_VAR Hint Applies No
    Type Integer
    Default Value 0
    Minimum Value 0
    Maximum Value (64-bit platforms) 18446744073709551615
    Maximum Value (32-bit platforms) 4294967295

    This option places an upper limit on the total size in bytes of all relay logs on the slave. A value of 0 means no limit. This is useful for a slave server host that has limited disk space. When the limit is reached, the I/O thread stops reading binary log events from the master server until the SQL thread has caught up and deleted some unused relay logs. Note that this limit is not absolute: There are cases where the SQL thread needs more events before it can delete relay logs. In that case, the I/O thread exceeds the limit until it becomes possible for the SQL thread to delete some relay logs because not doing so would cause a deadlock. You should not set --relay-log-space-limit to less than twice the value of --max-relay-log-size (or --max-binlog-size if --max-relay-log-size is 0). In that case, there is a chance that the I/O thread waits for free space because --relay-log-space-limit is exceeded, but the SQL thread has no relay log to purge and is unable to satisfy the I/O thread. This forces the I/O thread to ignore --relay-log-space-limit temporarily.

  • --replicate-do-db=db_name

    Property Value
    Command-Line Format --replicate-do-db=name
    Type String

    Creates a replication filter using the name of a database. Such filters can also be created using CHANGE REPLICATION FILTER REPLICATE_DO_DB.

    This option supports channel specific replication filters, enabling multi-source replication slaves to use specific filters for different sources. To configure a channel specific replication filter on a channel named channel_1 use --replicate-do-db:channel_1:db_name. In this case, the first colon is interpreted as a separator and subsequent colons are literal colons. See Section 17.2.5.4, “Replication Channel Based Filters” for more information.

    Note

    Global replication filters cannot be used on a MySQL server instance that is configured for Group Replication, because filtering transactions on some servers would make the group unable to reach agreement on a consistent state. Channel specific replication filters can be used on replication channels that are not directly involved with Group Replication, such as where a group member also acts as a replication slave to a master that is outside the group. They cannot be used on the group_replication_applier or group_replication_recovery channels.

    The precise effect of this replication filter depends on whether statement-based or row-based replication is in use.

    Statement-based replication.  Tell the slave SQL thread to restrict replication to statements where the default database (that is, the one selected by USE) is db_name. To specify more than one database, use this option multiple times, once for each database; however, doing so does not replicate cross-database statements such as UPDATE some_db.some_table SET foo='bar' while a different database (or no database) is selected.

    Warning

    To specify multiple databases you must use multiple instances of this option. Because database names can contain commas, if you supply a comma separated list then the list is treated as the name of a single database.

    An example of what does not work as you might expect when using statement-based replication: If the slave is started with --replicate-do-db=sales and you issue the following statements on the master, the UPDATE statement is not replicated:

    USE prices;
    UPDATE sales.january SET amount=amount+1000;
    

    The main reason for this check just the default database behavior is that it is difficult from the statement alone to know whether it should be replicated (for example, if you are using multiple-table DELETE statements or multiple-table UPDATE statements that act across multiple databases). It is also faster to check only the default database rather than all databases if there is no need.

    Row-based replication.  Tells the slave SQL thread to restrict replication to database db_name. Only tables belonging to db_name are changed; the current database has no effect on this. Suppose that the slave is started with --replicate-do-db=sales and row-based replication is in effect, and then the following statements are run on the master:

    USE prices;
    UPDATE sales.february SET amount=amount+100;
    

    The february table in the sales database on the slave is changed in accordance with the UPDATE statement; this occurs whether or not the USE statement was issued. However, issuing the following statements on the master has no effect on the slave when using row-based replication and --replicate-do-db=sales:

    USE prices;
    UPDATE prices.march SET amount=amount-25;
    

    Even if the statement USE prices were changed to USE sales, the UPDATE statement's effects would still not be replicated.

    Another important difference in how --replicate-do-db is handled in statement-based replication as opposed to row-based replication occurs with regard to statements that refer to multiple databases. Suppose that the slave is started with --replicate-do-db=db1, and the following statements are executed on the master:

    USE db1;
    UPDATE db1.table1 SET col1 = 10, db2.table2 SET col2 = 20;
    

    If you are using statement-based replication, then both tables are updated on the slave. However, when using row-based replication, only table1 is affected on the slave; since table2 is in a different database, table2 on the slave is not changed by the UPDATE. Now suppose that, instead of the USE db1 statement, a USE db4 statement had been used:

    USE db4;
    UPDATE db1.table1 SET col1 = 10, db2.table2 SET col2 = 20;
    

    In this case, the UPDATE statement would have no effect on the slave when using statement-based replication. However, if you are using row-based replication, the UPDATE would change table1 on the slave, but not table2—in other words, only tables in the database named by --replicate-do-db are changed, and the choice of default database has no effect on this behavior.

    If you need cross-database updates to work, use --replicate-wild-do-table=db_name.% instead. See Section 17.2.5, “How Servers Evaluate Replication Filtering Rules”.

    Note

    This option affects replication in the same manner that --binlog-do-db affects binary logging, and the effects of the replication format on how --replicate-do-db affects replication behavior are the same as those of the logging format on the behavior of --binlog-do-db.

    This option has no effect on BEGIN, COMMIT, or ROLLBACK statements.

  • --replicate-ignore-db=db_name

    Property Value
    Command-Line Format --replicate-ignore-db=name
    Type String

    Creates a replication filter using the name of a database. Such filters can also be created using CHANGE REPLICATION FILTER REPLICATE_IGNORE_DB.

    This option supports channel specific replication filters, enabling multi-source replication slaves to use specific filters for different sources. To configure a channel specific replication filter on a channel named channel_1 use --replicate-ignore-db:channel_1:db_name. In this case, the first colon is interpreted as a separator and subsequent colons are literal colons. See Section 17.2.5.4, “Replication Channel Based Filters” for more information.

    Note

    Global replication filters cannot be used on a MySQL server instance that is configured for Group Replication, because filtering transactions on some servers would make the group unable to reach agreement on a consistent state. Channel specific replication filters can be used on replication channels that are not directly involved with Group Replication, such as where a group member also acts as a replication slave to a master that is outside the group. They cannot be used on the group_replication_applier or group_replication_recovery channels.

    To specify more than one database to ignore, use this option multiple times, once for each database. Because database names can contain commas, if you supply a comma separated list then the list will be treated as the name of a single database.

    As with --replicate-do-db, the precise effect of this filtering depends on whether statement-based or row-based replication is in use, and are described in the next several paragraphs.

    Statement-based replication.  Tells the slave SQL thread not to replicate any statement where the default database (that is, the one selected by USE) is db_name.

    Row-based replication.  Tells the slave SQL thread not to update any tables in the database db_name. The default database has no effect.

    When using statement-based replication, the following example does not work as you might expect. Suppose that the slave is started with --replicate-ignore-db=sales and you issue the following statements on the master:

    USE prices;
    UPDATE sales.january SET amount=amount+1000;
    

    The UPDATE statement is replicated in such a case because --replicate-ignore-db applies only to the default database (determined by the USE statement). Because the sales database was specified explicitly in the statement, the statement has not been filtered. However, when using row-based replication, the UPDATE statement's effects are not propagated to the slave, and the slave's copy of the sales.january table is unchanged; in this instance, --replicate-ignore-db=sales causes all changes made to tables in the master's copy of the sales database to be ignored by the slave.

    You should not use this option if you are using cross-database updates and you do not want these updates to be replicated. See Section 17.2.5, “How Servers Evaluate Replication Filtering Rules”.

    If you need cross-database updates to work, use --replicate-wild-ignore-table=db_name.% instead. See Section 17.2.5, “How Servers Evaluate Replication Filtering Rules”.

    Note

    This option affects replication in the same manner that --binlog-ignore-db affects binary logging, and the effects of the replication format on how --replicate-ignore-db affects replication behavior are the same as those of the logging format on the behavior of --binlog-ignore-db.

    This option has no effect on BEGIN, COMMIT, or ROLLBACK statements.

  • --replicate-do-table=db_name.tbl_name

    Property Value
    Command-Line Format --replicate-do-table=name
    Type String

    Creates a replication filter by telling the slave SQL thread to restrict replication to a given table. To specify more than one table, use this option multiple times, once for each table. This works for both cross-database updates and default database updates, in contrast to --replicate-do-db. See Section 17.2.5, “How Servers Evaluate Replication Filtering Rules”. You can also create such a filter by issuing a CHANGE REPLICATION FILTER REPLICATE_DO_TABLE statement.

    This option supports channel specific replication filters, enabling multi-source replication slaves to use specific filters for different sources. To configure a channel specific replication filter on a channel named channel_1 use --replicate-do-table:channel_1:db_name.tbl_name. In this case, the first colon is interpreted as a separator and subsequent colons are literal colons. See Section 17.2.5.4, “Replication Channel Based Filters” for more information.

    Note

    Global replication filters cannot be used on a MySQL server instance that is configured for Group Replication, because filtering transactions on some servers would make the group unable to reach agreement on a consistent state. Channel specific replication filters can be used on replication channels that are not directly involved with Group Replication, such as where a group member also acts as a replication slave to a master that is outside the group. They cannot be used on the group_replication_applier or group_replication_recovery channels.

    This option affects only statements that apply to tables. It does not affect statements that apply only to other database objects, such as stored routines. To filter statements operating on stored routines, use one or more of the --replicate-*-db options.

  • --replicate-ignore-table=db_name.tbl_name

    Property Value
    Command-Line Format --replicate-ignore-table=name
    Type String

    Creates a replication filter by telling the slave SQL thread not to replicate any statement that updates the specified table, even if any other tables might be updated by the same statement. To specify more than one table to ignore, use this option multiple times, once for each table. This works for cross-database updates, in contrast to --replicate-ignore-db. See Section 17.2.5, “How Servers Evaluate Replication Filtering Rules”. You can also create such a filter by issuing a CHANGE REPLICATION FILTER REPLICATE_IGNORE_TABLE statement.

    This option supports channel specific replication filters, enabling multi-source replication slaves to use specific filters for different sources. To configure a channel specific replication filter on a channel named channel_1 use --replicate-ignore-table:channel_1:db_name.tbl_name. In this case, the first colon is interpreted as a separator and subsequent colons are literal colons. See Section 17.2.5.4, “Replication Channel Based Filters” for more information.

    Note

    Global replication filters cannot be used on a MySQL server instance that is configured for Group Replication, because filtering transactions on some servers would make the group unable to reach agreement on a consistent state. Channel specific replication filters can be used on replication channels that are not directly involved with Group Replication, such as where a group member also acts as a replication slave to a master that is outside the group. They cannot be used on the group_replication_applier or group_replication_recovery channels.

    This option affects only statements that apply to tables. It does not affect statements that apply only to other database objects, such as stored routines. To filter statements operating on stored routines, use one or more of the --replicate-*-db options.

  • --replicate-rewrite-db=from_name->to_name

    Property Value
    Command-Line Format --replicate-rewrite-db=old_name->new_name
    Type String

    Tells the slave to create a replication filter that translates the default database (that is, the one selected by USE) to to_name if it was from_name on the master. Only statements involving tables are affected (not statements such as CREATE DATABASE, DROP DATABASE, and ALTER DATABASE), and only if from_name is the default database on the master. To specify multiple rewrites, use this option multiple times. The server uses the first one with a from_name value that matches. The database name translation is done before the --replicate-* rules are tested. You can also create such a filter by issuing a CHANGE REPLICATION FILTER REPLICATE_REWRITE_DB statement.

    If you use this option on the command line and the > character is special to your command interpreter, quote the option value. For example:

    shell> mysqld --replicate-rewrite-db="olddb->newdb"
    

    This option supports channel specific replication filters, enabling multi-source replication slaves to use specific filters for different sources. Specify the channel name followed by a colon, followed by the filter specification. The first colon is interpreted as a separator, and any subsequent colons are interpreted as literal colons. For example, to configure a channel specific replication filter on a channel named channel_1, use:

    shell> mysqld --replicate-rewrite-db=channel_1:db_name1->db_name2
    

    If you use a colon but do not specify a channel name, the option configures the replication filter for the default replication channel. See Section 17.2.5.4, “Replication Channel Based Filters” for more information.

    Note

    Global replication filters cannot be used on a MySQL server instance that is configured for Group Replication, because filtering transactions on some servers would make the group unable to reach agreement on a consistent state. Channel specific replication filters can be used on replication channels that are not directly involved with Group Replication, such as where a group member also acts as a replication slave to a master that is outside the group. They cannot be used on the group_replication_applier or group_replication_recovery channels.

    Statements in which table names are qualified with database names when using this option do not work with table-level replication filtering options such as --replicate-do-table. Suppose we have a database named a on the master, one named b on the slave, each containing a table t, and have started the master with --replicate-rewrite-db='a->b'. At a later point in time, we execute DELETE FROM a.t. In this case, no relevant filtering rule works, for the reasons shown here:

    1. --replicate-do-table=a.t does not work because the slave has table t in database b.

    2. --replicate-do-table=b.t does not match the original statement and so is ignored.

    3. --replicate-do-table=*.t is handled identically to --replicate-do-table=a.t, and thus does not work, either.

    Similarly, the --replication-rewrite-db option does not work with cross-database updates.

  • --replicate-same-server-id

    Property Value
    Command-Line Format --replicate-same-server-id[={OFF|ON}]
    Type Boolean
    Default Value OFF

    This option is for use on replication slaves. The default is 0 (FALSE). With this option set to 1 (TRUE), the slave does not skip events that have its own server ID. This setting is normally useful only in rare configurations.

    When binary logging is enabled on a replication slave, the combination of the --replicate-same-server-id and --log-slave-updates options on the slave can cause infinite loops in replication if the server is part of a circular replication topology. (In MySQL 8.0, binary logging is enabled by default, and slave update logging is the default when binary logging is enabled.) However, the use of global transaction identifiers (GTIDs) prevents this situation by skipping the execution of transactions that have already been applied. If gtid_mode=ON is set on the slave, you can start the server with this combination of options, but you cannot change to any other GTID mode while the server is running. If any other GTID mode is set, the server does not start with this combination of options.

    By default, the slave I/O thread does not write binary log events to the relay log if they have the slave's server ID (this optimization helps save disk usage). If you want to use --replicate-same-server-id, be sure to start the slave with this option before you make the slave read its own events that you want the slave SQL thread to execute.

  • --replicate-wild-do-table=db_name.tbl_name

    Property Value
    Command-Line Format --replicate-wild-do-table=name
    Type String

    Creates a replication filter by telling the slave thread to restrict replication to statements where any of the updated tables match the specified database and table name patterns. Patterns can contain the % and _ wildcard characters, which have the same meaning as for the LIKE pattern-matching operator. To specify more than one table, use this option multiple times, once for each table. This works for cross-database updates. See Section 17.2.5, “How Servers Evaluate Replication Filtering Rules”. You can also create such a filter by issuing a CHANGE REPLICATION FILTER REPLICATE_WILD_DO_TABLE statement.

    This option supports channel specific replication filters, enabling multi-source replication slaves to use specific filters for different sources. To configure a channel specific replication filter on a channel named channel_1 use --replicate-wild-do-table:channel_1:db_name.tbl_name. In this case, the first colon is interpreted as a separator and subsequent colons are literal colons. See Section 17.2.5.4, “Replication Channel Based Filters” for more information.

    Note

    Global replication filters cannot be used on a MySQL server instance that is configured for Group Replication, because filtering transactions on some servers would make the group unable to reach agreement on a consistent state. Channel specific replication filters can be used on replication channels that are not directly involved with Group Replication, such as where a group member also acts as a replication slave to a master that is outside the group. They cannot be used on the group_replication_applier or group_replication_recovery channels.

    This option applies to tables, views, and triggers. It does not apply to stored procedures and functions, or events. To filter statements operating on the latter objects, use one or more of the --replicate-*-db options.

    As an example, --replicate-wild-do-table=foo%.bar% replicates only updates that use a table where the database name starts with foo and the table name starts with bar.

    If the table name pattern is %, it matches any table name and the option also applies to database-level statements (CREATE DATABASE, DROP DATABASE, and ALTER DATABASE). For example, if you use --replicate-wild-do-table=foo%.%, database-level statements are replicated if the database name matches the pattern foo%.

    To include literal wildcard characters in the database or table name patterns, escape them with a backslash. For example, to replicate all tables of a database that is named my_own%db, but not replicate tables from the my1ownAABCdb database, you should escape the _ and % characters like this: --replicate-wild-do-table=my\_own\%db. If you use the option on the command line, you might need to double the backslashes or quote the option value, depending on your command interpreter. For example, with the bash shell, you would need to type --replicate-wild-do-table=my\\_own\\%db.

  • --replicate-wild-ignore-table=db_name.tbl_name

    Property Value
    Command-Line Format --replicate-wild-ignore-table=name
    Type String

    Creates a replication filter which keeps the slave thread from replicating a statement in which any table matches the given wildcard pattern. To specify more than one table to ignore, use this option multiple times, once for each table. This works for cross-database updates. See Section 17.2.5, “How Servers Evaluate Replication Filtering Rules”. You can also create such a filter by issuing a CHANGE REPLICATION FILTER REPLICATE_WILD_IGNORE_TABLE statement.

    This option supports channel specific replication filters, enabling multi-source replication slaves to use specific filters for different sources. To configure a channel specific replication filter on a channel named channel_1 use --replicate-wild-ignore:channel_1:db_name.tbl_name. In this case, the first colon is interpreted as a separator and subsequent colons are literal colons. See Section 17.2.5.4, “Replication Channel Based Filters” for more information.

    Note

    Global replication filters cannot be used on a MySQL server instance that is configured for Group Replication, because filtering transactions on some servers would make the group unable to reach agreement on a consistent state. Channel specific replication filters can be used on replication channels that are not directly involved with Group Replication, such as where a group member also acts as a replication slave to a master that is outside the group. They cannot be used on the group_replication_applier or group_replication_recovery channels.

    As an example, --replicate-wild-ignore-table=foo%.bar% does not replicate updates that use a table where the database name starts with foo and the table name starts with bar. For information about how matching works, see the description of the --replicate-wild-do-table option. The rules for including literal wildcard characters in the option value are the same as for --replicate-wild-ignore-table as well.

  • --report-host=host_name

    Property Value
    Command-Line Format --report-host=host_name
    System Variable report_host
    Scope Global
    Dynamic No
    SET_VAR Hint Applies No
    Type String

    The host name or IP address of the slave to be reported to the master during slave registration. This value appears in the output of SHOW SLAVE HOSTS on the master server. Leave the value unset if you do not want the slave to register itself with the master.

    Note

    It is not sufficient for the master to simply read the IP address of the slave from the TCP/IP socket after the slave connects. Due to NAT and other routing issues, that IP may not be valid for connecting to the slave from the master or other hosts.

  • --report-password=password

    Property Value
    Command-Line Format --report-password=name
    System Variable report_password
    Scope Global
    Dynamic No
    SET_VAR Hint Applies No
    Type String

    The account password of the slave to be reported to the master during slave registration. This value appears in the output of SHOW SLAVE HOSTS on the master server if the master was started with --show-slave-auth-info.

    Although the name of this option might imply otherwise, --report-password is not connected to the MySQL user privilege system and so is not necessarily (or even likely to be) the same as the password for the MySQL replication user account.

  • --report-port=slave_port_num

    Property Value
    Command-Line Format --report-port=port_num
    System Variable report_port
    Scope Global
    Dynamic No
    SET_VAR Hint Applies No
    Type Integer
    Default Value [slave_port]
    Minimum Value 0
    Maximum Value 65535

    The TCP/IP port number for connecting to the slave, to be reported to the master during slave registration. Set this only if the slave is listening on a nondefault port or if you have a special tunnel from the master or other clients to the slave. If you are not sure, do not use this option.

    The default value for this option is the port number actually used by the slave (Bug #13333431). This is also the default value displayed by SHOW SLAVE HOSTS.

  • --report-user=user_name

    Property Value
    Command-Line Format --report-user=name
    System Variable report_user
    Scope Global
    Dynamic No
    SET_VAR Hint Applies No
    Type String

    The account user name of the slave to be reported to the master during slave registration. This value appears in the output of SHOW SLAVE HOSTS on the master server if the master was started with --show-slave-auth-info.

    Although the name of this option might imply otherwise, --report-user is not connected to the MySQL user privilege system and so is not necessarily (or even likely to be) the same as the name of the MySQL replication user account.

  • --slave-checkpoint-group=#

    Property Value
    Command-Line Format --slave-checkpoint-group=#
    Type Integer
    Default Value 512
    Minimum Value 32
    Maximum Value 524280
    Block Size 8

    Sets the maximum number of transactions that can be processed by a multithreaded slave before a checkpoint operation is called to update its status as shown by SHOW SLAVE STATUS. Setting this option has no effect on slaves for which multithreading is not enabled.

    Note

    Multithreaded slaves are not currently supported by NDB Cluster, which silently ignores the setting for this option. See Section 22.6.3, “Known Issues in NDB Cluster Replication”, for more information.

    This option works in combination with the --slave-checkpoint-period option in such a way that, when either limit is exceeded, the checkpoint is executed and the counters tracking both the number of transactions and the time elapsed since the last checkpoint are reset.

    The minimum allowed value for this option is 32, unless the server was built using -DWITH_DEBUG, in which case the minimum value is 1. The effective value is always a multiple of 8; you can set it to a value that is not such a multiple, but the server rounds it down to the next lower multiple of 8 before storing the value. (Exception: No such rounding is performed by the debug server.) Regardless of how the server was built, the default value is 512, and the maximum allowed value is 524280.

  • --slave-checkpoint-period=#

    Property Value
    Command-Line Format --slave-checkpoint-period=#
    Type Integer
    Default Value 300
    Minimum Value 1
    Maximum Value 4G

    Sets the maximum time (in milliseconds) that is allowed to pass before a checkpoint operation is called to update the status of a multithreaded slave as shown by SHOW SLAVE STATUS. Setting this option has no effect on slaves for which multithreading is not enabled.

    Note

    Multithreaded slaves are not currently supported by NDB Cluster, which silently ignores the setting for this option. See Section 22.6.3, “Known Issues in NDB Cluster Replication”, for more information.

    This option works in combination with the --slave-checkpoint-group option in such a way that, when either limit is exceeded, the checkpoint is executed and the counters tracking both the number of transactions and the time elapsed since the last checkpoint are reset.

    The minimum allowed value for this option is 1, unless the server was built using -DWITH_DEBUG, in which case the minimum value is 0. Regardless of how the server was built, the default value is 300, and the maximum possible value is 4294967296 (4GB).

  • --slave-parallel-workers

    Property Value
    Command-Line Format --slave-parallel-workers=#
    Type Integer
    Default Value 0
    Minimum Value 0
    Maximum Value 1024

    Enables multithreading on the slave and sets the number of slave applier threads for executing replication transactions in parallel. When the value is a number greater than 0, the slave is a multithreaded slave with the specified number of applier threads, plus a coordinator thread to manage them. If you are using multiple replication channels, each channel has this number of threads.

    Retrying of transactions is supported when multithreading is enabled on a slave. When slave_preserve_commit_order=1, transactions on a slave are externalized on the slave in the same order as they appear in the slave's relay log. The way in which transactions are distributed among applier threads is configured by --slave-parallel-type.

    To disable parallel execution, set this option to 0, which gives the slave a single applier thread and no coordinator thread. With this setting, the --slave-parallel-type and slave_preserve_commit_order options have no effect and are ignored.

    Note

    Multithreaded slaves are not currently supported by NDB Cluster, which silently ignores the setting for this option. See Section 22.6.3, “Known Issues in NDB Cluster Replication”, for more information.

  • --slave-pending-jobs-size-max=#

    Property Value
    Command-Line Format --slave-pending-jobs-size-max=#
    Type Integer
    Default Value (>= 8.0.12) 128M
    Default Value (<= 8.0.11) 16M
    Minimum Value 1024
    Maximum Value 16EiB
    Block Size 1024

    For multithreaded slaves, this option sets the maximum amount of memory (in bytes) available to slave worker queues holding events not yet applied. Setting this option has no effect on slaves for which multithreading is not enabled.

    The minimum possible value for this option is 1024 bytes; the default is 128MB. The maximum possible value is 18446744073709551615 (16 exbibytes). Values that are not exact multiples of 1024 bytes are rounded down to the next lower multiple of 1024 bytes prior to being stored.

    The value of this variable is a soft limit and can be set to match the normal workload. If an unusually large event exceeds this size, the transaction is held until all the slave workers have empty queues, and then processed. All subsequent transactions are held until the large transaction has been completed.

  • --skip-slave-start

    Property Value
    Command-Line Format --skip-slave-start[={OFF|ON}]
    Type Boolean
    Default Value OFF

    Tells the slave server not to start the slave threads when the server starts. To start the threads later, use a START SLAVE statement.

  • --slave-load-tmpdir=dir_name

    Property Value
    Command-Line Format --slave-load-tmpdir=dir_name
    System Variable slave_load_tmpdir
    Scope Global
    Dynamic No
    SET_VAR Hint Applies No
    Type Directory name
    Default Value Value of --tmpdir

    The name of the directory where the slave creates temporary files. This option is by default equal to the value of the tmpdir system variable, or the default that applies when that system variable is not specified. When the slave SQL thread replicates a LOAD DATA statement, it extracts the file to be loaded from the relay log into temporary files, and then loads these into the table. If the file loaded on the master is huge, the temporary files on the slave are huge, too. Therefore, it might be advisable to use this option to tell the slave to put temporary files in a directory located in some file system that has a lot of available space. In that case, the relay logs are huge as well, so you might also want to use the --relay-log option to place the relay logs in that file system.

    The directory specified by this option should be located in a disk-based file system (not a memory-based file system) so that the temporary files used to replicate LOAD DATA can survive machine restarts. The directory also should not be one that is cleared by the operating system during the system startup process. However, replication can now continue after a restart if the temporary files have been removed.

  • slave-max-allowed-packet=bytes

    Property Value
    Command-Line Format --slave-max-allowed-packet=#
    Type Integer
    Default Value 1073741824
    Minimum Value 1024
    Maximum Value 1073741824

    This option sets the maximum packet size in bytes that the slave SQL and I/O threads can handle. It is possible for a replication master to write binary log events longer than its max_allowed_packet setting once the event header is added. The setting for slave_max_allowed_packet must be larger than the max_allowed_packet setting on the master, so that large updates using row-based replication do not cause replication to fail.

    The corresponding server variable slave_max_allowed_packet always has a value that is a positive integer multiple of 1024; if you set it to some value that is not such a multiple, the value is automatically rounded down to the next highest multiple of 1024. (For example, if you start the server with --slave-max-allowed-packet=10000, the value used is 9216; setting 0 as the value causes 1024 to be used.) A truncation warning is issued in such cases.

    The maximum (and default) value is 1073741824 (1 GB); the minimum is 1024.

  • --slave-net-timeout=seconds

    Property Value
    Command-Line Format --slave-net-timeout=#
    System Variable slave_net_timeout
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Integer
    Default Value 60
    Minimum Value 1

    The number of seconds to wait for more data or a heartbeat signal from the master before the slave considers the connection broken, aborts the read, and tries to reconnect. The default value is 60 seconds (one minute). The first retry occurs immediately after the timeout. The interval between retries is controlled by the MASTER_CONNECT_RETRY option for the CHANGE MASTER TO statement, and the number of reconnection attempts is limited by the MASTER_RETRY_COUNT option for the CHANGE MASTER TO statement.

    The heartbeat interval, which stops the connection timeout occurring in the absence of data if the connection is still good, is controlled by the MASTER_HEARTBEAT_PERIOD option for the CHANGE MASTER TO statement. The heartbeat interval defaults to half the value of --slave-net-timeout, and it is recorded in the master info log and shown in the replication_connection_configuration Performance Schema table. Note that a change to the value or default setting of --slave-net-timeout does not automatically change the heartbeat interval, whether that has been set explicitly or is using a previously calculated default. If the connection timeout is changed, you must also issue CHANGE MASTER TO to adjust the heartbeat interval to an appropriate value so that it occurs before the connection timeout.

  • --slave-parallel-type=type

    Property Value
    Command-Line Format --slave-parallel-type=type
    Type Enumeration
    Default Value DATABASE
    Valid Values

    DATABASE

    LOGICAL_CLOCK

    When using a multithreaded slave (slave_parallel_workers is greater than 0), this option specifies the policy used to decide which transactions are allowed to execute in parallel on the slave. The option has no effect on slaves for which multithreading is not enabled. The possible values are:

    • LOGICAL_CLOCK: Transactions that are part of the same binary log group commit on a master are applied in parallel on a slave. The dependencies between transactions are tracked based on their timestamps to provide additional parallelization where possible. When this value is set, the binlog_transaction_dependency_tracking system variable can be used on the master to specify that write sets are used for parallelization in place of timestamps, if a write set is available for the transaction and gives improved results compared to timestamps.

    • DATABASE: Transactions that update different databases are applied in parallel. This value is only appropriate if data is partitioned into multiple databases which are being updated independently and concurrently on the master. There must be no cross-database constraints, as such constraints may be violated on the slave.

    When slave_preserve_commit_order=1 is set, you can only use LOGICAL_CLOCK.

    If your replication topology uses multiple levels of slaves, LOGICAL_CLOCK may achieve less parallelization for each level the slave is away from the master. You can reduce this effect by using binlog_transaction_dependency_tracking on the master to specify that write sets are used instead of timestamps for parallelization where possible.

  • slave-rows-search-algorithms=list

    Property Value
    Command-Line Format --slave-rows-search-algorithms=list
    Type Set
    Default Value (>= 8.0.2) INDEX_SCAN,HASH_SCAN
    Default Value (<= 8.0.1) TABLE_SCAN,INDEX_SCAN
    Valid Values

    TABLE_SCAN,INDEX_SCAN

    INDEX_SCAN,HASH_SCAN

    TABLE_SCAN,HASH_SCAN

    TABLE_SCAN,INDEX_SCAN,HASH_SCAN (equivalent to INDEX_SCAN,HASH_SCAN)

    When preparing batches of rows for row-based logging and replication, this option controls how the rows are searched for matches, in particular whether hash scans are used. The option sets the initial value for the slave_rows_search_algorithms system variable.

    Specify a comma-separated list of the following combinations of 2 values from the list INDEX_SCAN, TABLE_SCAN, HASH_SCAN. For the option, the list need not be quoted, but must contain no spaces, whether or not quotes are used. The recommended combinations (lists) and their effects are shown in the following table:

    Index used / option value INDEX_SCAN,HASH_SCAN INDEX_SCAN,TABLE_SCAN
    Primary key or unique key Index scan Index scan
    (Other) Key Hash scan over index Index scan
    No index Hash scan Table scan
    • The default value is INDEX_SCAN,HASH_SCAN. With this setting, hashing is used for any searches that do not use a primary or unique key. Specifying INDEX_SCAN,HASH_SCAN has the same effect as specifying INDEX_SCAN,TABLE_SCAN,HASH_SCAN, which is allowed.

    • To remove hashing, set INDEX_SCAN,TABLE_SCAN. With this setting, all searches that can use indexes do use them, and searches without any indexes use table scans.

    • Do not use the combination TABLE_SCAN,HASH_SCAN. This setting forces hashing for all searches. It has no advantage over INDEX_SCAN,HASH_SCAN, and it can lead to record not found errors or duplicate key errors in the case of a single event containing multiple updates to the same row, or updates that are order-dependent.

    The order in which the algorithms are specified in the list does not make any difference in the order in which they are displayed by a SELECT or SHOW VARIABLES statement.

    It is possible to specify a single value, but this is not optimal, because setting a single value limits searches to using only that algorithm. In particular, setting INDEX_SCAN alone is not recommended, as in that case searches are unable to find rows at all if no index is present.

  • --slave-skip-errors=[err_code1,err_code2,...|all|ddl_exist_errors]

    Property Value
    Command-Line Format --slave-skip-errors=name
    System Variable slave_skip_errors
    Scope Global
    Dynamic No
    SET_VAR Hint Applies No
    Type String
    Default Value OFF
    Valid Values

    OFF

    [list of error codes]

    all

    ddl_exist_errors

    Normally, replication stops when an error occurs on the slave, which gives you the opportunity to resolve the inconsistency in the data manually. This option causes the slave SQL thread to continue replication when a statement returns any of the errors listed in the option value.

    Do not use this option unless you fully understand why you are getting errors. If there are no bugs in your replication setup and client programs, and no bugs in MySQL itself, an error that stops replication should never occur. Indiscriminate use of this option results in slaves becoming hopelessly out of synchrony with the master, with you having no idea why this has occurred.

    For error codes, you should use the numbers provided by the error message in your slave error log and in the output of SHOW SLAVE STATUS. Appendix B, Errors, Error Codes, and Common Problems, lists server error codes.

    The shorthand value ddl_exist_errors is equivalent to the error code list 1007,1008,1050,1051,1054,1060,1061,1068,1094,1146.

    You can also (but should not) use the very nonrecommended value of all to cause the slave to ignore all error messages and keeps going regardless of what happens. Needless to say, if you use all, there are no guarantees regarding the integrity of your data. Please do not complain (or file bug reports) in this case if the slave's data is not anywhere close to what it is on the master. You have been warned.

    Examples:

    --slave-skip-errors=1062,1053
    --slave-skip-errors=all
    --slave-skip-errors=ddl_exist_errors
    
  • --slave-sql-verify-checksum={0|1}

    Property Value
    Command-Line Format --slave-sql-verify-checksum[={OFF|ON}]
    Type Boolean
    Default Value ON

    When this option is enabled, the slave examines checksums read from the relay log, in the event of a mismatch, the slave stops with an error.

The following options are used internally by the MySQL test suite for replication testing and debugging. They are not intended for use in a production setting.

  • --abort-slave-event-count

    Property Value
    Command-Line Format --abort-slave-event-count=#
    Type Integer
    Default Value 0
    Minimum Value 0

    When this option is set to some positive integer value other than 0 (the default) it affects replication behavior as follows: After the slave SQL thread has started, value log events are permitted to be executed; after that, the slave SQL thread does not receive any more events, just as if the network connection from the master were cut. The slave thread continues to run, and the output from SHOW SLAVE STATUS displays Yes in both the Slave_IO_Running and the Slave_SQL_Running columns, but no further events are read from the relay log.

  • --disconnect-slave-event-count

    Property Value
    Command-Line Format --disconnect-slave-event-count=#
    Type Integer
    Default Value 0
Options for Logging Slave Status to Tables

Replication slave status information is logged to an InnoDB table in the mysql database. Before MySQL 8.0, this information could alternatively be logged to a file in the data directory, but the use of that format is now deprecated. Writing of the master info log and the relay log info log can be configured separately using the two server options listed here:

  • --master-info-repository={TABLE|FILE}

    Property Value
    Command-Line Format --master-info-repository={FILE|TABLE}
    Type String
    Default Value (>= 8.0.2) TABLE
    Default Value (<= 8.0.1) FILE
    Valid Values

    FILE

    TABLE

    This option determines whether the slave server logs master status and connection information to an InnoDB table in the mysql database, or to a file in the data directory.

    The default setting is TABLE. As an InnoDB table, the master info log is named mysql.slave_master_info. The TABLE setting is required when multiple replication channels are configured.

    The FILE setting is deprecated, and will be removed in a future release. As a file, the master info log is named master.info by default, and you can change this name using the --master-info-file option.

    The setting for the location of this slave status log has a direct influence on the effect had by the setting of the sync_master_info system variable. You can only change the setting when no replication threads are executing.

  • --relay-log-info-repository={TABLE|FILE}

    Property Value
    Command-Line Format --relay-log-info-repository={FILE|TABLE}
    Type String
    Default Value (>= 8.0.2) TABLE
    Default Value (<= 8.0.1) FILE
    Valid Values

    FILE

    TABLE

    This option determines whether the slave server logs its position in the relay logs to an InnoDB table in the mysql database, or to a file in the data directory.

    The default setting is TABLE. As an InnoDB table, the relay log info log is named mysql.slave_relay_log_info. The TABLE setting is required when multiple replication channels are configured. The TABLE setting for the relay log info log is also required to make replication resilient to unexpected halts, for which the --relay-log-recovery option must also be enabled. See Making replication resilient to unexpected halts for more information.

    The FILE setting is deprecated, and will be removed in a future release. As a file, the relay log info log is named relay-log.info by default, and you can change this name using the --relay-log-info-file option.

    The setting for the location of this slave status log has a direct influence on the effect had by the setting of the sync_relay_log_info system variable. You can only change the setting when no replication threads are executing.

The slave status log tables and their contents are considered local to a given MySQL Server. They are not replicated, and changes to them are not written to the binary log.

For more information, see Section 17.2.4, “Replication Relay and Status Logs”.

System Variables Used on Replication Slaves

The following list describes system variables for controlling replication slave servers. They can be set at server startup and some of them can be changed at runtime using SET. Server options used with replication slaves are listed earlier in this section.

  • init_slave

    Property Value
    Command-Line Format --init-slave=name
    System Variable init_slave
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type String

    This variable is similar to init_connect, but is a string to be executed by a slave server each time the SQL thread starts. The format of the string is the same as for the init_connect variable. The setting of this variable takes effect for subsequent START SLAVE statements.

    Note

    The SQL thread sends an acknowledgment to the client before it executes init_slave. Therefore, it is not guaranteed that init_slave has been executed when START SLAVE returns. See Section 13.4.2.6, “START SLAVE Syntax”, for more information.

  • log_slow_slave_statements

    Property Value
    Command-Line Format --log-slow-slave-statements[={OFF|ON}]
    System Variable log_slow_slave_statements
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Boolean
    Default Value OFF

    When the slow query log is enabled, this variable enables logging for queries that have taken more than long_query_time seconds to execute on the slave. Note that if row-based replication is in use (binlog_format=ROW), log_slow_slave_statements has no effect. Queries are only added to the slave's slow query log when they are logged in statement format in the binary log, that is, when binlog_format=STATEMENT is set, or when binlog_format=MIXED is set and the statement is logged in statement format. Slow queries that are logged in row format when binlog_format=MIXED is set, or that are logged when binlog_format=ROW is set, are not added to the slave's slow query log, even if log_slow_slave_statements is enabled.

    Setting log_slow_slave_statements has no immediate effect. The state of the variable applies on all subsequent START SLAVE statements. Also note that the global setting for long_query_time applies for the lifetime of the SQL thread. If you change that setting, you must stop and restart the slave's SQL thread to implement the change there (for example, by issuing STOP SLAVE and START SLAVE statements with the SQL_THREAD option).

  • master_info_repository

    Property Value
    Command-Line Format --master-info-repository={FILE|TABLE}
    System Variable master_info_repository
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type String
    Default Value (>= 8.0.2) TABLE
    Default Value (<= 8.0.1) FILE
    Valid Values

    FILE

    TABLE

    The setting of this variable determines whether the slave server logs master status and connection information to an InnoDB table in the mysql database, or to a file in the data directory.

    The default setting is TABLE. As an InnoDB table, the master info log is named mysql.slave_master_info. The TABLE setting is required when multiple replication channels are configured.

    The FILE setting is deprecated, and will be removed in a future release. As a file, the master info log is named master.info by default, and you can change this name using the --master-info-file option.

    The setting for the location of this slave status log has a direct influence on the effect had by the setting of the sync_master_info system variable. You can only change the setting when no replication threads are executing.

  • max_relay_log_size

    Property Value
    Command-Line Format --max-relay-log-size=#
    System Variable max_relay_log_size
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Integer
    Default Value 0
    Minimum Value 0
    Maximum Value 1073741824

    If a write by a replication slave to its relay log causes the current log file size to exceed the value of this variable, the slave rotates the relay logs (closes the current file and opens the next one). If max_relay_log_size is 0, the server uses max_binlog_size for both the binary log and the relay log. If max_relay_log_size is greater than 0, it constrains the size of the relay log, which enables you to have different sizes for the two logs. You must set max_relay_log_size to between 4096 bytes and 1GB (inclusive), or to 0. The default value is 0. See Section 17.2.2, “Replication Implementation Details”.

  • relay_log

    Property Value
    Command-Line Format --relay-log=file_name
    System Variable relay_log
    Scope Global
    Dynamic No
    SET_VAR Hint Applies No
    Type File name

    The base name for relay log files, with no paths and no file extension. For the default replication channel, the default base name for relay logs is host_name-relay-bin. For non-default replication channels, the default base name for relay logs is host_name-relay-bin-channel, where channel is the name of the replication channel recorded in this relay log.

  • relay_log_basename

    Property Value
    System Variable relay_log_basename
    Scope Global
    Dynamic No
    SET_VAR Hint Applies No
    Type File name
    Default Value datadir + '/' + hostname + '-relay-bin'

    Holds the name and complete path to the relay log file. This variable is set by the server and is read only.

  • relay_log_index

    Property Value
    Command-Line Format --relay-log-index=file_name
    System Variable relay_log_index
    Scope Global
    Dynamic No
    SET_VAR Hint Applies No
    Type File name
    Default Value *host_name*-relay-bin.index

    The name of the relay log index file for the default replication channel. The default name is host_name-relay-bin.index.

  • relay_log_info_file

    Property Value
    Command-Line Format --relay-log-info-file=file_name
    System Variable relay_log_info_file
    Scope Global
    Dynamic No
    SET_VAR Hint Applies No
    Type File name
    Default Value relay-log.info

    The name of the relay log info log, when relay_log_info_repository=FILE is set. The default name is relay-log.info in the data directory. relay_log_info_repository=FILE is now deprecated.

  • relay_log_info_repository

    Property Value
    Command-Line Format --relay-log-info-repository=value
    System Variable relay_log_info_repository
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type String
    Default Value (>= 8.0.2) TABLE
    Default Value (<= 8.0.1) FILE
    Valid Values

    FILE

    TABLE

    The setting of this variable determines whether the slave server logs its position in the relay logs to an InnoDB table in the mysql database, or to a file in the data directory.

    The default setting is TABLE. As an InnoDB table, the relay log info log is named mysql.slave_relay_log_info. The TABLE setting is required when multiple replication channels are configured. The TABLE setting for the relay log info log is also required to make replication resilient to unexpected halts, for which the --relay-log-recovery option must also be enabled. See Making replication resilient to unexpected halts for more information.

    The FILE setting is deprecated, and will be removed in a future release. As a file, the relay log info log is named relay-log.info by default, and you can change this name using the --relay-log-info-file option.

    The setting for the location of this slave status log has a direct influence on the effect had by the setting of the sync_relay_log_info system variable. You can only change the setting when no replication threads are executing.

  • relay_log_purge

    Property Value
    Command-Line Format --relay-log-purge[={OFF|ON}]
    System Variable relay_log_purge
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Boolean
    Default Value ON

    Disables or enables automatic purging of relay log files as soon as they are not needed any more. The default value is 1 (ON).

  • relay_log_recovery

    Property Value
    Command-Line Format --relay-log-recovery[={OFF|ON}]
    System Variable relay_log_recovery
    Scope Global
    Dynamic No
    SET_VAR Hint Applies No
    Type Boolean
    Default Value OFF

    Enables automatic relay log recovery immediately following server startup. The recovery process creates a new relay log file, initializes the SQL thread position to this new relay log, and initializes the I/O thread to the SQL thread position. Reading of the relay log from the master then continues. This global variable is read-only; its value can be changed by starting the slave with the --relay-log-recovery option, which should be used following a crash on the replication slave to ensure that no possibly corrupted relay logs are processed, and must be used in order to guarantee a crash-safe slave. The default value is 0 (disabled).

    This variable also interacts with relay-log-purge, which controls purging of logs when they are no longer needed. Enabling the --relay-log-recovery option when relay-log-purge is disabled risks reading the relay log from files that were not purged, leading to data inconsistency, and is therefore not crash-safe.

    When relay_log_recovery is enabled and the slave has stopped due to errors encountered while running in multithreaded mode, you can use START SLAVE UNTIL SQL_AFTER_MTS_GAPS to ensure that all gaps are processed before switching back to single-threaded mode or executing a CHANGE MASTER TO statement.

  • relay_log_space_limit

    Property Value
    Command-Line Format --relay-log-space-limit=#
    System Variable relay_log_space_limit
    Scope Global
    Dynamic No
    SET_VAR Hint Applies No
    Type Integer
    Default Value 0
    Minimum Value 0
    Maximum Value (64-bit platforms) 18446744073709551615
    Maximum Value (32-bit platforms) 4294967295

    The maximum amount of space to use for all relay logs.

  • report_host

    Property Value
    Command-Line Format --report-host=host_name
    System Variable report_host
    Scope Global
    Dynamic No
    SET_VAR Hint Applies No
    Type String

    The value of the --report-host option.

  • report_password

    Property Value
    Command-Line Format --report-password=name
    System Variable report_password
    Scope Global
    Dynamic No
    SET_VAR Hint Applies No
    Type String

    The value of the --report-password option. Not the same as the password used for the MySQL replication user account.

  • report_port

    Property Value
    Command-Line Format --report-port=port_num
    System Variable report_port
    Scope Global
    Dynamic No
    SET_VAR Hint Applies No
    Type Integer
    Default Value [slave_port]
    Minimum Value 0
    Maximum Value 65535

    The value of the --report-port option.

  • report_user

    Property Value
    Command-Line Format --report-user=name
    System Variable report_user
    Scope Global
    Dynamic No
    SET_VAR Hint Applies No
    Type String

    The value of the --report-user option. Not the same as the name for the MySQL replication user account.

  • rpl_read_size

    Property Value
    Command-Line Format --rpl-read-size=#
    Introduced 8.0.11
    System Variable rpl_read_size
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Integer
    Default Value 8192
    Minimum Value 8192
    Maximum Value 4294967295

    The rpl_read_size system variable controls the minimum amount of data in bytes that is read from the binary log files and relay log files. If heavy disk I/O activity for these files is impeding performance for the database, increasing the read size might reduce file reads and I/O stalls when the file data is not currently cached by the operating system.

    The minimum and default value for rpl_read_size is 8192 bytes. The value must be a multiple of 4KB. Note that a buffer the size of this value is allocated for each thread that reads from the binary log and relay log files, including dump threads on masters and coordinator threads on slaves. Setting a large value might therefore have an impact on memory consumption for servers.

  • rpl_semi_sync_slave_enabled

    Property Value
    Command-Line Format --rpl-semi-sync-slave-enabled[={OFF|ON}]
    System Variable rpl_semi_sync_slave_enabled
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Boolean
    Default Value OFF

    Controls whether semisynchronous replication is enabled on the slave. To enable or disable the plugin, set this variable to ON or OFF (or 1 or 0), respectively. The default is OFF.

    This variable is available only if the slave-side semisynchronous replication plugin is installed.

  • rpl_semi_sync_slave_trace_level

    Property Value
    Command-Line Format --rpl-semi-sync-slave-trace-level=#
    System Variable rpl_semi_sync_slave_trace_level
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Integer
    Default Value 32

    The semisynchronous replication debug trace level on the slave. See rpl_semi_sync_master_trace_level for the permissible values.

    This variable is available only if the slave-side semisynchronous replication plugin is installed.

  • rpl_stop_slave_timeout

    Property Value
    Command-Line Format --rpl-stop-slave-timeout=seconds
    System Variable rpl_stop_slave_timeout
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Integer
    Default Value 31536000
    Minimum Value 2
    Maximum Value 31536000

    You can control the length of time (in seconds) that STOP SLAVE waits before timing out by setting this variable. This can be used to avoid deadlocks between STOP SLAVE and other slave SQL statements using different client connections to the slave.

    The maximum and default value of rpl_stop_slave_timeout is 31536000 seconds (1 year). The minimum is 2 seconds. Changes to this variable take effect for subsequent STOP SLAVE statements.

    This variable affects only the client that issues a STOP SLAVE statement. When the timeout is reached, the issuing client returns an error message stating that the command execution is incomplete. The client then stops waiting for the slave threads to stop, but the slave threads continue to try to stop, and the STOP SLAVE instruction remains in effect. Once the slave threads are no longer busy, the STOP SLAVE statement is executed and the slave stops.

  • slave_checkpoint_group

    Property Value
    Command-Line Format --slave-checkpoint-group=#
    System Variable slave_checkpoint_group
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Integer
    Default Value 512
    Minimum Value 32
    Maximum Value 524280
    Block Size 8

    Sets the maximum number of transactions that can be processed by a multithreaded slave before a checkpoint operation is called to update its status as shown by SHOW SLAVE STATUS. Setting this variable has no effect on slaves for which multithreading is not enabled. Setting this variable has no immediate effect. The state of the variable applies on all subsequent START SLAVE commands.

    Note

    Multithreaded slaves are not currently supported by NDB Cluster, which silently ignores the setting for this variable. See Section 22.6.3, “Known Issues in NDB Cluster Replication”, for more information.

    This variable works in combination with the slave_checkpoint_period system variable in such a way that, when either limit is exceeded, the checkpoint is executed and the counters tracking both the number of transactions and the time elapsed since the last checkpoint are reset.

    The minimum allowed value for this variable is 32, unless the server was built using -DWITH_DEBUG, in which case the minimum value is 1. The effective value is always a multiple of 8; you can set it to a value that is not such a multiple, but the server rounds it down to the next lower multiple of 8 before storing the value. (Exception: No such rounding is performed by the debug server.) Regardless of how the server was built, the default value is 512, and the maximum allowed value is 524280.

  • slave_checkpoint_period

    Property Value
    Command-Line Format --slave-checkpoint-period=#
    System Variable slave_checkpoint_period
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Integer
    Default Value 300
    Minimum Value 1
    Maximum Value 4G

    Sets the maximum time (in milliseconds) that is allowed to pass before a checkpoint operation is called to update the status of a multithreaded slave as shown by SHOW SLAVE STATUS. Setting this variable has no effect on slaves for which multithreading is not enabled. Setting this variable takes effect for all replication channels immediately, including running channels.

    Note

    Multithreaded slaves are not currently supported by NDB Cluster, which silently ignores the setting for this variable. See Section 22.6.3, “Known Issues in NDB Cluster Replication”, for more information.

    This variable works in combination with the slave_checkpoint_group system variable in such a way that, when either limit is exceeded, the checkpoint is executed and the counters tracking both the number of transactions and the time elapsed since the last checkpoint are reset.

    The minimum allowed value for this variable is 1, unless the server was built using -DWITH_DEBUG, in which case the minimum value is 0. Regardless of how the server was built, the default value is 300, and the maximum possible value is 4294967296 (4GB).

  • slave_compressed_protocol

    Property Value
    Command-Line Format --slave-compressed-protocol[={OFF|ON}]
    Deprecated 8.0.18
    System Variable slave_compressed_protocol
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Boolean
    Default Value OFF

    Whether to use compression of the master/slave protocol if both master and slave support it. If this variable is disabled (the default), connections are uncompressed. Changes to this variable take effect on subsequent connection attempts; this includes after issuing a START SLAVE statement, as well as reconnections made by a running I/O thread (for example, after setting the MASTER_RETRY_COUNT option for the CHANGE MASTER TO statement).

    As of MySQL 8.0.18, if slave_compressed_protocol is enabled, it takes precedence over any MASTER_COMPRESSION_ALGORITHMS option specified for the CHANGE MASTER TO statement. In this case, connections to the master use zlib compression if both the master and slave support that algorithm. If slave_compressed_protocol is disabled, the value of MASTER_COMPRESSION_ALGORITHMS applies. For more information, see Section 4.2.6, “Connection Compression Control”.

    As of MySQL 8.0.18, this system variable is deprecated. It will be removed in a future MySQL version. See Legacy Connection Compression Configuration.

  • slave_exec_mode

    Property Value
    Command-Line Format --slave-exec-mode=mode
    System Variable slave_exec_mode
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Enumeration
    Default Value

    IDEMPOTENT (NDB)

    STRICT (Other)

    Valid Values

    IDEMPOTENT

    STRICT

    Controls how a slave thread resolves conflicts and errors during replication. IDEMPOTENT mode causes suppression of duplicate-key and no-key-found errors; STRICT means no such suppression takes place.

    IDEMPOTENT mode is intended for use in multi-master replication, circular replication, and some other special replication scenarios for NDB Cluster Replication. (See Section 22.6.10, “NDB Cluster Replication: Multi-Master and Circular Replication”, and Section 22.6.11, “NDB Cluster Replication Conflict Resolution”, for more information.) NDB Cluster ignores any value explicitly set for slave_exec_mode, and always treats it as IDEMPOTENT.

    In MySQL Server 8.0, STRICT mode is the default value.

    Setting this variable takes immediate effect for all replication channels, including running channels.

    For storage engines other than NDB, IDEMPOTENT mode should be used only when you are absolutely sure that duplicate-key errors and key-not-found errors can safely be ignored. It is meant to be used in fail-over scenarios for NDB Cluster where multi-master replication or circular replication is employed, and is not recommended for use in other cases.

  • slave_load_tmpdir

    Property Value
    Command-Line Format --slave-load-tmpdir=dir_name
    System Variable slave_load_tmpdir
    Scope Global
    Dynamic No
    SET_VAR Hint Applies No
    Type Directory name
    Default Value Value of --tmpdir

    The name of the directory where the slave creates temporary files. Setting this variable takes effect for all replication channels immediately, including running channels. This system variable is by default equal to the value of the tmpdir system variable, or the default that applies when that system variable is not specified.

    When the slave SQL thread replicates a LOAD DATA statement, it extracts the file to be loaded from the relay log into temporary files, and then loads these into the table. If the file loaded on the master is huge, the temporary files on the slave are huge, too. Therefore, it might be advisable to use this option to tell the slave to put temporary files in a directory located in some file system that has a lot of available space. In that case, the relay logs are huge as well, so you might also want to use the --relay-log option to place the relay logs in that file system.

    The directory specified by this option should be located in a disk-based file system (not a memory-based file system) so that the temporary files used to replicate LOAD DATA can survive machine restarts. The directory also should not be one that is cleared by the operating system during the system startup process. However, replication can now continue after a restart if the temporary files have been removed.

  • slave_max_allowed_packet

    Property Value
    Command-Line Format --slave-max-allowed-packet=#
    System Variable slave_max_allowed_packet
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Integer
    Default Value 1073741824
    Minimum Value 1024
    Maximum Value 1073741824

    This option sets the maximum packet size in bytes that the slave SQL and I/O threads can handle. Setting this variable takes effect for all replication channels immediately, including running channels. It is possible for a replication master to write binary log events longer than its max_allowed_packet setting once the event header is added. The setting for slave_max_allowed_packet must be larger than the max_allowed_packet setting on the master, so that large updates using row-based replication do not cause replication to fail.

    This global variable always has a value that is a positive integer multiple of 1024; if you set it to some value that is not, the value is rounded down to the next highest multiple of 1024 for it is stored or used; setting slave_max_allowed_packet to 0 causes 1024 to be used. (A truncation warning is issued in all such cases.) The default and maximum value is 1073741824 (1 GB); the minimum is 1024.

    slave_max_allowed_packet can also be set at startup, using the --slave-max-allowed-packet option.

  • slave_net_timeout

    Property Value
    Command-Line Format --slave-net-timeout=#
    System Variable slave_net_timeout
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Integer
    Default Value 60
    Minimum Value 1

    The number of seconds to wait for more data or a heartbeat signal from the master before the slave considers the connection broken, aborts the read, and tries to reconnect. Setting this variable has no immediate effect. The state of the variable applies on all subsequent START SLAVE commands.

    The default value is 60 seconds (one minute). The first retry occurs immediately after the timeout. The interval between retries is controlled by the MASTER_CONNECT_RETRY option for the CHANGE MASTER TO statement, and the number of reconnection attempts is limited by the MASTER_RETRY_COUNT option for the CHANGE MASTER TO statement.

    The heartbeat interval, which stops the connection timeout occurring in the absence of data if the connection is still good, is controlled by the MASTER_HEARTBEAT_PERIOD option for the CHANGE MASTER TO statement. The heartbeat interval defaults to half the value of slave_net_timeout, and it is recorded in the master info log and shown in the replication_connection_configuration Performance Schema table. Note that a change to the value or default setting of slave_net_timeout does not automatically change the heartbeat interval, whether that has been set explicitly or is using a previously calculated default. If the connection timeout is changed, you must also issue CHANGE MASTER TO to adjust the heartbeat interval to an appropriate value so that it occurs before the connection timeout.

  • slave_parallel_type=type

    Property Value
    Command-Line Format --slave-parallel-type=type
    Type Enumeration
    Default Value DATABASE
    Valid Values

    DATABASE

    LOGICAL_CLOCK

    When using a multithreaded slave (slave_parallel_workers is greater than 0), this variable specifies the policy used to decide which transactions are allowed to execute in parallel on the slave. The variable has no effect on slaves for which multithreading is not enabled. The possible values are:

    • LOGICAL_CLOCK: Transactions that are part of the same binary log group commit on a master are applied in parallel on a slave. The dependencies between transactions are tracked based on their timestamps to provide additional parallelization where possible. When this value is set, the binlog_transaction_dependency_tracking system variable can be used on the master to specify that write sets are used for parallelization in place of timestamps, if a write set is available for the transaction and gives improved results compared to timestamps.

    • DATABASE: Transactions that update different databases are applied in parallel. This value is only appropriate if data is partitioned into multiple databases which are being updated independently and concurrently on the master. There must be no cross-database constraints, as such constraints may be violated on the slave.

    When slave_preserve_commit_order=1 is set, you can only use LOGICAL_CLOCK.

    If your replication topology uses multiple levels of slaves, LOGICAL_CLOCK may achieve less parallelization for each level the slave is away from the master. You can reduce this effect by using binlog_transaction_dependency_tracking on the master to specify that write sets are used instead of timestamps for parallelization where possible.

  • slave_parallel_workers

    Property Value
    Command-Line Format --slave-parallel-workers=#
    System Variable slave_parallel_workers
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Integer
    Default Value 0
    Minimum Value 0
    Maximum Value 1024

    Enables multithreading on the slave and sets the number of slave applier threads for executing replication transactions in parallel. When the value is a number greater than 0, the slave is a multithreaded slave with the specified number of applier threads, plus a coordinator thread to manage them. If you are using multiple replication channels, each channel has this number of threads.

    Note

    Multithreaded slaves are not currently supported by NDB Cluster, which silently ignores the setting for this variable. See Section 22.6.3, “Known Issues in NDB Cluster Replication”, for more information.

    Retrying of transactions is supported when multithreading is enabled on a slave. When slave_preserve_commit_order=1, transactions on a slave are externalized on the slave in the same order as they appear in the slave's relay log. The way in which transactions are distributed among applier threads is configured by --slave-parallel-type.

    To disable parallel execution, set this option to 0, which gives the slave a single applier thread and no coordinator thread. With this setting, the --slave-parallel-type and slave_preserve_commit_order options have no effect and are ignored.

    Setting slave_parallel_workers has no immediate effect. The state of the variable applies on all subsequent START SLAVE statements.

  • slave_pending_jobs_size_max

    Property Value
    Command-Line Format --slave-pending-jobs-size-max=#
    System Variable slave_pending_jobs_size_max
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Integer
    Default Value (>= 8.0.12) 128M
    Default Value (<= 8.0.11) 16M
    Minimum Value 1024
    Maximum Value 16EiB
    Block Size 1024

    For multithreaded slaves, this variable sets the maximum amount of memory (in bytes) available to slave worker queues holding events not yet applied. Setting this variable has no effect on slaves for which multithreading is not enabled. Setting this variable has no immediate effect. The state of the variable applies on all subsequent START SLAVE commands.

    The minimum possible value for this variable is 1024 bytes; the default is 128MB. The maximum possible value is 18446744073709551615 (16 exbibytes). Values that are not exact multiples of 1024 bytes are rounded down to the next lower multiple of 1024 bytes prior to being stored.

    The value of this variable is a soft limit and can be set to match the normal workload. If an unusually large event exceeds this size, the transaction is held until all the slave workers have empty queues, and then processed. All subsequent transactions are held until the large transaction has been completed.

  • slave_preserve_commit_order

    Property Value
    Command-Line Format --slave-preserve-commit-order[={OFF|ON}]
    System Variable slave_preserve_commit_order
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Boolean
    Default Value OFF

    For multithreaded slaves, the setting 1 for this variable ensures that transactions are externalized on the slave in the same order as they appear in the slave's relay log, and prevents gaps in the sequence of transactions that have been executed from the relay log. This variable has no effect on slaves for which multithreading is not enabled. Note that slave_preserve_commit_order=1 does not preserve the order of non-transactional DML updates, so these might commit before transactions that precede them in the relay log, which might result in gaps.

    slave_preserve_commit_order=1 requires that --log-bin and --log-slave-updates are enabled on the slave, and --slave-parallel-type is set to LOGICAL_CLOCK.

    With slave_preserve_commit_order enabled, the executing thread waits until all previous transactions are committed before committing. While the slave thread is waiting for other workers to commit their transactions it reports its status as Waiting for preceding transaction to commit. With this mode, a multithreaded slave never enters a state that the master was not in. This supports the use of replication for read scale-out. See Section 17.3.5, “Using Replication for Scale-Out”.

    Before changing this variable, all replication threads (for all replication channels if you are using multiple replication channels) must be stopped. If slave_preserve_commit_order=0 is set, the transactions that the slave applies in parallel may commit out of order. Therefore, checking for the most recently executed transaction does not guarantee that all previous transactions from the master have been executed on the slave. There is a chance of gaps in the sequence of transactions that have been executed from the slave's relay log. This has implications for logging and recovery when using a multithreaded slave. Note that the setting slave_preserve_commit_order=1 prevents gaps, but does not prevent master log position lag (where Exec_master_log_pos is behind the position up to which transactions have been executed). See Section 17.4.1.33, “Replication and Transaction Inconsistencies” for more information.

  • slave_rows_search_algorithms

    Property Value
    Command-Line Format --slave-rows-search-algorithms=value
    System Variable slave_rows_search_algorithms
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Set
    Default Value (>= 8.0.2) INDEX_SCAN,HASH_SCAN
    Default Value (<= 8.0.1) TABLE_SCAN,INDEX_SCAN
    Valid Values

    TABLE_SCAN,INDEX_SCAN

    INDEX_SCAN,HASH_SCAN

    TABLE_SCAN,HASH_SCAN

    TABLE_SCAN,INDEX_SCAN,HASH_SCAN (equivalent to INDEX_SCAN,HASH_SCAN)

    When preparing batches of rows for row-based logging and replication, this variable controls how the rows are searched for matches, in particular whether hash scans are used. Setting this variable takes effect for all replication channels immediately, including running channels. The initial setting for the system variable can be specified using the --slave-rows-search-algorithms option.

    Specify a comma-separated list of the following combinations of 2 values from the list INDEX_SCAN, TABLE_SCAN, HASH_SCAN. For the system variable, the value is expected as a string, so the value must be quoted. In addition, the value must not contain any spaces. The recommended combinations (lists) and their effects are shown in the following table:

    Index used / option value INDEX_SCAN,HASH_SCAN INDEX_SCAN,TABLE_SCAN
    Primary key or unique key Index scan Index scan
    (Other) Key Hash scan over index Index scan
    No index Hash scan Table scan
    • The default value is INDEX_SCAN,HASH_SCAN. With this setting, hashing is used for any searches that do not use a primary or unique key. Specifying INDEX_SCAN,HASH_SCAN has the same effect as specifying INDEX_SCAN,TABLE_SCAN,HASH_SCAN, which is allowed.

    • To remove hashing, set INDEX_SCAN,TABLE_SCAN. With this setting, all searches that can use indexes do use them, and searches without any indexes use table scans.

    • Do not use the combination TABLE_SCAN,HASH_SCAN. This setting forces hashing for all searches. It has no advantage over INDEX_SCAN,HASH_SCAN, and it can lead to record not found errors or duplicate key errors in the case of a single event containing multiple updates to the same row, or updates that are order-dependent.

    The order in which the algorithms are specified in the list does not make any difference in the order in which they are displayed by a SELECT or SHOW VARIABLES statement.

    It is possible to specify a single value, but this is not optimal, because setting a single value limits searches to using only that algorithm. In particular, setting INDEX_SCAN alone is not recommended, as in that case searches are unable to find rows at all if no index is present.

  • slave_skip_errors

    Property Value
    Command-Line Format --slave-skip-errors=name
    System Variable slave_skip_errors
    Scope Global
    Dynamic No
    SET_VAR Hint Applies No
    Type String
    Default Value OFF
    Valid Values

    OFF

    [list of error codes]

    all

    ddl_exist_errors

    Normally, replication stops when an error occurs on the slave, which gives you the opportunity to resolve the inconsistency in the data manually. This variable causes the slave SQL thread to continue replication when a statement returns any of the errors listed in the variable value.

  • slave_sql_verify_checksum

    Property Value
    Command-Line Format --slave-sql-verify-checksum[={OFF|ON}]
    System Variable slave_sql_verify_checksum
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Boolean
    Default Value ON

    Cause the slave SQL thread to verify data using the checksums read from the relay log. In the event of a mismatch, the slave stops with an error. Setting this variable takes effect for all replication channels immediately, including running channels.

    Note

    The slave I/O thread always reads checksums if possible when accepting events from over the network.

  • slave_transaction_retries

    Property Value
    Command-Line Format --slave-transaction-retries=#
    System Variable slave_transaction_retries
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Integer
    Default Value 10
    Minimum Value 0
    Maximum Value (64-bit platforms) 18446744073709551615
    Maximum Value (32-bit platforms) 4294967295

    Sets the maximum number of times for replication slave SQL threads on a single-threaded or multithreaded slave to automatically retry failed transactions before stopping. Setting this variable takes effect for all replication channels immediately, including running channels. The default value is 10. Setting the variable to 0 disables automatic retrying of transactions.

    If a replication slave SQL thread fails to execute a transaction because of an InnoDB deadlock or because the transaction's execution time exceeded InnoDB's innodb_lock_wait_timeout or NDB's TransactionDeadlockDetectionTimeout or TransactionInactiveTimeout, it automatically retries slave_transaction_retries times before stopping with an error. Transactions with a non-temporary error are not retried.

    The Performance Schema table replication_applier_status shows the number of retries that took place on each replication channel, in the COUNT_TRANSACTIONS_RETRIES column. The Performance Schema table replication_applier_status_by_worker shows detailed information on transaction retries by individual applier threads on a single-threaded or multithreaded replication slave, and identifies the errors that caused the last transaction and the transaction currently in progress to be reattempted.

  • slave_type_conversions

    Property Value
    Command-Line Format --slave-type-conversions=set
    System Variable slave_type_conversions
    Scope Global
    Dynamic No
    SET_VAR Hint Applies No
    Type Set
    Default Value
    Valid Values

    ALL_LOSSY

    ALL_NON_LOSSY

    ALL_SIGNED

    ALL_UNSIGNED

    Controls the type conversion mode in effect on the slave when using row-based replication. Its value is a comma-delimited set of zero or more elements from the list: ALL_LOSSY, ALL_NON_LOSSY, ALL_SIGNED, ALL_UNSIGNED. Set this variable to an empty string to disallow type conversions between the master and the slave. Setting this variable takes effect for all replication channels immediately, including running channels.

    For additional information on type conversion modes applicable to attribute promotion and demotion in row-based replication, see Row-based replication: attribute promotion and demotion.

  • sql_slave_skip_counter

    Property Value
    System Variable sql_slave_skip_counter
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Integer

    The number of events from the master that a slave server should skip. Setting the option has no immediate effect. The variable applies to the next START SLAVE statement; the next START SLAVE statement also changes the value back to 0. When this variable is set to a nonzero value and there are multiple replication channels configured, the START SLAVE statement can only be used with the FOR CHANNEL channel clause.

    This option is incompatible with GTID-based replication, and must not be set to a nonzero value when --gtid-mode=ON. If you need to skip transactions when employing GTIDs, use gtid_executed from the master instead. See Injecting empty transactions, for information about how to do this.

    Important

    If skipping the number of events specified by setting this variable would cause the slave to begin in the middle of an event group, the slave continues to skip until it finds the beginning of the next event group and begins from that point. For more information, see Section 13.4.2.5, “SET GLOBAL sql_slave_skip_counter Syntax”.

  • sync_master_info

    Property Value
    Command-Line Format --sync-master-info=#
    System Variable sync_master_info
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Integer
    Default Value 10000
    Minimum Value 0
    Maximum Value (64-bit platforms) 18446744073709551615
    Maximum Value (32-bit platforms) 4294967295

    The effects of this variable on a replication slave depend on whether the slave's master_info_repository is set to FILE or TABLE, as explained in the following paragraphs.

    master_info_repository = FILE.  If the value of sync_master_info is greater than 0, the slave synchronizes its master.info file to disk (using fdatasync()) after every sync_master_info events. If it is 0, the MySQL server performs no synchronization of the master.info file to disk; instead, the server relies on the operating system to flush its contents periodically as with any other file.

    master_info_repository = TABLE.  If the value of sync_master_info is greater than 0, the slave updates its master info repository table after every sync_master_info events. If it is 0, the table is never updated.

    The default value for sync_master_info is 10000. Setting this variable takes effect for all replication channels immediately, including running channels.

  • sync_relay_log

    Property Value
    Command-Line Format --sync-relay-log=#
    System Variable sync_relay_log
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Integer
    Default Value 10000
    Minimum Value 0
    Maximum Value (64-bit platforms) 18446744073709551615
    Maximum Value (32-bit platforms) 4294967295

    If the value of this variable is greater than 0, the MySQL server synchronizes its relay log to disk (using fdatasync()) after every sync_relay_log events are written to the relay log. Setting this variable takes effect for all replication channels immediately, including running channels.

    Setting sync_relay_log to 0 causes no synchronization to be done to disk; in this case, the server relies on the operating system to flush the relay log's contents from time to time as for any other file.

    A value of 1 is the safest choice because in the event of a crash you lose at most one event from the relay log. However, it is also the slowest choice (unless the disk has a battery-backed cache, which makes synchronization very fast).

  • sync_relay_log_info

    Property Value
    Command-Line Format --sync-relay-log-info=#
    System Variable sync_relay_log_info
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Integer
    Default Value 10000
    Minimum Value 0
    Maximum Value (64-bit platforms) 18446744073709551615
    Maximum Value (32-bit platforms) 4294967295

    The default value for sync_relay_log_info is 10000. Setting this variable takes effect for all replication channels immediately, including running channels.

    The effects of this variable on the replication slave depend on the server's relay_log_info_repository setting (FILE or TABLE). If the setting is TABLE, the effects of the variable also depend on whether the storage engine used by the relay log info table is transactional (such as InnoDB) or not transactional (MyISAM). The effects of these factors on the behavior of the server for sync_relay_log_info values of zero and greater than zero are as follows:

    sync_relay_log_info = 0
    • If relay_log_info_repository is set to FILE, the MySQL server performs no synchronization of the relay-log.info file to disk; instead, the server relies on the operating system to flush its contents periodically as with any other file.

    • If relay_log_info_repository is set to TABLE, and the storage engine for that table is transactional, the table is updated after each transaction. (The sync_relay_log_info setting is effectively ignored in this case.)

    • If relay_log_info_repository is set to TABLE, and the storage engine for that table is not transactional, the table is never updated.

    sync_relay_log_info = N > 0
    • If relay_log_info_repository is set to FILE, the slave synchronizes its relay-log.info file to disk (using fdatasync()) after every N transactions.

    • If relay_log_info_repository is set to TABLE, and the storage engine for that table is transactional, the table is updated after each transaction. (The sync_relay_log_info setting is effectively ignored in this case.)

    • If relay_log_info_repository is set to TABLE, and the storage engine for that table is not transactional, the table is updated after every N events.

17.1.6.4 Binary Logging Options and Variables

You can use the mysqld options and system variables that are described in this section to affect the operation of the binary log as well as to control which statements are written to the binary log. For additional information about the binary log, see Section 5.4.4, “The Binary Log”. For additional information about using MySQL server options and system variables, see Section 5.1.7, “Server Command Options”, and Section 5.1.8, “Server System Variables”.

Startup Options Used with Binary Logging

The following list describes startup options for enabling and configuring the binary log. System variables used with binary logging are discussed later in this section.

  • --binlog-row-event-max-size=N

    Property Value
    Command-Line Format --binlog-row-event-max-size=#
    System Variable (>= 8.0.14) binlog_row_event_max_size
    Scope (>= 8.0.14) Global
    Dynamic (>= 8.0.14) No
    SET_VAR Hint Applies (>= 8.0.14) No
    Type Integer
    Default Value 8192
    Minimum Value 256
    Maximum Value (64-bit platforms) 18446744073709551615
    Maximum Value (32-bit platforms) 4294967295

    When row-based binary logging is used, this setting is a soft limit on the maximum size of a row-based binary log event, in bytes. Where possible, rows stored in the binary log are grouped into events with a size not exceeding the value of this setting. If an event cannot be split, the maximum size can be exceeded. The value must be (or else gets rounded down to) a multiple of 256. The default is 8192 bytes.

  • --log-bin[=base_name]

    Property Value
    Command-Line Format --log-bin=file_name
    Type File name

    Specifies the base name to use for binary log files. With binary logging enabled, the server logs all statements that change data to the binary log, which is used for backup and replication. The binary log is a sequence of files with a base name and numeric extension. The --log-bin option value is the base name for the log sequence. The server creates binary log files in sequence by adding a numeric suffix to the base name.

    If you do not supply the --log-bin option, MySQL uses binlog as the default base name for the binary log files. For compatibility with earlier releases, if you supply the --log-bin option with no string or with an empty string, the base name defaults to host_name-bin, using the name of the host machine.

    The default location for binary log files is the data directory. You can use the --log-bin option to specify an alternative location, by adding a leading absolute path name to the base name to specify a different directory. When the server reads an entry from the binary log index file, which tracks the binary log files that have been used, it checks whether the entry contains a relative path. If it does, the relative part of the path is replaced with the absolute path set using the --log-bin option. An absolute path recorded in the binary log index file remains unchanged; in such a case, the index file must be edited manually to enable a new path or paths to be used. The binary log file base name and any specified path are available as the log_bin_basename system variable.

    In earlier MySQL versions, binary logging was disabled by default, and was enabled if you specified the --log-bin option. From MySQL 8.0, binary logging is enabled by default, whether or not you specify the --log-bin option. The exception is if you use mysqld to initialize the data directory manually by invoking it with the --initialize or --initialize-insecure option, when binary logging is disabled by default. It is possible to enable binary logging in this case by specifying the --log-bin option. When binary logging is enabled, the log_bin system variable, which shows the status of binary logging on the server, is set to ON.

    To disable binary logging, you can specify the --skip-log-bin or --disable-log-bin option at startup. If either of these options is specified and --log-bin is also specified, the option specified later takes precedence. When binary logging is disabled, the log_bin system variable is set to OFF.

    When GTIDs are in use on the server, if you disable binary logging when restarting the server after an abnormal shutdown, some GTIDs are likely to be lost, causing replication to fail. In a normal shutdown, the set of GTIDs from the current binary log file is saved in the mysql.gtid_executed table. Following an abnormal shutdown where this did not happen, during recovery the GTIDs are added to the table from the binary log file, provided that binary logging is still enabled. If binary logging is disabled for the server restart, the server cannot access the binary log file to recover the GTIDs, so replication cannot be started. Binary logging can be disabled safely after a normal shutdown.

    The --log-slave-updates and --slave-preserve-commit-order options require binary logging. If you disable binary logging, either omit these options, or specify --skip-log-slave-updates and --skip-slave-preserve-commit-order. MySQL disables these options by default when --skip-log-bin or --disable-log-bin is specified. If you specify --log-slave-updates or --slave-preserve-commit-order together with --skip-log-bin or --disable-log-bin, a warning or error message is issued.

    In MySQL 5.7, a server ID had to be specified when binary logging was enabled, or the server would not start. In MySQL 8.0, the server_id system variable is set to 1 by default. The server can now be started with this default server ID when binary logging is enabled, but an informational message is issued if you do not specify a server ID explicitly using the --server-id option. For servers that are used in a replication topology, you must specify a unique nonzero server ID for each server.

    For information on the format and management of the binary log, see Section 5.4.4, “The Binary Log”.

  • --log-bin-index[=file_name]

    Property Value
    Command-Line Format --log-bin-index=file_name
    System Variable log_bin_index
    Scope Global
    Dynamic No
    SET_VAR Hint Applies No
    Type File name

    The name for the binary log index file, which contains the names of the binary log files. By default, it has the same location and base name as the value specified for the binary log files using the --log-bin option, plus the extension .index. If you do not specify --log-bin, the default binary log index file name is binlog.index. If you specify --log-bin option with no string or an empty string, the default binary log index file name is host_name-bin.index, using the name of the host machine.

    For information on the format and management of the binary log, see Section 5.4.4, “The Binary Log”.

  • --log-bin-trust-function-creators[={0|1}]

    Property Value
    Command-Line Format --log-bin-trust-function-creators[={OFF|ON}]
    System Variable log_bin_trust_function_creators
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Boolean
    Default Value OFF

    This option sets the corresponding log_bin_trust_function_creators system variable. If no argument is given, the option sets the variable to 1. log_bin_trust_function_creators affects how MySQL enforces restrictions on stored function and trigger creation. See Section 24.7, “Stored Program Binary Logging”.

  • --log-bin-use-v1-row-events[={0|1}]

    Property Value
    Command-Line Format --log-bin-use-v1-row-events[={OFF|ON}]
    System Variable log_bin_use_v1_row_events
    Scope Global
    Dynamic No
    SET_VAR Hint Applies No
    Type Boolean
    Default Value OFF

    MySQL 8.0 uses Version 2 binary log row events, which cannot be read by MySQL Server releases prior to MySQL 5.6.6. Setting this option to 1 causes mysqld to write the binary log using Version 1 logging events, which is the only version of binary log events used in those releases, and thus produce binary logs that can be read by slaves at those releases. Setting --log-bin-use-v1-row-events to 0 (the default) causes mysqld to use Version 2 binary log events.

    The value used for this option can be obtained from the read-only log_bin_use_v1_row_events system variable.

    --log-bin-use-v1-row-events is chiefly of interest when setting up replication conflict detection and resolution using NDB$EPOCH_TRANS() as the conflict detection function, which requires Version 2 binary log row events. Thus, this option and --ndb-log-transaction-id are not compatible.

    For more information, see Section 22.6.11, “NDB Cluster Replication Conflict Resolution”.

Statement selection options.  The options in the following list affect which statements are written to the binary log, and thus sent by a replication master server to its slaves. There are also options for slave servers that control which statements received from the master should be executed or ignored. For details, see Section 17.1.6.3, “Replication Slave Options and Variables”.

  • --binlog-do-db=db_name

    Property Value
    Command-Line Format --binlog-do-db=name
    Type String

    This option affects binary logging in a manner similar to the way that --replicate-do-db affects replication.

    The effects of this option depend on whether the statement-based or row-based logging format is in use, in the same way that the effects of --replicate-do-db depend on whether statement-based or row-based replication is in use. You should keep in mind that the format used to log a given statement may not necessarily be the same as that indicated by the value of binlog_format. For example, DDL statements such as CREATE TABLE and ALTER TABLE are always logged as statements, without regard to the logging format in effect, so the following statement-based rules for --binlog-do-db always apply in determining whether or not the statement is logged.

    Statement-based logging.  Only those statements are written to the binary log where the default database (that is, the one selected by USE) is db_name. To specify more than one database, use this option multiple times, once for each database; however, doing so does not cause cross-database statements such as UPDATE some_db.some_table SET foo='bar' to be logged while a different database (or no database) is selected.

    Warning

    To specify multiple databases you must use multiple instances of this option. Because database names can contain commas, the list will be treated as the name of a single database if you supply a comma-separated list.

    An example of what does not work as you might expect when using statement-based logging: If the server is started with --binlog-do-db=sales and you issue the following statements, the UPDATE statement is not logged:

    USE prices;
    UPDATE sales.january SET amount=amount+1000;
    

    The main reason for this just check the default database behavior is that it is difficult from the statement alone to know whether it should be replicated (for example, if you are using multiple-table DELETE statements or multiple-table UPDATE statements that act across multiple databases). It is also faster to check only the default database rather than all databases if there is no need.

    Another case which may not be self-evident occurs when a given database is replicated even though it was not specified when setting the option. If the server is started with --binlog-do-db=sales, the following UPDATE statement is logged even though prices was not included when setting --binlog-do-db:

            
    USE sales;
    UPDATE prices.discounts SET percentage = percentage + 10;
    

    Because sales is the default database when the UPDATE statement is issued, the UPDATE is logged.

    Row-based logging.  Logging is restricted to database db_name. Only changes to tables belonging to db_name are logged; the default database has no effect on this. Suppose that the server is started with --binlog-do-db=sales and row-based logging is in effect, and then the following statements are executed:

    USE prices;
    UPDATE sales.february SET amount=amount+100;
    

    The changes to the february table in the sales database are logged in accordance with the UPDATE statement; this occurs whether or not the USE statement was issued. However, when using the row-based logging format and --binlog-do-db=sales, changes made by the following UPDATE are not logged:

    USE prices;
    UPDATE prices.march SET amount=amount-25;
    

    Even if the USE prices statement were changed to USE sales, the UPDATE statement's effects would still not be written to the binary log.

    Another important difference in --binlog-do-db handling for statement-based logging as opposed to the row-based logging occurs with regard to statements that refer to multiple databases. Suppose that the server is started with --binlog-do-db=db1, and the following statements are executed:

    USE db1;
    UPDATE db1.table1 SET col1 = 10, db2.table2 SET col2 = 20;
    

    If you are using statement-based logging, the updates to both tables are written to the binary log. However, when using the row-based format, only the changes to table1 are logged; table2 is in a different database, so it is not changed by the UPDATE. Now suppose that, instead of the USE db1 statement, a USE db4 statement had been used:

    USE db4;
    UPDATE db1.table1 SET col1 = 10, db2.table2 SET col2 = 20;
    

    In this case, the UPDATE statement is not written to the binary log when using statement-based logging. However, when using row-based logging, the change to table1 is logged, but not that to table2—in other words, only changes to tables in the database named by --binlog-do-db are logged, and the choice of default database has no effect on this behavior.

  • --binlog-ignore-db=db_name

    Property Value
    Command-Line Format --binlog-ignore-db=name
    Type String

    This option affects binary logging in a manner similar to the way that --replicate-ignore-db affects replication.

    The effects of this option depend on whether the statement-based or row-based logging format is in use, in the same way that the effects of --replicate-ignore-db depend on whether statement-based or row-based replication is in use. You should keep in mind that the format used to log a given statement may not necessarily be the same as that indicated by the value of binlog_format. For example, DDL statements such as CREATE TABLE and ALTER TABLE are always logged as statements, without regard to the logging format in effect, so the following statement-based rules for --binlog-ignore-db always apply in determining whether or not the statement is logged.

    Statement-based logging.  Tells the server to not log any statement where the default database (that is, the one selected by USE) is db_name.

    When there is no default database, no --binlog-ignore-db options are applied, and such statements are always logged. (Bug #11829838, Bug #60188)

    Row-based format.  Tells the server not to log updates to any tables in the database db_name. The current database has no effect.

    When using statement-based logging, the following example does not work as you might expect. Suppose that the server is started with --binlog-ignore-db=sales and you issue the following statements:

    USE prices;
    UPDATE sales.january SET amount=amount+1000;
    

    The UPDATE statement is logged in such a case because --binlog-ignore-db applies only to the default database (determined by the USE statement). Because the sales database was specified explicitly in the statement, the statement has not been filtered. However, when using row-based logging, the UPDATE statement's effects are not written to the binary log, which means that no changes to the sales.january table are logged; in this instance, --binlog-ignore-db=sales causes all changes made to tables in the master's copy of the sales database to be ignored for purposes of binary logging.

    To specify more than one database to ignore, use this option multiple times, once for each database. Because database names can contain commas, the list will be treated as the name of a single database if you supply a comma-separated list.

    You should not use this option if you are using cross-database updates and you do not want these updates to be logged.

Checksum options.  MySQL supports reading and writing of binary log checksums. These are enabled using the two options listed here:

  • --binlog-checksum={NONE|CRC32}

    Property Value
    Command-Line Format --binlog-checksum=type
    Type String
    Default Value CRC32
    Valid Values

    NONE

    CRC32

    Enabling this option causes the master to write checksums for events written to the binary log. Set to NONE to disable, or the name of the algorithm to be used for generating checksums; currently, only CRC32 checksums are supported, and CRC32 is the default. You cannot change the setting for this option within a transaction.

  • --master-verify-checksum={0|1}

    Property Value
    Command-Line Format --master-verify-checksum[={OFF|ON}]
    Type Boolean
    Default Value OFF

    Enabling this option causes the master to verify events from the binary log using checksums, and to stop with an error in the event of a mismatch. Disabled by default.

To control reading of checksums by the slave (from the relay) log, use the --slave-sql-verify-checksum option.

Testing and debugging options.  The following binary log options are used in replication testing and debugging. They are not intended for use in normal operations.

  • --max-binlog-dump-events=N

    Property Value
    Command-Line Format --max-binlog-dump-events=#
    Type Integer
    Default Value 0

    This option is used internally by the MySQL test suite for replication testing and debugging.

  • --sporadic-binlog-dump-fail

    Property Value
    Command-Line Format --sporadic-binlog-dump-fail[={OFF|ON}]
    Type Boolean
    Default Value OFF

    This option is used internally by the MySQL test suite for replication testing and debugging.

System Variables Used with Binary Logging

The following list describes system variables for controlling binary logging. They can be set at server startup and some of them can be changed at runtime using SET. Server options used to control binary logging are listed earlier in this section.

  • binlog_cache_size

    Property Value
    Command-Line Format --binlog-cache-size=#
    System Variable binlog_cache_size
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Integer
    Default Value 32768
    Minimum Value 4096
    Maximum Value (64-bit platforms) 18446744073709551615
    Maximum Value (32-bit platforms) 4294967295

    The size of the memory buffer to hold changes to the binary log during a transaction. When binary logging is enabled on the server (with the log_bin system variable set to ON), a binary log cache is allocated for each client if the server supports any transactional storage engines. If the data for the transaction exceeds the space in the memory buffer, the excess data is stored in a temporary file. When binary log encryption is active on the server, the memory buffer is not encrypted, but (from MySQL 8.0.17) any temporary file used to hold the binary log cache is encrypted. After each transaction is committed, the binary log cache is reset by clearing the memory buffer and truncating the temporary file if used.

    If you often use large transactions, you can increase this cache size to get better performance by reducing or eliminating the need to write to temporary files. The Binlog_cache_use and Binlog_cache_disk_use status variables can be useful for tuning the size of this variable. See Section 5.4.4, “The Binary Log”.

    binlog_cache_size sets the size for the transaction cache only; the size of the statement cache is governed by the binlog_stmt_cache_size system variable.

  • binlog_checksum

    Property Value
    Command-Line Format --binlog-checksum=name
    System Variable binlog_checksum
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type String
    Default Value CRC32
    Valid Values

    NONE

    CRC32

    When enabled, this variable causes the master to write a checksum for each event in the binary log. binlog_checksum supports the values NONE (disabled) and CRC32. The default is CRC32. You cannot change the value of binlog_checksum within a transaction.

    When binlog_checksum is disabled (value NONE), the server verifies that it is writing only complete events to the binary log by writing and checking the event length (rather than a checksum) for each event.

    Changing the value of this variable causes the binary log to be rotated; checksums are always written to an entire binary log file, and never to only part of one.

    Setting this variable on the master to a value unrecognized by the slave causes the slave to set its own binlog_checksum value to NONE, and to stop replication with an error. (Bug #13553750, Bug #61096) If backward compatibility with older slaves is a concern, you may want to set the value explicitly to NONE.

  • binlog_direct_non_transactional_updates

    Property Value
    Command-Line Format --binlog-direct-non-transactional-updates[={OFF|ON}]
    System Variable binlog_direct_non_transactional_updates
    Scope Global, Session
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Boolean
    Default Value OFF

    Due to concurrency issues, a slave can become inconsistent when a transaction contains updates to both transactional and nontransactional tables. MySQL tries to preserve causality among these statements by writing nontransactional statements to the transaction cache, which is flushed upon commit. However, problems arise when modifications done to nontransactional tables on behalf of a transaction become immediately visible to other connections because these changes may not be written immediately into the binary log.

    The binlog_direct_non_transactional_updates variable offers one possible workaround to this issue. By default, this variable is disabled. Enabling binlog_direct_non_transactional_updates causes updates to nontransactional tables to be written directly to the binary log, rather than to the transaction cache.

    As of MySQL 8.0.14, setting the session value of this system variable is a restricted operation. The session user must have privileges sufficient to set restricted session variables. See Section 5.1.9.1, “System Variable Privileges”.

    binlog_direct_non_transactional_updates works only for statements that are replicated using the statement-based binary logging format; that is, it works only when the value of binlog_format is STATEMENT, or when binlog_format is MIXED and a given statement is being replicated using the statement-based format. This variable has no effect when the binary log format is ROW, or when binlog_format is set to MIXED and a given statement is replicated using the row-based format.

    Important

    Before enabling this variable, you must make certain that there are no dependencies between transactional and nontransactional tables; an example of such a dependency would be the statement INSERT INTO myisam_table SELECT * FROM innodb_table. Otherwise, such statements are likely to cause the slave to diverge from the master.

    This variable has no effect when the binary log format is ROW or MIXED.

  • binlog_encryption

    Property Value
    Command-Line Format --binlog-encryption[={OFF|ON}]
    Introduced 8.0.14
    System Variable binlog_encryption
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Boolean
    Default Value OFF

    Enables encryption for binary log files and relay log files on this server. OFF is the default. ON sets encryption on for binary log files and relay log files. Binary logging does not need to be enabled on the server to enable encryption, so you can encrypt the relay log files on a slave that has no binary log. To use encryption, a keyring plugin must be installed and configured to supply MySQL Server's keyring service. For instructions to do this, see Section 6.4.4, “The MySQL Keyring”. Any supported keyring plugin can be used to store binary log encryption keys.

    When you first start the server with binary log encryption enabled, a new binary log encryption key is generated before the binary log and relay logs are initialized. This key is used to encrypt a file password for each binary log file (if the server has binary logging enabled) and relay log file (if the server has replication channels), and further keys generated from the file passwords are used to encrypt the data in the files. Relay log files are encrypted for all channels, including Group Replication applier channels and new channels that are created after encryption is activated. The binary log index file and relay log index file are never encrypted.

    If you activate encryption while the server is running, a new binary log encryption key is generated at that time. The exception is if encryption was active previously on the server and was then disabled, in which case the binary log encryption key that was in use before is used again. The binary log file and relay log files are rotated immediately, and file passwords for the new files and all subsequent binary log files and relay log files are encrypted using this binary log encryption key. Existing binary log files and relay log files still present on the server are not automatically encrypted, but you can purge them if they are no longer needed.

    If you deactivate encryption by changing the binlog_encryption system variable to OFF, the binary log file and relay log files are rotated immediately and all subsequent logging is unencrypted. Previously encrypted files are not automatically decrypted, but the server is still able to read them. SUPER privileges or the BINLOG_ENCRYPTION_ADMIN privilege are required to activate or deactivate encryption while the server is running. Group Replication applier channels are not included in the relay log rotation request, so unencrypted logging for these channels does not start until their logs are rotated in normal use.

    For more information on binary log file and relay log file encryption, see Section 17.3.10, “Encrypting Binary Log Files and Relay Log Files”.

  • binlog_error_action

    Property Value
    Command-Line Format --binlog-error-action[=value]
    System Variable binlog_error_action
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Enumeration
    Default Value ABORT_SERVER
    Valid Values

    IGNORE_ERROR

    ABORT_SERVER

    Controls what happens when the server encounters an error such as not being able to write to, flush or synchronize the binary log, which can cause the master's binary log to become inconsistent and replication slaves to lose synchronization.

    This variable defaults to ABORT_SERVER, which makes the server halt logging and shut down whenever it encounters such an error with the binary log. On restart, recovery proceeds as in the case of an unexpected server halt (see Section 17.3.2, “Handling an Unexpected Halt of a Replication Slave”).

    When binlog_error_action is set to IGNORE_ERROR, if the server encounters such an error it continues the ongoing transaction, logs the error then halts logging, and continues performing updates. To resume binary logging log_bin must be enabled again, which requires a server restart. This setting provides backward compatibility with older versions of MySQL.

  • binlog_expire_logs_seconds

    Property Value
    Command-Line Format --binlog-expire-logs-seconds=#
    Introduced 8.0.1
    System Variable binlog_expire_logs_seconds
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Integer
    Default Value (>= 8.0.11) 2592000
    Default Value (<= 8.0.4) 0
    Minimum Value 0
    Maximum Value 4294967295

    Sets the binary log expiration period in seconds. After their expiration period ends, binary log files can be automatically removed. Possible removals happen at startup and when the binary log is flushed. Log flushing occurs as indicated in Section 5.4, “MySQL Server Logs”.

    The default binary log expiration period is 2592000 seconds, which equals 30 days (30*24*60*60 seconds). The default applies if neither binlog_expire_logs_seconds nor the deprecated system variable expire_logs_days has a value set at startup. If a non-zero value for one of the variables binlog_expire_logs_seconds or expire_logs_days is set at startup, this value is used as the binary log expiration period. If a non-zero value for both of those variables is set at startup, the value for binlog_expire_logs_seconds is used as the binary log expiration period, and the value for expire_logs_days is ignored with a warning message.

    To disable automatic purging of the binary log, specify a value of 0 explicitly for binlog_expire_logs_seconds, and do not specify a value for expire_logs_days. For compatibility with earlier releases, automatic purging is also disabled if you specify a value of 0 explicitly for expire_logs_days and do not specify a value for binlog_expire_logs_seconds. In that case, the default for binlog_expire_logs_seconds is not applied.

    To remove binary log files manually, use the PURGE BINARY LOGS statement. See Section 13.4.1.1, “PURGE BINARY LOGS Syntax”.

  • binlog_format

    Property Value
    Command-Line Format --binlog-format=format
    System Variable binlog_format
    Scope Global, Session
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Enumeration
    Default Value ROW
    Valid Values

    ROW

    STATEMENT

    MIXED

    This variable sets the binary logging format, and can be any one of STATEMENT, ROW, or MIXED. See Section 17.2.1, “Replication Formats”.

    binlog_format can be set at startup or at runtime, except that under some conditions, changing this variable at runtime is not possible or causes replication to fail, as described later.

    The default is ROW. Exception: In NDB Cluster, the default is MIXED; statement-based replication is not supported for NDB Cluster.

    Setting the session value of this system variable is a restricted operation. The session user must have privileges sufficient to set restricted session variables. See Section 5.1.9.1, “System Variable Privileges”.

    The rules governing when changes to this variable take effect and how long the effect lasts are the same as for other MySQL server system variables. For more information, see Section 13.7.5.1, “SET Syntax for Variable Assignment”.

    When MIXED is specified, statement-based replication is used, except for cases where only row-based replication is guaranteed to lead to proper results. For example, this happens when statements contain user-defined functions (UDF) or the UUID() function.

    For details of how stored programs (stored procedures and functions, triggers, and events) are handled when each binary logging format is set, see Section 24.7, “Stored Program Binary Logging”.

    There are exceptions when you cannot switch the replication format at runtime:

    • The replication format cannot be changed from within a stored function or a trigger.

    • If a session has open temporary tables, the replication format cannot be changed for the session (SET @@SESSION.binlog_format).

    • If any replication channel has open temporary tables, the replication format cannot be changed globally (SET @@GLOBAL.binlog_format or SET @@PERSIST.binlog_format).

    • If any replication channel applier thread is currently running, the replication format cannot be changed globally (SET @@GLOBAL.binlog_format or SET @@PERSIST.binlog_format).

    Trying to switch the replication format in any of these cases (or attempting to set the current replication format) results in an error. You can, however, use PERSIST_ONLY (SET @@PERSIST_ONLY.binlog_format) to change the replication format at any time, because this action does not modify the runtime global system variable value, and takes effect only after a server restart.

    Switching the replication format at runtime is not recommended when any temporary tables exist, because temporary tables are logged only when using statement-based replication, whereas with row-based replication and mixed replication, they are not logged.

    Changing the logging format on a replication master does not cause a replication slave to change its logging format to match. Switching the replication format while replication is ongoing can cause issues if a replication slave has binary logging enabled, and the change results in the slave using STATEMENT format logging while the master is using ROW or MIXED format logging. A replication slave is not able to convert binary log entries received in ROW logging format to STATEMENT format for use in its own binary log, so this situation can cause replication to fail. For more information, see Section 5.4.4.2, “Setting The Binary Log Format”.

    The binary log format affects the behavior of the following server options:

    These effects are discussed in detail in the descriptions of the individual options.

  • binlog_group_commit_sync_delay

    Property Value
    Command-Line Format --binlog-group-commit-sync-delay=#
    System Variable binlog_group_commit_sync_delay
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Integer
    Default Value 0
    Minimum Value 0
    Maximum Value 1000000

    Controls how many microseconds the binary log commit waits before synchronizing the binary log file to disk. By default binlog_group_commit_sync_delay is set to 0, meaning that there is no delay. Setting binlog_group_commit_sync_delay to a microsecond delay enables more transactions to be synchronized together to disk at once, reducing the overall time to commit a group of transactions because the larger groups require fewer time units per group.

    When sync_binlog=0 or sync_binlog=1 is set, the delay specified by binlog_group_commit_sync_delay is applied for every binary log commit group before synchronization (or in the case of sync_binlog=0, before proceeding). When sync_binlog is set to a value n greater than 1, the delay is applied after every n binary log commit groups.

    Setting binlog_group_commit_sync_delay can increase the number of parallel committing transactions on any server that has (or might have after a failover) a replication slave, and therefore can increase parallel execution on the replication slaves. To benefit from this effect, the slave servers must have slave_parallel_type=LOGICAL_CLOCK set, and the effect is more significant when binlog_transaction_dependency_tracking=COMMIT_ORDER is also set. It is important to take into account both the master's throughput and the slaves' throughput when you are tuning the setting for binlog_group_commit_sync_delay.

    Setting binlog_group_commit_sync_delay can also reduce the number of fsync() calls to the binary log on any server (master or slave) that has a binary log.

    Note that setting binlog_group_commit_sync_delay increases the latency of transactions on the server, which might affect client applications. Also, on highly concurrent workloads, it is possible for the delay to increase contention and therefore reduce throughput. Typically, the benefits of setting a delay outweigh the drawbacks, but tuning should always be carried out to determine the optimal setting.

  • binlog_group_commit_sync_no_delay_count

    Property Value
    Command-Line Format --binlog-group-commit-sync-no-delay-count=#
    System Variable binlog_group_commit_sync_no_delay_count
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Integer
    Default Value 0
    Minimum Value 0
    Maximum Value 1000000

    The maximum number of transactions to wait for before aborting the current delay as specified by binlog_group_commit_sync_delay. If binlog_group_commit_sync_delay is set to 0, then this option has no effect.

  • binlog_max_flush_queue_time

    Property Value
    Command-Line Format --binlog-max-flush-queue-time=#
    Deprecated Yes
    System Variable binlog_max_flush_queue_time
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Integer
    Default Value 0
    Minimum Value 0
    Maximum Value 100000

    binlog_max_flush_queue_time is deprecated, and is marked for eventual removal in a future MySQL release. Formerly, this system variable controlled the time in microseconds to continue reading transactions from the flush queue before proceeding with group commit. It no longer has any effect.

  • binlog_order_commits

    Property Value
    Command-Line Format --binlog-order-commits[={OFF|ON}]
    System Variable binlog_order_commits
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Boolean
    Default Value ON

    When this variable is enabled on a replication master (which is the default), transaction commit instructions issued to storage engines are serialized on a single thread, so that transactions are always committed in the same order as they are written to the binary log. Disabling this variable permits transaction commit instructions to be issued using multiple threads. Used in combination with binary log group commit, this prevents the commit rate of a single transaction being a bottleneck to throughput, and might therefore produce a performance improvement.

    Transactions are written to the binary log at the point when all the storage engines involved have confirmed that the transaction is prepared to commit. The binary log group commit logic then commits a group of transactions after their binary log write has taken place. When binlog_order_commits is disabled, because multiple threads are used for this process, transactions in a commit group might be committed in a different order from their order in the binary log. (Transactions from a single client always commit in chronological order.) In many cases this does not matter, as operations carried out in separate transactions should produce consistent results, and if that is not the case, a single transaction ought to be used instead.

    If you want to ensure that the transaction history on the master and on a multithreaded replication slave remains identical, set slave_preserve_commit_order=1 on the replication slave.

  • binlog_rotate_encryption_master_key_at_startup

    Property Value
    Command-Line Format --binlog-rotate-encryption-master-key-at-startup[={OFF|ON}]
    Introduced 8.0.14
    System Variable binlog_rotate_encryption_master_key_at_startup
    Scope Global
    Dynamic No
    SET_VAR Hint Applies No
    Type Boolean
    Default Value OFF

    Specifies whether or not the binary log master key is rotated at server startup. The binary log master key is the binary log encryption key that is used to encrypt file passwords for the binary log files and relay log files on the server. When a server is started for the first time with binary log encryption enabled (binlog_encryption=ON), a new binary log encryption key is generated and used as the binary log master key. If the binlog_rotate_encryption_master_key_at_startup system variable is also set to ON, whenever the server is restarted, a further binary log encryption key is generated and used as the binary log master key for all subsequent binary log files and relay log files. If the binlog_rotate_encryption_master_key_at_startup system variable is set to OFF, which is the default, the existing binary log master key is used again after the server restarts. For more information on binary log encryption keys and the binary log master key, see Section 17.3.10, “Encrypting Binary Log Files and Relay Log Files”.

  • binlog_row_event_max_size

    Property Value
    Command-Line Format --binlog-row-event-max-size=#
    System Variable (>= 8.0.14) binlog_row_event_max_size
    Scope (>= 8.0.14) Global
    Dynamic (>= 8.0.14) No
    SET_VAR Hint Applies (>= 8.0.14) No
    Type Integer
    Default Value 8192
    Minimum Value 256
    Maximum Value (64-bit platforms) 18446744073709551615
    Maximum Value (32-bit platforms) 4294967295

    When row-based binary logging is used, this setting is a soft limit on the maximum size of a row-based binary log event, in bytes. Where possible, rows stored in the binary log are grouped into events with a size not exceeding the value of this setting. If an event cannot be split, the maximum size can be exceeded. The value must be (or else gets rounded down to) a multiple of 256. The default is 8192 bytes.

    This global system variable is read-only and can be set only at server startup. Its value can therefore only be modified by using the PERSIST_ONLY keyword or the @@persist_only qualifier with the SET statement.

  • binlog_row_image

    Property Value
    Command-Line Format --binlog-row-image=image_type
    System Variable binlog_row_image
    Scope Global, Session
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Enumeration
    Default Value full
    Valid Values

    full (Log all columns)

    minimal (Log only changed columns, and columns needed to identify rows)

    noblob (Log all columns, except for unneeded BLOB and TEXT columns)

    For MySQL row-based replication, this variable determines how row images are written to the binary log.

    Setting the session value of this system variable is a restricted operation. The session user must have privileges sufficient to set restricted session variables. See Section 5.1.9.1, “System Variable Privileges”.

    In MySQL row-based replication, each row change event contains two images, a before image whose columns are matched against when searching for the row to be updated, and an after image containing the changes. Normally, MySQL logs full rows (that is, all columns) for both the before and after images. However, it is not strictly necessary to include every column in both images, and we can often save disk, memory, and network usage by logging only those columns which are actually required.

    Note

    When deleting a row, only the before image is logged, since there are no changed values to propagate following the deletion. When inserting a row, only the after image is logged, since there is no existing row to be matched. Only when updating a row are both the before and after images required, and both written to the binary log.

    For the before image, it is necessary only that the minimum set of columns required to uniquely identify rows is logged. If the table containing the row has a primary key, then only the primary key column or columns are written to the binary log. Otherwise, if the table has a unique key all of whose columns are NOT NULL, then only the columns in the unique key need be logged. (If the table has neither a primary key nor a unique key without any NULL columns, then all columns must be used in the before image, and logged.) In the after image, it is necessary to log only the columns which have actually changed.

    You can cause the server to log full or minimal rows using the binlog_row_image system variable. This variable actually takes one of three possible values, as shown in the following list:

    • full: Log all columns in both the before image and the after image.

    • minimal: Log only those columns in the before image that are required to identify the row to be changed; log only those columns in the after image where a value was specified by the SQL statement, or generated by auto-increment.

    • noblob: Log all columns (same as full), except for BLOB and TEXT columns that are not required to identify rows, or that have not changed.

    Note

    This variable is not supported by NDB Cluster; setting it has no effect on the logging of NDB tables.

    The default value is full.

    When using minimal or noblob, deletes and updates are guaranteed to work correctly for a given table if and only if the following conditions are true for both the source and destination tables:

    • All columns must be present and in the same order; each column must use the same data type as its counterpart in the other table.

    • The tables must have identical primary key definitions.

    (In other words, the tables must be identical with the possible exception of indexes that are not part of the tables' primary keys.)

    If these conditions are not met, it is possible that the primary key column values in the destination table may prove insufficient to provide a unique match for a delete or update. In this event, no warning or error is issued; the master and slave silently diverge, thus breaking consistency.

    Setting this variable has no effect when the binary logging format is STATEMENT. When binlog_format is MIXED, the setting for binlog_row_image is applied to changes that are logged using row-based format, but this setting no effect on changes logged as statements.

    Setting binlog_row_image on either the global or session level does not cause an implicit commit; this means that this variable can be changed while a transaction is in progress without affecting the transaction.

  • binlog_row_metadata

    Property Value
    Command-Line Format --binlog-row-metadata=metadata_type
    Introduced 8.0.1
    System Variable binlog_row_metadata
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Enumeration
    Default Value MINIMAL
    Valid Values

    FULL (All metadata is included)

    MINIMAL (Limit included metadata)

    Configures the amount of table metadata added to the binary log when using row-based logging. When set to MINIMAL, the default, only metadata related to SIGNED flags, column character set and geometry types are logged. When set to FULL complete metadata for tables is logged, such as column name, ENUM or SET string values, PRIMARY KEY information, and so on.

    The extended metadata serves the following purposes:

    • Slaves use the metadata to transfer data when its table structure is different from the master's.

    • External software can use the metadata to decode row events and store the data into external databases, such as a data warehouse.

  • binlog_row_value_options

    Property Value
    Command-Line Format --binlog-row-value-options=#
    Introduced 8.0.3
    System Variable binlog_row_value_options
    Scope Global, Session
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Set
    Default Value ''
    Valid Values PARTIAL_JSON

    When set to PARTIAL_JSON, this enables use of a space-efficient binary log format for updates that modify only a small portion of a JSON document, which causes row-based replication to write only the modified parts of the JSON document to the after-image for the update in the binary log (rather than writing the full document). This works for an UPDATE statement which modifies a JSON column using any sequence of JSON_SET(), JSON_REPLACE(), and JSON_REMOVE(). If the modification requires more space than the full document, or if the server is unable to generate a partial update, the full document is used instead.

    Setting the session value of this system variable is a restricted operation. The session user must have privileges sufficient to set restricted session variables. See Section 5.1.9.1, “System Variable Privileges”.

    PARTIAL_JSON is the only supported value; to unset binlog_row_value_options, set its value to the empty string.

    binlog_row_value_options=PARTIAL_JSON takes effect only when binary logging is enabled and binlog_format is set to ROW or MIXED. Statement-based replication always logs only the modified parts of the JSON document, regardless of any value set for binlog_row_value_options. To maximize the amount of space saved, use binlog_row_image=NOBLOB or binlog_row_image=MINIMAL together with this option. binlog_row_image=FULL saves less space than either of these, since the full JSON document is stored in the before-image, and the partial update is stored only in the after-image.

    binlog_row_value_options=PARTIAL_JSON overrides any setting for the log_bin_use_v1_row_events variable. If that option is enabled, the event format required by binlog_row_value_options=PARTIAL_JSON is still used.

    mysqlbinlog output includes partial JSON updates in the form of events encoded as base-64 strings using BINLOG statements. If the --verbose option is specified, mysqlbinlog displays the partial JSON updates as readable JSON using pseudo-SQL statements.

    MySQL Replication generates an error if a modification cannot be applied to the JSON document on the slave. This includes a failure to find the path. Be aware that, even with this and other safety checks, if a JSON document on a slave has diverged from that on the master and a partial update is applied, it remains theoretically possible to produce a valid but unexpected JSON document on the slave.

    Replicating to older MySQL versions.  When replicating to a slave that uses MySQL 8.0.2 or a previous version from a master running MySQL 8.0.3 or later, binlog_row_value_options must be disabled (that is, set to ''). This is because logging of JSON partial updates uses a binary log event type introduced in MySQL 8.0.3; this event type is not recognized by previous versions of MySQL.

  • binlog_rows_query_log_events

    Property Value
    Command-Line Format --binlog-rows-query-log-events[={OFF|ON}]
    System Variable binlog_rows_query_log_events
    Scope Global, Session
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Boolean
    Default Value OFF

    This system variable affects row-based logging only. When enabled, it causes the server to write informational log events such as row query log events into its binary log. This information can be used for debugging and related purposes, such as obtaining the original query issued on the master when it cannot be reconstructed from the row updates.

    Setting the session value of this system variable is a restricted operation. The session user must have privileges sufficient to set restricted session variables. See Section 5.1.9.1, “System Variable Privileges”.

    These informational events are normally ignored by MySQL programs reading the binary log and so cause no issues when replicating or restoring from backup. To view them, increase the verbosity level by using mysqlbinlog's --verbose option twice, either as -vv or --verbose --verbose.

  • binlog_stmt_cache_size

    Property Value
    Command-Line Format --binlog-stmt-cache-size=#
    System Variable binlog_stmt_cache_size
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Integer
    Default Value 32768
    Minimum Value 4096
    Maximum Value (64-bit platforms) 18446744073709551615
    Maximum Value (32-bit platforms) 4294967295

    The size of the memory buffer for the binary log to hold nontransactional statements issued during a transaction. When binary logging is enabled on the server (with the log_bin system variable set to ON), separate binary log transaction and statement caches are allocated for each client if the server supports any transactional storage engines. If the data for the nontransactional statements used in the transaction exceeds the space in the memory buffer, the excess data is stored in a temporary file. When binary log encryption is active on the server, the memory buffer is not encrypted, but (from MySQL 8.0.17) any temporary file used to hold the binary log cache is encrypted. After each transaction is committed, the binary log statement cache is reset by clearing the memory buffer and truncating the temporary file if used.

    If you often use large nontransactional statements during transactions, you can increase this cache size to get better performance by reducing or eliminating the need to write to temporary files. The Binlog_stmt_cache_use and Binlog_stmt_cache_disk_use status variables can be useful for tuning the size of this variable. See Section 5.4.4, “The Binary Log”.

    The binlog_cache_size system variable sets the size for the transaction cache.

  • binlog_transaction_dependency_tracking

    Property Value
    Command-Line Format --binlog-transaction-dependency-tracking=value
    Introduced 8.0.1
    System Variable binlog_transaction_dependency_tracking
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Enumeration
    Default Value COMMIT_ORDER
    Valid Values

    COMMIT_ORDER

    WRITESET

    WRITESET_SESSION

    The source of dependency information that the master uses to determine which transactions can be executed in parallel by the slave's multithreaded applier. This variable can take one of the three values described in the following list:

    • COMMIT_ORDER: Dependency information is generated from the master's commit timestamps. This is the default. This mode is also used for any transactions without write sets, even if this variable's is WRITESET or WRITESET_SESSION; this is also the case for transactions updating tables without primary keys and transactions updating tables having foreign key constraints.

    • WRITESET: Dependency information is generated from the master's write set, and any transactions which write different tuples can be parallelized.

    • WRITESET_SESSION: Dependency information is generated from the master's write set, but no two updates from the same session can be reordered.

    WRITESET and WRITESET_SESSION modes do not deliver any transaction dependencies that are newer than those that would have been returned in COMMIT_ORDER mode.

    The value of this variable cannot be set to anything other than COMMIT_ORDER if transaction_write_set_extraction is OFF. You should also note that the value of transaction_write_set_extraction cannot be changed if the current value of binlog_transaction_dependency_tracking is WRITESET or WRITESET_SESSION.

    The number of row hashes to be kept and checked for the latest transaction to have changed a given row is determined by the value of binlog_transaction_dependency_history_size.

  • binlog_transaction_dependency_history_size

    Property Value
    Command-Line Format --binlog-transaction-dependency-history-size=#
    Introduced 8.0.1
    System Variable binlog_transaction_dependency_history_size
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Integer
    Default Value 25000
    Minimum Value 1
    Maximum Value 1000000

    Sets an upper limit on the number of row hashes which are kept in memory and used for looking up the transaction that last modified a given row. Once this number of hashes has been reached, the history is purged.

  • expire_logs_days

    Property Value
    Command-Line Format --expire-logs-days=#
    Deprecated 8.0.3
    System Variable expire_logs_days
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Integer
    Default Value (>= 8.0.11) 0
    Default Value (>= 8.0.2, <= 8.0.4) 30
    Default Value (<= 8.0.1) 0
    Minimum Value 0
    Maximum Value 99

    Specifies the number of days before automatic removal of binary log files. expire_logs_days is deprecated, and will be removed in a future release. Instead, use binlog_expire_logs_seconds, which sets the binary log expiration period in seconds. If you do not set a value for either system variable, the default expiration period is 30 days. Possible removals happen at startup and when the binary log is flushed. Log flushing occurs as indicated in Section 5.4, “MySQL Server Logs”.

    Any non-zero value that you specify for expire_logs_days is ignored if binlog_expire_logs_seconds is also specified, and the value of binlog_expire_logs_seconds is used instead as the binary log expiration period. A warning message is issued in this situation. A non-zero value for expire_logs_days is only applied as the binary log expiration period if binlog_expire_logs_seconds is not specified or is specified as 0.

    To disable automatic purging of the binary log, specify a value of 0 explicitly for binlog_expire_logs_seconds, and do not specify a value for expire_logs_days. For compatibility with earlier releases, automatic purging is also disabled if you specify a value of 0 explicitly for expire_logs_days and do not specify a value for binlog_expire_logs_seconds. In that case, the default for binlog_expire_logs_seconds is not applied.

    To remove binary log files manually, use the PURGE BINARY LOGS statement. See Section 13.4.1.1, “PURGE BINARY LOGS Syntax”.

  • log_bin

    Property Value
    System Variable log_bin
    Scope Global
    Dynamic No
    SET_VAR Hint Applies No
    Type Boolean

    Shows the status of binary logging on the server, either enabled (ON) or disabled (OFF). With binary logging enabled, the server logs all statements that change data to the binary log, which is used for backup and replication. ON means that the binary log is available, OFF means that it is not in use. The --log-bin option can be used to specify a base name and location for the binary log.

    In earlier MySQL versions, binary logging was disabled by default, and was enabled if you specified the --log-bin option. From MySQL 8.0, binary logging is enabled by default, with the log_bin system variable set to ON, whether or not you specify the --log-bin option. The exception is if you use mysqld to initialize the data directory manually by invoking it with the --initialize or --initialize-insecure option, when binary logging is disabled by default. It is possible to enable binary logging in this case by specifying the --log-bin option.

    If the --skip-log-bin or --disable-log-bin option is specified at startup, binary logging is disabled, with the log_bin system variable set to OFF. If either of these options is specified and --log-bin is also specified, the option specified later takes precedence.

    For information on the format and management of the binary log, see Section 5.4.4, “The Binary Log”.

  • log_bin_basename

    Property Value
    System Variable log_bin_basename
    Scope Global
    Dynamic No
    SET_VAR Hint Applies No
    Type File name

    Holds the base name and path for the binary log files, which can be set with the --log-bin server option. In MySQL 8.0, if the --log-bin option is not supplied, the default base name is binlog. For compatibility with MySQL 5.7, if the --log-bin option is supplied with no string or with an empty string, the default base name is host_name-bin, using the name of the host machine. The default location is the data directory.

  • log_bin_index

    Property Value
    Command-Line Format --log-bin-index=file_name
    System Variable log_bin_index
    Scope Global
    Dynamic No
    SET_VAR Hint Applies No
    Type File name

    Holds the base name and path for the binary log index file, which can be set with the --log-bin-index server option.

  • log_bin_trust_function_creators

    Property Value
    Command-Line Format --log-bin-trust-function-creators[={OFF|ON}]
    System Variable log_bin_trust_function_creators
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Boolean
    Default Value OFF

    This variable applies when binary logging is enabled. It controls whether stored function creators can be trusted not to create stored functions that will cause unsafe events to be written to the binary log. If set to 0 (the default), users are not permitted to create or alter stored functions unless they have the SUPER privilege in addition to the CREATE ROUTINE or ALTER ROUTINE privilege. A setting of 0 also enforces the restriction that a function must be declared with the DETERMINISTIC characteristic, or with the READS SQL DATA or NO SQL characteristic. If the variable is set to 1, MySQL does not enforce these restrictions on stored function creation. This variable also applies to trigger creation. See Section 24.7, “Stored Program Binary Logging”.

  • log_bin_use_v1_row_events

    Property Value
    Command-Line Format --log-bin-use-v1-row-events[={OFF|ON}]
    System Variable log_bin_use_v1_row_events
    Scope Global
    Dynamic No
    SET_VAR Hint Applies No
    Type Boolean
    Default Value OFF

    Shows whether Version 2 binary logging is in use. A value of 1 shows that the server is writing the binary log using Version 1 logging events (the only version of binary log events used in previous releases), and thus producing a binary log that can be read by older slaves. 0 indicates that Version 2 binary log events are in use.

    This variable is read-only. To switch between Version 1 and Version 2 binary event binary logging, it is necessary to restart mysqld with the --log-bin-use-v1-row-events option.

    Other than when performing upgrades of NDB Cluster Replication, --log-bin-use-v1-events is chiefly of interest when setting up replication conflict detection and resolution using NDB$EPOCH_TRANS(), which requires Version 2 binary row event logging. Thus, this option and --ndb-log-transaction-id are not compatible.

    Note

    MySQL NDB Cluster 8.0 uses Version 2 binary log row events by default. You should keep this mind when planning upgrades or downgrades, and for setups using NDB Cluster Replication.

    For more information, see Section 22.6.11, “NDB Cluster Replication Conflict Resolution”.

  • log_builtin_as_identified_by_password

    Property Value
    Command-Line Format --log-builtin-as-identified-by-password[={OFF|ON}]
    Removed 8.0.11
    System Variable log_builtin_as_identified_by_password
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Boolean
    Default Value OFF

    This system variable was removed in MySQL 8.0.11.

  • log_slave_updates

    Property Value
    Command-Line Format --log-slave-updates[={OFF|ON}]
    System Variable log_slave_updates
    Scope Global
    Dynamic No
    SET_VAR Hint Applies No
    Type Boolean
    Default Value (>= 8.0.3) ON
    Default Value (<= 8.0.2) OFF

    Whether updates received by a slave server from a master server should be logged to the slave's own binary log. Binary logging must be enabled on the slave for this variable to have any effect. See Section 17.1.6, “Replication and Binary Logging Options and Variables”.

    This system variable is set on by default, and is read-only. If you need to prevent the slave server from logging updates, specify --skip-log-slave-updates when you start the slave, or specify log_slave_updates=OFF in the configuration file for the slave.

  • log_statements_unsafe_for_binlog

    Property Value
    Command-Line Format --log-statements-unsafe-for-binlog[={OFF|ON}]
    System Variable log_statements_unsafe_for_binlog
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Boolean
    Default Value ON

    If error 1592 is encountered, controls whether the generated warnings are added to the error log or not.

  • master_verify_checksum

    Property Value
    Command-Line Format --master-verify-checksum[={OFF|ON}]
    System Variable master_verify_checksum
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Boolean
    Default Value OFF

    Enabling this variable causes the master to examine checksums when reading from the binary log. master_verify_checksum is disabled by default; in this case, the master uses the event length from the binary log to verify events, so that only complete events are read from the binary log.

  • max_binlog_cache_size

    Property Value
    Command-Line Format --max-binlog-cache-size=#
    System Variable max_binlog_cache_size
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Integer
    Default Value 18446744073709551615
    Minimum Value 4096
    Maximum Value 18446744073709551615

    If a transaction requires more than this many bytes of memory, the server generates a Multi-statement transaction required more than 'max_binlog_cache_size' bytes of storage error. The minimum value is 4096. The maximum possible value is 16EiB (exbibytes). The maximum recommended value is 4GB; this is due to the fact that MySQL currently cannot work with binary log positions greater than 4GB.

    max_binlog_cache_size sets the size for the transaction cache only; the upper limit for the statement cache is governed by the max_binlog_stmt_cache_size system variable.

    The visibility to sessions of max_binlog_cache_size matches that of the binlog_cache_size system variable; in other words, changing its value affects only new sessions that are started after the value is changed.

  • max_binlog_size

    Property Value
    Command-Line Format --max-binlog-size=#
    System Variable max_binlog_size
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Integer
    Default Value 1073741824
    Minimum Value 4096
    Maximum Value 1073741824

    If a write to the binary log causes the current log file size to exceed the value of this variable, the server rotates the binary logs (closes the current file and opens the next one). The minimum value is 4096 bytes. The maximum and default value is 1GB. Encrypted binary log files have an additional 512-byte header, which is included in max_binlog_size.

    A transaction is written in one chunk to the binary log, so it is never split between several binary logs. Therefore, if you have big transactions, you might see binary log files larger than max_binlog_size.

    If max_relay_log_size is 0, the value of max_binlog_size applies to relay logs as well.

    With GTIDs in use on the server, when max_binlog_size is reached, if the system table mysql.gtid_executed cannot be accessed to write the GTIDs from the current binary log file, the binary log cannot be rotated. In this situation, the server responds according to its binlog_error_action setting. If IGNORE_ERROR is set, an error is logged on the server and binary logging is halted, or if ABORT_SERVER is set, the server shuts down.

  • max_binlog_stmt_cache_size

    Property Value
    Command-Line Format --max-binlog-stmt-cache-size=#
    System Variable max_binlog_stmt_cache_size
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Integer
    Default Value 18446744073709547520
    Minimum Value 4096
    Maximum Value 18446744073709547520

    If nontransactional statements within a transaction require more than this many bytes of memory, the server generates an error. The minimum value is 4096. The maximum and default values are 4GB on 32-bit platforms and 16EB (exabytes) on 64-bit platforms.

    max_binlog_stmt_cache_size sets the size for the statement cache only; the upper limit for the transaction cache is governed exclusively by the max_binlog_cache_size system variable.

  • original_commit_timestamp

    Property Value
    Introduced 8.0.1
    System Variable original_commit_timestamp
    Scope Session
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Numeric

    For internal use by replication. When re-executing a transaction on a slave, this is set to the time when the transaction was committed on the original master, measured in microseconds since the epoch. This allows the original commit timestamp to be propagated throughout a replication topology.

    Setting the session value of this system variable is a restricted operation. The session user must have privileges sufficient to set restricted session variables. See Section 5.1.9.1, “System Variable Privileges”.

  • sql_log_bin

    Property Value
    System Variable sql_log_bin
    Scope Session
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Boolean
    Default Value ON

    This variable controls whether logging to the binary log is enabled for the current session (assuming that the binary log itself is enabled). The default value is ON. To disable or enable binary logging for the current session, set the session sql_log_bin variable to OFF or ON.

    Set this variable to OFF for a session to temporarily disable binary logging while making changes to the master you do not want replicated to the slave.

    Setting the session value of this system variable is a restricted operation. The session user must have privileges sufficient to set restricted session variables. See Section 5.1.9.1, “System Variable Privileges”.

    It is not possible to set the session value of sql_log_bin within a transaction or subquery.

    Setting this variable to OFF prevents GTIDs from being assigned to transactions in the binary log. If you are using GTIDs for replication, this means that even when binary logging is later enabled again, the GTIDs written into the log from this point do not account for any transactions that occurred in the meantime, so in effect those transactions are lost.

  • sync_binlog

    Property Value
    Command-Line Format --sync-binlog=#
    System Variable sync_binlog
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Integer
    Default Value 1
    Minimum Value 0
    Maximum Value 4294967295

    Controls how often the MySQL server synchronizes the binary log to disk.

    • sync_binlog=0: Disables synchronization of the binary log to disk by the MySQL server. Instead, the MySQL server relies on the operating system to flush the binary log to disk from time to time as it does for any other file. This setting provides the best performance, but in the event of a power failure or operating system crash, it is possible that the server has committed transactions that have not been synchronized to the binary log.

    • sync_binlog=1: Enables synchronization of the binary log to disk before transactions are committed. This is the safest setting but can have a negative impact on performance due to the increased number of disk writes. In the event of a power failure or operating system crash, transactions that are missing from the binary log are only in a prepared state. This permits the automatic recovery routine to roll back the transactions, which guarantees that no transaction is lost from the binary log.

    • sync_binlog=N, where N is a value other than 0 or 1: The binary log is synchronized to disk after N binary log commit groups have been collected. In the event of a power failure or operating system crash, it is possible that the server has committed transactions that have not been flushed to the binary log. This setting can have a negative impact on performance due to the increased number of disk writes. A higher value improves performance, but with an increased risk of data loss.

    For the greatest possible durability and consistency in a replication setup that uses InnoDB with transactions, use these settings:

    Caution

    Many operating systems and some disk hardware fool the flush-to-disk operation. They may tell mysqld that the flush has taken place, even though it has not. In this case, the durability of transactions is not guaranteed even with the recommended settings, and in the worst case, a power outage can corrupt InnoDB data. Using a battery-backed disk cache in the SCSI disk controller or in the disk itself speeds up file flushes, and makes the operation safer. You can also try to disable the caching of disk writes in hardware caches.

  • transaction_write_set_extraction

    Property Value
    Command-Line Format --transaction-write-set-extraction[=value]
    System Variable transaction_write_set_extraction
    Scope Global, Session
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Enumeration
    Default Value (>= 8.0.2) XXHASH64
    Default Value OFF
    Valid Values

    OFF

    MURMUR32

    XXHASH64

    Defines the algorithm used to hash the writes extracted during a transaction. If you are using Group Replication, this variable must be set to XXHASH64 because the process of extracting the writes from a transaction is required for conflict detection on all group members. See Section 18.9.1, “Group Replication Requirements”.

    As of MySQL 8.0.14, setting the session value of this system variable is a restricted operation. The session user must have privileges sufficient to set restricted session variables. See Section 5.1.9.1, “System Variable Privileges”.

    Note

    The value of this variable cannot be changed when binlog_transaction_dependency_tracking is set to either of WRITESET or WRITESET_SESSION.

17.1.6.5 Global Transaction ID Options and Variables

The MySQL Server options and system variables described in this section are used to monitor and control Global Transaction Identifiers (GTIDs).

For additional information, see Section 17.1.3, “Replication with Global Transaction Identifiers”.

Startup Options Used with GTID Replication

The following server startup options are used with GTID-based replication:

  • --enforce-gtid-consistency

    Property Value
    Command-Line Format --enforce-gtid-consistency[=value]
    System Variable enforce_gtid_consistency
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Enumeration
    Default Value OFF
    Valid Values

    OFF

    ON

    WARN

    When enabled, the server enforces GTID consistency by allowing execution of only statements that can be safely logged using a GTID. You must set this option to ON before enabling GTID based replication.

    The values that --enforce-gtid-consistency can be configured to are:

    • OFF: all transactions are allowed to violate GTID consistency.

    • ON: no transaction is allowed to violate GTID consistency.

    • WARN: all transactions are allowed to violate GTID consistency, but a warning is generated in this case.

    Setting --enforce-gtid-consistency without a value is an alias for --enforce-gtid-consistency=ON. This impacts on the behavior of the variable, see enforce_gtid_consistency.

    Only statements that can be logged using GTID safe statements can be logged when enforce-gtid-consistency is set to ON, so the operations listed here cannot be used with this option:

    • CREATE TABLE ... SELECT statements

    • CREATE TEMPORARY TABLE or DROP TEMPORARY TABLE statements inside transactions

    • Transactions or statements that update both transactional and nontransactional tables. There is an exception that nontransactional DML is allowed in the same transaction or in the same statement as transactional DML, if all nontransactional tables are temporary.

    --enforce-gtid-consistency only takes effect if binary logging takes place for a statement. If binary logging is disabled on the server, or if statements are not written to the binary log because they are removed by a filter, GTID consistency is not checked or enforced for the statements that are not logged.

    For more information, see Section 17.1.3.6, “Restrictions on Replication with GTIDs”.

  • --gtid-mode

    Property Value
    Command-Line Format --gtid-mode=MODE
    System Variable gtid_mode
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Enumeration
    Default Value OFF
    Valid Values

    OFF

    OFF_PERMISSIVE

    ON_PERMISSIVE

    ON

    This option specifies whether global transaction identifiers (GTIDs) are used to identify transactions. Setting this option to --gtid-mode=ON requires that enforce-gtid-consistency be set to ON. The gtid_mode variable is dynamic and enables GTID based replication to be configured online. Before using this feature, see Section 17.1.5, “Changing Replication Modes on Online Servers”.

  • --gtid-executed-compression-period

    Property Value
    Command-Line Format --gtid-executed-compression-period=#
    Type Integer
    Default Value 1000
    Minimum Value 0
    Maximum Value 4294967295

    Compress the mysql.gtid_executed table each time this many transactions have taken place. A setting of 0 means that this table is not compressed. No compression of the table occurs when binary logging is enabled, therefore the option has no effect unless log_bin is OFF.

    See mysql.gtid_executed Table Compression, for more information.

System Variables Used with GTID Replication

The following system variables are used with GTID-based replication:

  • binlog_gtid_simple_recovery

    Property Value
    Command-Line Format --binlog-gtid-simple-recovery[={OFF|ON}]
    System Variable binlog_gtid_simple_recovery
    Scope Global
    Dynamic No
    SET_VAR Hint Applies No
    Type Boolean
    Default Value ON

    This variable controls how binary log files are iterated during the search for GTIDs when MySQL starts or restarts.

    When binlog_gtid_simple_recovery=TRUE, which is the default in MySQL 8.0, the values of gtid_executed and gtid_purged are computed at startup based on the values of Previous_gtids_log_event in the most recent and oldest binary log files. For a description of the computation, see The gtid_purged System Variable. This setting accesses only two binary log files during server restart. If all binary logs on the server were generated using MySQL 5.7.8 or later, binlog_gtid_simple_recovery=TRUE can always safely be used.

    If any binary logs from MySQL 5.7.7 or older are present on the server (for example, following an upgrade of an older server to MySQL 8.0), with binlog_gtid_simple_recovery=TRUE, gtid_executed and gtid_purged might be initialized incorrectly in the following two situations:

    • The newest binary log was generated by MySQL 5.7.5 or earlier, and gtid_mode was ON for some binary logs but OFF for the newest binary log.

    • A SET @@GLOBAL.gtid_purged statement was issued on MySQL 5.7.7 or earlier, and the binary log that was active at the time of the SET @@GLOBAL.gtid_purged statement has not yet been purged.

    If an incorrect GTID set is computed in either situation, it will remain incorrect even if the server is later restarted with binlog_gtid_simple_recovery=FALSE. If either of these situations apply or might apply on the server, set binlog_gtid_simple_recovery=FALSE before starting or restarting the server.

    When binlog_gtid_simple_recovery=FALSE is set, the method of computing gtid_executed and gtid_purged as described in The gtid_purged System Variable is changed to iterate the binary log files as follows:

    • Instead of using the value of Previous_gtids_log_event and GTID log events from the newest binary log file, the computation for gtid_executed iterates from the newest binary log file, and uses the value of Previous_gtids_log_event and any GTID log events from the first binary log file where it finds a Previous_gtids_log_event value. If the server's most recent binary log files do not have GTID log events, for example if gtid_mode=ON was used but the server was later changed to gtid_mode=OFF, this process can take a long time.

    • Instead of using the value of Previous_gtids_log_event from the oldest binary log file, the computation for gtid_purged iterates from the oldest binary log file, and uses the value of Previous_gtids_log_event from the first binary log file where it finds either a nonempty Previous_gtids_log_event value, or at least one GTID log event (indicating that the use of GTIDs starts at that point). If the server's older binary log files do not have GTID log events, for example if gtid_mode=ON was only set recently on the server, this process can take a long time.

  • enforce_gtid_consistency

    Property Value
    Command-Line Format --enforce-gtid-consistency[=value]
    System Variable enforce_gtid_consistency
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Enumeration
    Default Value OFF
    Valid Values

    OFF

    ON

    WARN

    Depending on the value of this variable, the server enforces GTID consistency by allowing execution of only statements that can be safely logged using a GTID. You must set this variable to ON before enabling GTID based replication.

    The values that enforce_gtid_consistency can be configured to are:

    • OFF: all transactions are allowed to violate GTID consistency.

    • ON: no transaction is allowed to violate GTID consistency.

    • WARN: all transactions are allowed to violate GTID consistency, but a warning is generated in this case.

    enforce_gtid_consistency only takes effect if binary logging takes place for a statement. If binary logging is disabled on the server, or if statements are not written to the binary log because they are removed by a filter, GTID consistency is not checked or enforced for the statements that are not logged.

    For more information on statements that can be logged using GTID based replication, see --enforce-gtid-consistency.

    Prior to MySQL 5.7 and in early releases in that release series, the boolean enforce_gtid_consistency defaulted to OFF. To maintain compatibility with these earlier releases, the enumeration defaults to OFF, and setting --enforce-gtid-consistency without a value is interpreted as setting the value to ON. The variable also has multiple textual aliases for the values: 0=OFF=FALSE, 1=ON=TRUE,2=WARN. This differs from other enumeration types but maintains compatibility with the boolean type used in previous releases. These changes impact on what is returned by the variable. Using SELECT @@ENFORCE_GTID_CONSISTENCY, SHOW VARIABLES LIKE 'ENFORCE_GTID_CONSISTENCY', and SELECT * FROM INFORMATION_SCHEMA.VARIABLES WHERE 'VARIABLE_NAME' = 'ENFORCE_GTID_CONSISTENCY', all return the textual form, not the numeric form. This is an incompatible change, since @@ENFORCE_GTID_CONSISTENCY returns the numeric form for booleans but returns the textual form for SHOW and the Information Schema.

  • gtid_executed

    Property Value
    System Variable gtid_executed
    System Variable gtid_executed
    Scope Global
    Scope Global, Session
    Dynamic No
    Dynamic No
    SET_VAR Hint Applies No
    SET_VAR Hint Applies No
    Type String

    When used with global scope, this variable contains a representation of the set of all transactions executed on the server and GTIDs that have been set by a SET gtid_purged statement. This is the same as the value of the Executed_Gtid_Set column in the output of SHOW MASTER STATUS and SHOW SLAVE STATUS. The value of this variable is a GTID set, see GTID Sets for more information.

    When the server starts, @@GLOBAL.gtid_executed is initialized. See binlog_gtid_simple_recovery for more information on how binary logs are iterated to populate gtid_executed. GTIDs are then added to the set as transactions are executed, or if any SET gtid_purged statement is executed.

    The set of transactions that can be found in the binary logs at any given time is equal to GTID_SUBTRACT(@@GLOBAL.gtid_executed, @@GLOBAL.gtid_purged); that is, to all transactions in the binary log that have not yet been purged.

    Issuing RESET MASTER causes the global value (but not the session value) of this variable to be reset to an empty string. GTIDs are not otherwise removed from this set other than when the set is cleared due to RESET MASTER.

    In some older releases, this variable could also be used with session scope, where it contained a representation of the set of transactions that are written to the cache in the current session. The session scope is now deprecated.

  • gtid_executed_compression_period

    Property Value
    System Variable gtid_executed_compression_period
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Integer
    Default Value 1000
    Minimum Value 0
    Maximum Value 4294967295

    Compress the mysql.gtid_executed table each time this many transactions have been processed. A setting of 0 means that this table is not compressed. Since no compression of the table occurs when using the binary log, setting the value of the variable has no effect unless binary logging is disabled.

    See mysql.gtid_executed Table Compression, for more information.

  • gtid_mode

    Property Value
    System Variable gtid_mode
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Enumeration
    Default Value OFF
    Valid Values

    OFF

    OFF_PERMISSIVE

    ON_PERMISSIVE

    ON

    Controls whether GTID based logging is enabled and what type of transactions the logs can contain. You must have privileges sufficient to set global system variables. See Section 5.1.9.1, “System Variable Privileges”. enforce_gtid_consistency must be true before you can set gtid_mode=ON. Before modifying this variable, see Section 17.1.5, “Changing Replication Modes on Online Servers”.

    Logged transactions can be either anonymous or use GTIDs. Anonymous transactions rely on binary log file and position to identify specific transactions. GTID transactions have a unique identifier that is used to refer to transactions. The different modes are:

    • OFF: Both new and replicated transactions must be anonymous.

    • OFF_PERMISSIVE: New transactions are anonymous. Replicated transactions can be either anonymous or GTID transactions.

    • ON_PERMISSIVE: New transactions are GTID transactions. Replicated transactions can be either anonymous or GTID transactions.

    • ON: Both new and replicated transactions must be GTID transactions.

    Changes from one value to another can only be one step at a time. For example, if gtid_mode is currently set to OFF_PERMISSIVE, it is possible to change to OFF or ON_PERMISSIVE but not to ON.

    The values of gtid_purged and gtid_executed are persistent regardless of the value of gtid_mode. Therefore even after changing the value of gtid_mode, these variables contain the correct values.

  • gtid_next

    Property Value
    System Variable gtid_next
    Scope Session
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Enumeration
    Default Value AUTOMATIC
    Valid Values

    AUTOMATIC

    ANONYMOUS

    UUID:NUMBER

    This variable is used to specify whether and how the next GTID is obtained.

    Setting the session value of this system variable is a restricted operation. The session user must have privileges sufficient to set restricted session variables. See Section 5.1.9.1, “System Variable Privileges”.

    gtid_next can take any of the following values:

    • AUTOMATIC: Use the next automatically-generated global transaction ID.

    • ANONYMOUS: Transactions do not have global identifiers, and are identified by file and position only.

    • A global transaction ID in UUID:NUMBER format.

    Exactly which of the above options are valid depends on the setting of gtid_mode, see Section 17.1.5.1, “Replication Mode Concepts” for more information. Setting this variable has no effect if gtid_mode is OFF.

    After this variable has been set to UUID:NUMBER, and a transaction has been committed or rolled back, an explicit SET GTID_NEXT statement must again be issued before any other statement.

    DROP TABLE or DROP TEMPORARY TABLE fails with an explicit error when used on a combination of nontemporary tables with temporary tables, or of temporary tables using transactional storage engines with temporary tables using nontransactional storage engines.

  • gtid_owned

    Property Value
    System Variable gtid_owned
    Scope Global, Session
    Dynamic No
    SET_VAR Hint Applies No
    Type String

    This read-only variable is primarily for internal use. Its contents depend on its scope.

    • When used with global scope, gtid_owned holds a list of all the GTIDs that are currently in use on the server, with the IDs of the threads that own them. This variable is mainly useful for a multi-threaded replication slave to check whether a transaction is already being applied on another thread. An applier thread takes ownership of a transaction's GTID all the time it is processing the transaction, so @@global.gtid_owned shows the GTID and owner for the duration of processing. When a transaction has been committed (or rolled back), the applier thread releases ownership of the GTID.

    • When used with session scope, gtid_owned holds a single GTID that is currently in use by and owned by this session. This variable is mainly useful for testing and debugging the use of GTIDs when the client has explicitly assigned a GTID for the transaction by setting gtid_next. In this case, @@session.gtid_owned displays the GTID all the time the client is processing the transaction, until the transaction has been committed (or rolled back). When the client has finished processing the transaction, the variable is cleared. If gtid_next=AUTOMATIC is used for the session, gtid_owned is only populated briefly during the execution of the commit statement for the transaction, so it cannot be observed from the session concerned, although it will be listed if @@global.gtid_owned is read at the right point. If you have a requirement to track the GTIDs that are handled by a client in a session, you can enable the session state tracker controlled by the session_track_gtids system variable.

  • gtid_purged

    Property Value
    System Variable gtid_purged
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type String

    The global value of the gtid_purged system variable (@@GLOBAL.gtid_purged) is a GTID set consisting of the GTIDs of all the transactions that have been committed on the server, but do not exist in any binary log file on the server. gtid_purged is a subset of gtid_executed. The following categories of GTIDs are in gtid_purged:

    • GTIDs of replicated transactions that were committed with binary logging disabled on the slave.

    • GTIDs of transactions that were written to a binary log file that has now been purged.

    • GTIDs that were added explicitly to the set by the statement SET @@GLOBAL.gtid_purged.

    When the server starts, the global value of gtid_purged is initialized to a set of GTIDs. For information on how this GTID set is computed, see The gtid_purged System Variable. If binary logs from MySQL 5.7.7 or older are present on the server, you might need to set binlog_gtid_simple_recovery=FALSE in the server's configuration file to produce the correct computation. See the description for binlog_gtid_simple_recovery for details of the situations in which this setting is needed.

    Issuing RESET MASTER causes the value of gtid_purged to be reset to an empty string.

    You can set the value of gtid_purged in order to record on the server that the transactions in a certain GTID set have been applied, although they do not exist in any binary log on the server. An example use case for this action is when you are restoring a backup of one or more databases on a server, but you do not have the relevant binary logs containing the transactions on the server.

    From MySQL 8.0, there are two ways to set the value of gtid_purged. You can either replace the value of gtid_purged with your specified GTID set, or you can append your specified GTID set to the GTID set that is already held by gtid_purged. If the server has no existing GTIDs, for example an empty server that you are provisioning with a backup of an existing database, both methods have the same result. If you are restoring a backup that overlaps the transactions that are already on the server, for example replacing a corrupted table with a partial dump from the master made using mysqldump (which includes the GTIDs of all the transactions on the server, even though the dump is partial), use the first method of replacing the value of gtid_purged. If you are restoring a backup that is disjoint from the transactions that are already on the server, for example provisioning a multi-source replication slave using dumps from two different servers, use the second method of adding to the value of gtid_purged.

    • To replace the value of gtid_purged with your specified GTID set, use the following statement:

      SET @@GLOBAL.gtid_purged = 'gtid_set'

      gtid_set must be a superset of the current value of gtid_purged, and must not intersect with gtid_subtract(gtid_executed,gtid_purged). In other words, the new GTID set must include any GTIDs that were already in gtid_purged, and must not include any GTIDs in gtid_executed that have not yet been purged. gtid_set also cannot include any GTIDs that are in @@global.gtid_owned, that is, the GTIDs for transactions that are currently being processed on the server.

      The result is that the global value of gtid_purged is set equal to gtid_set, and the value of gtid_executed becomes the union of gtid_set and the previous value of gtid_executed.

    • To append your specified GTID set to gtid_purged, use the following statement with a plus sign (+) before the GTID set:

      SET @@GLOBAL.gtid_purged = '+gtid_set'

      gtid_set must not intersect with the current value of gtid_executed. In other words, the new GTID set must not include any GTIDs in gtid_executed, including transactions that are already also in gtid_purged. gtid_set also cannot include any GTIDs that are in @@global.gtid_owned, that is, the GTIDs for transactions that are currently being processed on the server.

      The result is that gtid_set is added to both gtid_executed and gtid_purged.

Note

If any binary logs from MySQL 5.7.7 or older are present on the server (for example, following an upgrade of an older server to MySQL 8.0), after issuing a SET @@GLOBAL.gtid_purged statement, you might need to set binlog_gtid_simple_recovery=FALSE in the server's configuration file before restarting the server, otherwise gtid_purged can be computed incorrectly. See the description for binlog_gtid_simple_recovery for details of the situations in which this setting is needed.

17.1.7 Common Replication Administration Tasks

Once replication has been started it executes without requiring much regular administration. This section describes how to check the status of replication and how to pause a slave.

17.1.7.1 Checking Replication Status

The most common task when managing a replication process is to ensure that replication is taking place and that there have been no errors between the slave and the master.

The SHOW SLAVE STATUS statement, which you must execute on each slave, provides information about the configuration and status of the connection between the slave server and the master server. From MySQL 5.7, the Performance Schema has replication tables that provide this information in a more accessible form. See Section 26.12.11, “Performance Schema Replication Tables”.

The SHOW STATUS statement also provided some information relating specifically to replication slaves. From MySQL 5.7, the following status variables previously monitored using SHOW STATUS were deprecated and moved to the Performance Schema replication tables:

The replication heartbeat information shown in the Performance Schema replication tables lets you check that the replication connection is active even if the master has not sent events to the slave recently. The master sends a heartbeat signal to a slave if there are no updates to, and no unsent events in, the binary log for a longer period than the heartbeat interval. The MASTER_HEARTBEAT_PERIOD setting on the master (set by the CHANGE MASTER TO statement) specifies the frequency of the heartbeat, which defaults to half of the connection timeout interval for the slave (slave_net_timeout). The replication_connection_status Performance Schema table shows when the most recent heartbeat signal was received by a replication slave, and how many heartbeat signals it has received.

If you are using the SHOW SLAVE STATUS statement to check on the status of an individual slave, the statement provides the following information:

mysql> SHOW SLAVE STATUS\G
*************************** 1. row ***************************
               Slave_IO_State: Waiting for master to send event
                  Master_Host: master1
                  Master_User: root
                  Master_Port: 3306
                Connect_Retry: 60
              Master_Log_File: mysql-bin.000004
          Read_Master_Log_Pos: 931
               Relay_Log_File: slave1-relay-bin.000056
                Relay_Log_Pos: 950
        Relay_Master_Log_File: mysql-bin.000004
             Slave_IO_Running: Yes
            Slave_SQL_Running: Yes
              Replicate_Do_DB:
          Replicate_Ignore_DB:
           Replicate_Do_Table:
       Replicate_Ignore_Table:
      Replicate_Wild_Do_Table:
  Replicate_Wild_Ignore_Table:
                   Last_Errno: 0
                   Last_Error:
                 Skip_Counter: 0
          Exec_Master_Log_Pos: 931
              Relay_Log_Space: 1365
              Until_Condition: None
               Until_Log_File:
                Until_Log_Pos: 0
           Master_SSL_Allowed: No
           Master_SSL_CA_File:
           Master_SSL_CA_Path:
              Master_SSL_Cert:
            Master_SSL_Cipher:
               Master_SSL_Key:
        Seconds_Behind_Master: 0
Master_SSL_Verify_Server_Cert: No
                Last_IO_Errno: 0
                Last_IO_Error:
               Last_SQL_Errno: 0
               Last_SQL_Error:
  Replicate_Ignore_Server_Ids: 0

The key fields from the status report to examine are:

  • Slave_IO_State: The current status of the slave. See Section 8.14.4, “Replication Slave I/O Thread States”, and Section 8.14.5, “Replication Slave SQL Thread States”, for more information.

  • Slave_IO_Running: Whether the I/O thread for reading the master's binary log is running. Normally, you want this to be Yes unless you have not yet started replication or have explicitly stopped it with STOP SLAVE.

  • Slave_SQL_Running: Whether the SQL thread for executing events in the relay log is running. As with the I/O thread, this should normally be Yes.

  • Last_IO_Error, Last_SQL_Error: The last errors registered by the I/O and SQL threads when processing the relay log. Ideally these should be blank, indicating no errors.

  • Seconds_Behind_Master: The number of seconds that the slave SQL thread is behind processing the master binary log. A high number (or an increasing one) can indicate that the slave is unable to handle events from the master in a timely fashion.

    A value of 0 for Seconds_Behind_Master can usually be interpreted as meaning that the slave has caught up with the master, but there are some cases where this is not strictly true. For example, this can occur if the network connection between master and slave is broken but the slave I/O thread has not yet noticed this—that is, slave_net_timeout has not yet elapsed.

    It is also possible that transient values for Seconds_Behind_Master may not reflect the situation accurately. When the slave SQL thread has caught up on I/O, Seconds_Behind_Master displays 0; but when the slave I/O thread is still queuing up a new event, Seconds_Behind_Master may show a large value until the SQL thread finishes executing the new event. This is especially likely when the events have old timestamps; in such cases, if you execute SHOW SLAVE STATUS several times in a relatively short period, you may see this value change back and forth repeatedly between 0 and a relatively large value.

Several pairs of fields provide information about the progress of the slave in reading events from the master binary log and processing them in the relay log:

  • (Master_Log_file, Read_Master_Log_Pos): Coordinates in the master binary log indicating how far the slave I/O thread has read events from that log.

  • (Relay_Master_Log_File, Exec_Master_Log_Pos): Coordinates in the master binary log indicating how far the slave SQL thread has executed events received from that log.

  • (Relay_Log_File, Relay_Log_Pos): Coordinates in the slave relay log indicating how far the slave SQL thread has executed the relay log. These correspond to the preceding coordinates, but are expressed in slave relay log coordinates rather than master binary log coordinates.

On the master, you can check the status of connected slaves using SHOW PROCESSLIST to examine the list of running processes. Slave connections have Binlog Dump in the Command field:

mysql> SHOW PROCESSLIST \G;
*************************** 4. row ***************************
     Id: 10
   User: root
   Host: slave1:58371
     db: NULL
Command: Binlog Dump
   Time: 777
  State: Has sent all binlog to slave; waiting for binlog to be updated
   Info: NULL

Because it is the slave that drives the replication process, very little information is available in this report.

For slaves that were started with the --report-host option and are connected to the master, the SHOW SLAVE HOSTS statement on the master shows basic information about the slaves. The output includes the ID of the slave server, the value of the --report-host option, the connecting port, and master ID:

mysql> SHOW SLAVE HOSTS;
+-----------+--------+------+-------------------+-----------+
| Server_id | Host   | Port | Rpl_recovery_rank | Master_id |
+-----------+--------+------+-------------------+-----------+
|        10 | slave1 | 3306 |                 0 |         1 |
+-----------+--------+------+-------------------+-----------+
1 row in set (0.00 sec)

17.1.7.2 Pausing Replication on the Slave

You can stop and start replication on the slave using the STOP SLAVE and START SLAVE statements.

To stop processing of the binary log from the master, use STOP SLAVE:

mysql> STOP SLAVE;

When replication is stopped, the slave I/O thread stops reading events from the master binary log and writing them to the relay log, and the SQL thread stops reading events from the relay log and executing them. You can pause the I/O or SQL thread individually by specifying the thread type:

mysql> STOP SLAVE IO_THREAD;
mysql> STOP SLAVE SQL_THREAD;

To start execution again, use the START SLAVE statement:

mysql> START SLAVE;

To start a particular thread, specify the thread type:

mysql> START SLAVE IO_THREAD;
mysql> START SLAVE SQL_THREAD;

For a slave that performs updates only by processing events from the master, stopping only the SQL thread can be useful if you want to perform a backup or other task. The I/O thread will continue to read events from the master but they are not executed. This makes it easier for the slave to catch up when you restart the SQL thread.

Stopping only the I/O thread enables the events in the relay log to be executed by the SQL thread up to the point where the relay log ends. This can be useful when you want to pause execution to catch up with events already received from the master, when you want to perform administration on the slave but also ensure that it has processed all updates to a specific point. This method can also be used to pause event receipt on the slave while you conduct administration on the master. Stopping the I/O thread but permitting the SQL thread to run helps ensure that there is not a massive backlog of events to be executed when replication is started again.

17.2 Replication Implementation

Replication is based on the master server keeping track of all changes to its databases (updates, deletes, and so on) in its binary log. The binary log serves as a written record of all events that modify database structure or content (data) from the moment the server was started. Typically, SELECT statements are not recorded because they modify neither database structure nor content.

Each slave that connects to the master requests a copy of the binary log. That is, it pulls the data from the master, rather than the master pushing the data to the slave. The slave also executes the events from the binary log that it receives. This has the effect of repeating the original changes just as they were made on the master. Tables are created or their structure modified, and data is inserted, deleted, and updated according to the changes that were originally made on the master.

Because each slave is independent, the replaying of the changes from the master's binary log occurs independently on each slave that is connected to the master. In addition, because each slave receives a copy of the binary log only by requesting it from the master, the slave is able to read and update the copy of the database at its own pace and can start and stop the replication process at will without affecting the ability to update to the latest database status on either the master or slave side.

For more information on the specifics of the replication implementation, see Section 17.2.2, “Replication Implementation Details”.

Masters and slaves report their status in respect of the replication process regularly so that you can monitor them. See Section 8.14, “Examining Thread Information”, for descriptions of all replicated-related states.

The master binary log is written to a local relay log on the slave before it is processed. The slave also records information about the current position with the master's binary log and the local relay log. See Section 17.2.4, “Replication Relay and Status Logs”.

Database changes are filtered on the slave according to a set of rules that are applied according to the various configuration options and variables that control event evaluation. For details on how these rules are applied, see Section 17.2.5, “How Servers Evaluate Replication Filtering Rules”.

17.2.1 Replication Formats

Replication works because events written to the binary log are read from the master and then processed on the slave. The events are recorded within the binary log in different formats according to the type of event. The different replication formats used correspond to the binary logging format used when the events were recorded in the master's binary log. The correlation between binary logging formats and the terms used during replication are:

  • When using statement-based binary logging, the master writes SQL statements to the binary log. Replication of the master to the slave works by executing the SQL statements on the slave. This is called statement-based replication (which can be abbreviated as SBR), which corresponds to the MySQL statement-based binary logging format.

  • When using row-based logging, the master writes events to the binary log that indicate how individual table rows are changed. Replication of the master to the slave works by copying the events representing the changes to the table rows to the slave. This is called row-based replication (which can be abbreviated as RBR).

    Row-based logging is the default method.

  • You can also configure MySQL to use a mix of both statement-based and row-based logging, depending on which is most appropriate for the change to be logged. This is called mixed-format logging. When using mixed-format logging, a statement-based log is used by default. Depending on certain statements, and also the storage engine being used, the log is automatically switched to row-based in particular cases. Replication using the mixed format is referred to as mixed-based replication or mixed-format replication. For more information, see Section 5.4.4.3, “Mixed Binary Logging Format”.

NDB Cluster.  The default binary logging format in MySQL NDB Cluster 8.0 is MIXED. You should note that NDB Cluster Replication always uses row-based replication, and that the NDB storage engine is incompatible with statement-based replication. See Section 22.6.2, “General Requirements for NDB Cluster Replication”, for more information.

When using MIXED format, the binary logging format is determined in part by the storage engine being used and the statement being executed. For more information on mixed-format logging and the rules governing the support of different logging formats, see Section 5.4.4.3, “Mixed Binary Logging Format”.

The logging format in a running MySQL server is controlled by setting the binlog_format server system variable. This variable can be set with session or global scope. The rules governing when and how the new setting takes effect are the same as for other MySQL server system variables. Setting the variable for the current session lasts only until the end of that session, and the change is not visible to other sessions. Setting the variable globally takes effect for clients that connect after the change, but not for any current client sessions, including the session where the variable setting was changed. To make the global system variable setting permanent so that it applies across server restarts, you must set it in an option file. For more information, see Section 13.7.5.1, “SET Syntax for Variable Assignment”.

There are conditions under which you cannot change the binary logging format at runtime or doing so causes replication to fail. See Section 5.4.4.2, “Setting The Binary Log Format”.

Changing the global binlog_format value requires privileges sufficient to set global system variables. Changing the session binlog_format value requires privileges sufficient to set restricted session system variables. See Section 5.1.9.1, “System Variable Privileges”.

The statement-based and row-based replication formats have different issues and limitations. For a comparison of their relative advantages and disadvantages, see Section 17.2.1.1, “Advantages and Disadvantages of Statement-Based and Row-Based Replication”.

With statement-based replication, you may encounter issues with replicating stored routines or triggers. You can avoid these issues by using row-based replication instead. For more information, see Section 24.7, “Stored Program Binary Logging”.

17.2.1.1 Advantages and Disadvantages of Statement-Based and Row-Based Replication

Each binary logging format has advantages and disadvantages. For most users, the mixed replication format should provide the best combination of data integrity and performance. If, however, you want to take advantage of the features specific to the statement-based or row-based replication format when performing certain tasks, you can use the information in this section, which provides a summary of their relative advantages and disadvantages, to determine which is best for your needs.

Advantages of statement-based replication
  • Proven technology.

  • Less data written to log files. When updates or deletes affect many rows, this results in much less storage space required for log files. This also means that taking and restoring from backups can be accomplished more quickly.

  • Log files contain all statements that made any changes, so they can be used to audit the database.

Disadvantages of statement-based replication
Advantages of row-based replication
  • All changes can be replicated. This is the safest form of replication.

    Note

    Statements that update the information in the mysql system schema—such as GRANT, REVOKE and the manipulation of triggers, stored routines (including stored procedures), and views—are all replicated to slaves using statement-based replication.

    For statements such as CREATE TABLE ... SELECT, a CREATE statement is generated from the table definition and replicated using statement-based format, while the row insertions are replicated using row-based format.

  • Fewer row locks are required on the master, which thus achieves higher concurrency, for the following types of statements:

  • Fewer row locks are required on the slave for any INSERT, UPDATE, or DELETE statement.

Disadvantages of row-based replication
  • RBR can generate more data that must be logged. To replicate a DML statement (such as an UPDATE or DELETE statement), statement-based replication writes only the statement to the binary log. By contrast, row-based replication writes each changed row to the binary log. If the statement changes many rows, row-based replication may write significantly more data to the binary log; this is true even for statements that are rolled back. This also means that making and restoring a backup can require more time. In addition, the binary log is locked for a longer time to write the data, which may cause concurrency problems. Use binlog_row_image=minimal to reduce the disadvantage considerably.

  • Deterministic UDFs that generate large BLOB values take longer to replicate with row-based replication than with statement-based replication. This is because the BLOB column value is logged, rather than the statement generating the data.

  • You cannot see on the slave what statements were received from the master and executed. However, you can see what data was changed using mysqlbinlog with the options --base64-output=DECODE-ROWS and --verbose.

    Alternatively, use the binlog_rows_query_log_events variable, which if enabled adds a Rows_query event with the statement to mysqlbinlog output when the -vv option is used.

  • For tables using the MyISAM storage engine, a stronger lock is required on the slave for INSERT statements when applying them as row-based events to the binary log than when applying them as statements. This means that concurrent inserts on MyISAM tables are not supported when using row-based replication.

17.2.1.2 Usage of Row-Based Logging and Replication

MySQL uses statement-based logging (SBL), row-based logging (RBL) or mixed-format logging. The type of binary log used impacts the size and efficiency of logging. Therefore the choice between row-based replication (RBR) or statement-based replication (SBR) depends on your application and environment. This section describes known issues when using a row-based format log, and describes some best practices using it in replication.

For additional information, see Section 17.2.1, “Replication Formats”, and Section 17.2.1.1, “Advantages and Disadvantages of Statement-Based and Row-Based Replication”.

For information about issues specific to NDB Cluster Replication (which depends on row-based replication), see Section 22.6.3, “Known Issues in NDB Cluster Replication”.

  • Row-based logging of temporary tables.  As noted in Section 17.4.1.30, “Replication and Temporary Tables”, temporary tables are not replicated when using row-based format or (from MySQL 8.0.4) mixed format. For more information, see Section 17.2.1.1, “Advantages and Disadvantages of Statement-Based and Row-Based Replication”.

    Temporary tables are not replicated when using row-based or mixed format because there is no need. In addition, because temporary tables can be read only from the thread which created them, there is seldom if ever any benefit obtained from replicating them, even when using statement-based format.

    You can switch from statement-based to row-based binary logging format at runtime even when temporary tables have been created. However, in MySQL 8.0, you cannot switch from row-based or mixed format for binary logging to statement-based format at runtime, because any CREATE TEMPORARY TABLE statements will have been omitted from the binary log in the previous mode.

    The MySQL server tracks the logging mode that was in effect when each temporary table was created. When a given client session ends, the server logs a DROP TEMPORARY TABLE IF EXISTS statement for each temporary table that still exists and was created when statement-based binary logging was in use. If row-based or mixed format binary logging was in use when the table was created, the DROP TEMPORARY TABLE IF EXISTS statement is not logged. In releases before MySQL 8.0.4 and 5.7.25, the DROP TEMPORARY TABLE IF EXISTS statement was logged regardless of the logging mode that was in effect.

    Nontransactional DML statements involving temporary tables are allowed when using binlog_format=ROW, as long as any nontransactional tables affected by the statements are temporary tables (Bug #14272672).

  • RBL and synchronization of nontransactional tables.  When many rows are affected, the set of changes is split into several events; when the statement commits, all of these events are written to the binary log. When executing on the slave, a table lock is taken on all tables involved, and then the rows are applied in batch mode. Depending on the engine used for the slave's copy of the table, this may or may not be effective.

  • Latency and binary log size.  RBL writes changes for each row to the binary log and so its size can increase quite rapidly. This can significantly increase the time required to make changes on the slave that match those on the master. You should be aware of the potential for this delay in your applications.

  • Reading the binary log.  mysqlbinlog displays row-based events in the binary log using the BINLOG statement (see Section 13.7.7.1, “BINLOG Syntax”). This statement displays an event as a base 64-encoded string, the meaning of which is not evident. When invoked with the --base64-output=DECODE-ROWS and --verbose options, mysqlbinlog formats the contents of the binary log to be human readable. When binary log events were written in row-based format and you want to read or recover from a replication or database failure you can use this command to read contents of the binary log. For more information, see Section 4.6.8.2, “mysqlbinlog Row Event Display”.

  • Binary log execution errors and slave_exec_mode.  Using slave_exec_mode=IDEMPOTENT is generally only useful with MySQL NDB Cluster replication, for which IDEMPOTENT is the default value. (See Section 22.6.10, “NDB Cluster Replication: Multi-Master and Circular Replication”). When slave_exec_mode is IDEMPOTENT, a failure to apply changes from RBL because the original row cannot be found does not trigger an error or cause replication to fail. This means that it is possible that updates are not applied on the slave, so that the master and slave are no longer synchronized. Latency issues and use of nontransactional tables with RBR when slave_exec_mode is IDEMPOTENT can cause the master and slave to diverge even further. For more information about slave_exec_mode, see Section 5.1.8, “Server System Variables”.

    For other scenarios, setting slave_exec_mode to STRICT is normally sufficient; this is the default value for storage engines other than NDB.

  • Filtering based on server ID not supported.  You can filter based on server ID by using the IGNORE_SERVER_IDS option for the CHANGE MASTER TO statement. This option works with statement-based and row-based logging formats, but is deprecated for use when GTID_MODE=ON is set. Another method to filter out changes on some slaves is to use a WHERE clause that includes the relation @@server_id <> id_value clause with UPDATE and DELETE statements. For example, WHERE @@server_id <> 1. However, this does not work correctly with row-based logging. To use the server_id system variable for statement filtering, use statement-based logging.

  • Database-level replication options.  The effects of the --replicate-do-db, --replicate-ignore-db, and --replicate-rewrite-db options differ considerably depending on whether row-based or statement-based logging is used. Therefore, it is recommended to avoid database-level options and instead use table-level options such as --replicate-do-table and --replicate-ignore-table. For more information about these options and the impact replication format has on how they operate, see Section 17.1.6, “Replication and Binary Logging Options and Variables”.

  • RBL, nontransactional tables, and stopped slaves.  When using row-based logging, if the slave server is stopped while a slave thread is updating a nontransactional table, the slave database can reach an inconsistent state. For this reason, it is recommended that you use a transactional storage engine such as InnoDB for all tables replicated using the row-based format. Use of STOP SLAVE or STOP SLAVE SQL_THREAD prior to shutting down the slave MySQL server helps prevent issues from occurring, and is always recommended regardless of the logging format or storage engine you use.

17.2.1.3 Determination of Safe and Unsafe Statements in Binary Logging

The safeness of a statement in MySQL replication refers to whether the statement and its effects can be replicated correctly using statement-based format. If this is true of the statement, we refer to the statement as safe; otherwise, we refer to it as unsafe.

In general, a statement is safe if it deterministic, and unsafe if it is not. However, certain nondeterministic functions are not considered unsafe (see Nondeterministic functions not considered unsafe, later in this section). In addition, statements using results from floating-point math functions—which are hardware-dependent—are always considered unsafe (see Section 17.4.1.12, “Replication and Floating-Point Values”).

Handling of safe and unsafe statements.  A statement is treated differently depending on whether the statement is considered safe, and with respect to the binary logging format (that is, the current value of binlog_format).

  • When using row-based logging, no distinction is made in the treatment of safe and unsafe statements.

  • When using mixed-format logging, statements flagged as unsafe are logged using the row-based format; statements regarded as safe are logged using the statement-based format.

  • When using statement-based logging, statements flagged as being unsafe generate a warning to this effect. Safe statements are logged normally.

Each statement flagged as unsafe generates a warning. If a large number of such statements were executed on the master, this could lead to excessively large error log files. To prevent this, MySQL has a warning suppression mechanism. Whenever the 50 most recent ER_BINLOG_UNSAFE_STATEMENT warnings have been generated more than 50 times in any 50-second period, warning suppression is enabled. When activated, this causes such warnings not to be written to the error log; instead, for each 50 warnings of this type, a note The last warning was repeated N times in last S seconds is written to the error log. This continues as long as the 50 most recent such warnings were issued in 50 seconds or less; once the rate has decreased below this threshold, the warnings are once again logged normally. Warning suppression has no effect on how the safety of statements for statement-based logging is determined, nor on how warnings are sent to the client. MySQL clients still receive one warning for each such statement.

For more information, see Section 17.2.1, “Replication Formats”.

Statements considered unsafe.  Statements with the following characteristics are considered unsafe:

  • Statements containing system functions that may return a different value on the slave.  These functions include FOUND_ROWS(), GET_LOCK(), IS_FREE_LOCK(), IS_USED_LOCK(), LOAD_FILE(), MASTER_POS_WAIT(), RAND(), RELEASE_LOCK(), ROW_COUNT(), SESSION_USER(), SLEEP(), SYSDATE(), SYSTEM_USER(), USER(), UUID(), and UUID_SHORT().

    Nondeterministic functions not considered unsafe.  Although these functions are not deterministic, they are treated as safe for purposes of logging and replication: CONNECTION_ID(), CURDATE(), CURRENT_DATE(), CURRENT_TIME(), CURRENT_TIMESTAMP(), CURTIME(),, LAST_INSERT_ID(), LOCALTIME(), LOCALTIMESTAMP(), NOW(), UNIX_TIMESTAMP(), UTC_DATE(), UTC_TIME(), and UTC_TIMESTAMP().

    For more information, see Section 17.4.1.14, “Replication and System Functions”.

  • References to system variables.  Most system variables are not replicated correctly using the statement-based format. See Section 17.4.1.38, “Replication and Variables”. For exceptions, see Section 5.4.4.3, “Mixed Binary Logging Format”.

  • UDFs.  Since we have no control over what a UDF does, we must assume that it is executing unsafe statements.

  • Fulltext plugin.  This plugin may behave differently on different MySQL servers; therefore, statements depending on it could have different results. For this reason, all statements relying on the fulltext plugin are treated as unsafe in MySQL.

  • Trigger or stored program updates a table having an AUTO_INCREMENT column.  This is unsafe because the order in which the rows are updated may differ on the master and the slave.

    In addition, an INSERT into a table that has a composite primary key containing an AUTO_INCREMENT column that is not the first column of this composite key is unsafe.

    For more information, see Section 17.4.1.1, “Replication and AUTO_INCREMENT”.

  • INSERT ... ON DUPLICATE KEY UPDATE statements on tables with multiple primary or unique keys.  When executed against a table that contains more than one primary or unique key, this statement is considered unsafe, being sensitive to the order in which the storage engine checks the keys, which is not deterministic, and on which the choice of rows updated by the MySQL Server depends.

    An INSERT ... ON DUPLICATE KEY UPDATE statement against a table having more than one unique or primary key is marked as unsafe for statement-based replication. (Bug #11765650, Bug #58637)

  • Updates using LIMIT.  The order in which rows are retrieved is not specified, and is therefore considered unsafe. See Section 17.4.1.18, “Replication and LIMIT”.

  • Accesses or references log tables.  The contents of the system log table may differ between master and slave.

  • Nontransactional operations after transactional operations.  Within a transaction, allowing any nontransactional reads or writes to execute after any transactional reads or writes is considered unsafe.

    For more information, see Section 17.4.1.34, “Replication and Transactions”.

  • Accesses or references self-logging tables.  All reads and writes to self-logging tables are considered unsafe. Within a transaction, any statement following a read or write to self-logging tables is also considered unsafe.

  • LOAD DATA statements.  LOAD DATA is treated as unsafe and when binlog_format=MIXED the statement is logged in row-based format. When binlog_format=STATEMENT LOAD DATA does not generate a warning, unlike other unsafe statements.

  • XA transactions.  If two XA transactions committed in parallel on the master are being prepared on the slave in the inverse order, locking dependencies can occur with statement-based replication that cannot be safely resolved, and it is possible for replication to fail with deadlock on the slave. When binlog_format=STATEMENT is set, DML statements inside XA transactions are flagged as being unsafe and generate a warning. When binlog_format=MIXED or binlog_format=ROW is set, DML statements inside XA transactions are logged using row-based replication, and the potential issue is not present.

  • DEFAULT clause that refers to a nondeterministic function.  If an expression default value refers to a nondeterministic function, any statement that causes the expression to be evaluated is unsafe for statement-based replication. This includes statements such as INSERT, UPDATE, and ALTER TABLE. Unlike most other unsafe statements, this category of statement cannot be replicated safely in row-based format. When binlog_format is set to STATEMENT, the statement is logged and executed but a warning message is written to the error log. When binlog_format is set to MIXED or ROW, the statement is not executed and an error message is written to the error log. For more information on the handling of explicit defaults, see Handling of Explicit Defaults as of MySQL 8.0.13.

For additional information, see Section 17.4.1, “Replication Features and Issues”.

17.2.2 Replication Implementation Details

MySQL replication capabilities are implemented using three threads, one on the master server and two on the slave:

  • Binlog dump thread.  The master creates a thread to send the binary log contents to a slave when the slave connects. This thread can be identified in the output of SHOW PROCESSLIST on the master as the Binlog Dump thread.

    The binary log dump thread acquires a lock on the master's binary log for reading each event that is to be sent to the slave. As soon as the event has been read, the lock is released, even before the event is sent to the slave.

  • Slave I/O thread.  When a START SLAVE statement is issued on a slave server, the slave creates an I/O thread, which connects to the master and asks it to send the updates recorded in its binary logs.

    The slave I/O thread reads the updates that the master's Binlog Dump thread sends (see previous item) and copies them to local files that comprise the slave's relay log.

    The state of this thread is shown as Slave_IO_running in the output of SHOW SLAVE STATUS or as Slave_running in the output of SHOW STATUS.

  • Slave SQL thread.  The slave creates an SQL thread to read the relay log that is written by the slave I/O thread and execute the events contained therein.

In the preceding description, there are three threads per master/slave connection. A master that has multiple slaves creates one binary log dump thread for each currently connected slave, and each slave has its own I/O and SQL threads.

A slave uses two threads to separate reading updates from the master and executing them into independent tasks. Thus, the task of reading statements is not slowed down if statement execution is slow. For example, if the slave server has not been running for a while, its I/O thread can quickly fetch all the binary log contents from the master when the slave starts, even if the SQL thread lags far behind. If the slave stops before the SQL thread has executed all the fetched statements, the I/O thread has at least fetched everything so that a safe copy of the statements is stored locally in the slave's relay logs, ready for execution the next time that the slave starts.

The SHOW PROCESSLIST statement provides information that tells you what is happening on the master and on the slave regarding replication. For information on master states, see Section 8.14.3, “Replication Master Thread States”. For slave states, see Section 8.14.4, “Replication Slave I/O Thread States”, and Section 8.14.5, “Replication Slave SQL Thread States”.

The following example illustrates how the three threads show up in the output from SHOW PROCESSLIST.

On the master server, the output from SHOW PROCESSLIST looks like this:

mysql> SHOW PROCESSLIST\G
*************************** 1. row ***************************
     Id: 2
   User: root
   Host: localhost:32931
     db: NULL
Command: Binlog Dump
   Time: 94
  State: Has sent all binlog to slave; waiting for binlog to
         be updated
   Info: NULL

Here, thread 2 is a Binlog Dump replication thread that services a connected slave. The State information indicates that all outstanding updates have been sent to the slave and that the master is waiting for more updates to occur. If you see no Binlog Dump threads on a master server, this means that replication is not running; that is, no slaves are currently connected.

On a slave server, the output from SHOW PROCESSLIST looks like this:

mysql> SHOW PROCESSLIST\G
*************************** 1. row ***************************
     Id: 10
   User: system user
   Host:
     db: NULL
Command: Connect
   Time: 11
  State: Waiting for master to send event
   Info: NULL
*************************** 2. row ***************************
     Id: 11
   User: system user
   Host:
     db: NULL
Command: Connect
   Time: 11
  State: Has read all relay log; waiting for the slave I/O
         thread to update it
   Info: NULL

The State information indicates that thread 10 is the I/O thread that is communicating with the master server, and thread 11 is the SQL thread that is processing the updates stored in the relay logs. At the time that SHOW PROCESSLIST was run, both threads were idle, waiting for further updates.

The value in the Time column can show how late the slave is compared to the master. See Section A.13, “MySQL 8.0 FAQ: Replication”. If sufficient time elapses on the master side without activity on the Binlog Dump thread, the master determines that the slave is no longer connected. As for any other client connection, the timeouts for this depend on the values of net_write_timeout and net_retry_count; for more information about these, see Section 5.1.8, “Server System Variables”.

The SHOW SLAVE STATUS statement provides additional information about replication processing on a slave server. See Section 17.1.7.1, “Checking Replication Status”.

17.2.3 Replication Channels

Replication channels represent the path of transactions flowing from a master to a slave. This section describes how channels can be used in a replication topology, and the impact they have on single-source replication.

To provide compatibility with previous versions, the MySQL server automatically creates on startup a default channel whose name is the empty string (""). This channel is always present; it cannot be created or destroyed by the user. If no other channels (having nonempty names) have been created, replication statements act on the default channel only, so that all replication statements from older slaves function as expected (see Section 17.2.3.2, “Compatibility with Previous Replication Statements”. Statements applying to replication channels as described in this section can be used only when there is at least one named channel.

A replication channel encompasses the path of transactions transmitted from a master to a slave. In multi-source replication a slave opens multiple channels, one per master, and each channel has its own relay log and applier (SQL) threads. Once transactions are received by a replication channel's receiver (I/O) thread, they are added to the channel's relay log file and passed through to an applier thread. This enables channels to function independently.

A replication channel is also associated with a host name and port. You can assign multiple channels to the same combination of host name and port. In MySQL 8.0, the maximum number of channels that can be added to one slave in a multi-source replication topology is 256. Each replication channel must have a unique (nonempty) name (see Section 17.2.3.4, “Replication Channel Naming Conventions”). Channels can be configured independently.

17.2.3.1 Commands for Operations on a Single Channel

To enable MySQL replication operations to act on individual replication channels, use the FOR CHANNEL channel clause with the following replication statements:

Similarly, an additional channel parameter is introduced for the following functions:

The following statements are disallowed for the group_replication_recovery channel:

The following statements are disallowed for the group_replication_applier channel:

FLUSH RELAY LOGS is now permitted for the group_replication_applier channel, but if the request is received while a transaction is being applied, the request is performed after the transaction ends. The requester must wait while the transaction is completed and the rotation takes place. This behavior prevents transactions from being split, which is not permitted for Group Replication.

17.2.3.2 Compatibility with Previous Replication Statements

When a replication slave has multiple channels and a FOR CHANNEL channel option is not specified, a valid statement generally acts on all available channels, with some specific exceptions.

For example, the following statements behave as expected for all except certain Group Replication channels:

  • START SLAVE starts replication threads for all channels, except the group_replication_recovery and group_replication_applier channels.

  • STOP SLAVE stops replication threads for all channels, except the group_replication_recovery and group_replication_applier channels.

  • SHOW SLAVE STATUS reports the status for all channels, except the group_replication_applier channel.

  • RESET SLAVE resets all channels.

Warning

Use RESET SLAVE with caution as this statement deletes all existing channels, purges their relay log files, and recreates only the default channel.

Some replication statements cannot operate on all channels. In this case, error 1964 Multiple channels exist on the slave. Please provide channel name as an argument. is generated. The following statements and functions generate this error when used in a multi-source replication topology and a FOR CHANNEL channel option is not used to specify which channel to act on:

Note that a default channel always exists in a single source replication topology, where statements and functions behave as in previous versions of MySQL.

17.2.3.3 Startup Options and Replication Channels

This section describes startup options which are impacted by the addition of replication channels.

The following startup options must be configured correctly to use multi-source replication.

The following startup options now affect all channels in a replication topology.

  • --log-slave-updates

    All transactions received by the slave (even from multiple sources) are written in the binary log.