Mysql：配置replication——摘自联机手册

Chapter 16. Replication

Replication enables data from one MySQL database server (called the master) to be replicated to one or more MySQL database servers (slaves). Replication is asynchronous - your replication slaves do not need to be connected permanently to receive updates from the master, which means that updates can occur over long-distance connections and even temporary solutions such as a dial-up service. Depending on the configuration, you can replicate all databases, selected databases, or even selected tables within a database.

The target uses for replication in MySQL include:

• Scale-out solutions - spreading the load among multiple slaves to improve performance. In this environment, all writes and updates must take place on the master server. Reads, however, may take place on one or more slaves. This model can improve the performance of writes (since the master is dedicated to updates), while dramatically increasing read speed across an increasing number of slaves.

• Data security - because data is replicated to the slave, and the slave can pause the replication process, it is possible to run backup services on the slave without corrupting the corresponding master data.

• Analytics - live data can be created on the master, while the analysis of the information can take place on the slave without affecting the performance of the master.

• Long-distance data distribution - if a branch office would like to work with a copy of your main data, you can use replication to create a local copy of the data for their use without requiring permanent access to the master.

Replication in MySQL features support for 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 MySQL Cluster (see Chapter 17, MySQL Cluster NDB 6.X/7.X).

There are a number of solutions available for setting up replication between two servers, but 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 16.1.1, “How to Set Up 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 may also use a third variety, Mixed Based Replication (MBR), which is the default mode within MySQL 5.1.12 and later. For more information on the different replication formats, see Section 16.1.2, “Replication Formats”.

Replication is controlled through a number of different options and variables. These control the core operation of the replication, timeouts and the databases and filters that can be applied on databases and tables. For more information on the available options, see Section 16.1.3, “Replication and Binary Logging Options and Variables”.

You can use replication to solve a number of different problems, including problems with performance, supporting the backup of different databases and for use as part of a larger solution to alleviate system failures. For information on how to address these issues, see Section 16.2, “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 problems and their resolution, including an FAQ, see Section 16.3, “Replication Notes and Tips”.

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 replication, see Section 16.4, “Replication Implementation”.

MySQL Enterprise.  The MySQL Enterprise Monitor provides numerous advisors that provide immediate feedback about replication-related problems. For more information, see http://www.mysql.com/products/enterprise/advisors.html.

16.1. Replication Configuration

Replication between servers in MySQL works through the use of the binary logging mechanism. 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.

The master is “dumb” in this scenario. Once binary logging has been enabled, all statements are recorded in the binary log. Each slave will receive 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; you cannot configure the master to log only certain events. If you do not 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.

Each slave keeps a record of the binary log file and position within the log 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 remembers the position within the binary log, it is possible for slaves to be disconnected, reconnect and then “catch up” by continuing from the recorded position.

Both the master and each slave must be configured with a unique ID (using the server-id option). In addition, the 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. The details are stored within the master.info file.

In this section the setup and configuration required for a replication environment is described, including step-by-step instructions for creating a new replication environment. The major components of this section are:

• For a guide to setting up two or more servers for replication see Section 16.1.1, “How to Set Up Replication”. This section deals with the setup of the systems and provides methods for copying data between the master and slaves.

• Events in the binary log are recorded using a number of formats. These are referred to as statement-based replication (SBR) or row-based replication (RBR). A third type, mixed-format replication (MIXED), uses SBR or RBR replication automatically to take advantage of the benefits of both SBR and RBR formats when appropriate. The different formats are discussed in Section 16.1.2, “Replication Formats”.

• Detailed information on the different configuration options and variables that apply to replication is provided in Section 16.1.3, “Replication and Binary Logging Options and Variables”.

• Once started, the replication process should require little administration or monitoring. However, for advice on common tasks that you may want to execute, see Section 16.1.4, “Common Replication Administration Tasks”.

16.1.1. How to Set Up Replication

This section describes how to set up complete replication of a MySQL server. There are a number of different methods for setting up replication, and the exact method that you use will depend on how you are setting up replication, and whether you already have data within your master database.

There are some generic tasks which may be required for all replication setups:

• You may want to create a separate user that will be used by your slaves to authenticate with the master to read the binary log for replication. The step is optional. See Section 16.1.1.1, “Creating a User for Replication”.

• You must configure the master to support the binary log and configure a unique ID. See Section 16.1.1.2, “Setting the Replication Master Configuration”.

• You must configure a unique ID for each slave that you want to connect to the master. See Section 16.1.1.3, “Setting the Replication Slave Configuration”.

• Before starting a data snapshot or the replication process, you should record the position of the binary log on the master. You will need this information when configuring the slave so that the slave knows where within the binary log to start executing events. See Section 16.1.1.4, “Obtaining the Master Replication Information”.

• If you already have data on your master and you want to synchronize your slave with this base data, then you will need to create a data snapshot of your database. You can create a snapshot using mysqldump (see Section 16.1.1.5, “Creating a Data Snapshot Using mysqldump”) or by copying the data files directly (see Section 16.1.1.6, “Creating a Data Snapshot Using Raw Data Files”).

• You will need to configure the slave with the master settings, such as the host name, login credentials and binary log name and positions. See Section 16.1.1.10, “Setting the Master Configuration on the Slave”.

Once you have configured the basic options, you will need to follow the instructions for your replication setup. A number of alternatives are provided:

• If you are establishing a new MySQL master and one or more slaves, then you need only set up the configuration, as you have no data to exchange. For guidance on setting up replication in this situation, see Section 16.1.1.7, “Setting Up Replication with New Master and Slaves”.

• If you are already running a MySQL server, and therefore already have data that will need to be transferred to your slaves before replication starts, have not previously configured the binary log and are able to shut down your MySQL server for a short period during the process, see Section 16.1.1.8, “Setting Up Replication with Existing Data”.

• If you are setting up additional slaves to an existing replication environment then you can set up the slaves without affecting the master. See Section 16.1.1.9, “Introducing Additional Slaves to an Existing Replication Environment”.

If you want to administer a MySQL replication setup, we suggest that you read this entire chapter through and try all statements mentioned in Section 12.6.1, “SQL Statements for Controlling Master Servers”, and Section 12.6.2, “SQL Statements for Controlling Slave Servers”. You should also familiarize yourself with the replication startup options described in Section 16.1.3, “Replication and Binary Logging Options and Variables”.

Note

Note that certain steps within the setup process require the SUPER privilege. If you do not have this privilege then enabling replication may not be possible.

16.1.1.1. Creating a User for Replication

Each slave must connect to the master using a standard MySQL user name and password, so there must be a user account on the master that the slave can use to connect. Any account can be used for this operation, providing it has been granted the REPLICATION SLAVE privilege.

You do not need to create a specific user for replication. However, you should be aware that the user name and password will be stored in plain text within the master.info file. Therefore, you may want to create a user that only has privileges for the replication process.

To create a user or grant an existing user the privileges required for replication, use the GRANT statement. If you create a user solely for the purposes of replication then that user needs only the REPLICATION SLAVE privilege. For example, to create a user, repl, that can connect for replication from any host within the mydomain.com domain, issue this statement on the master:

mysql> GRANT REPLICATION SLAVE ON *.*
    -> TO 'repl'@'%.mydomain.com' IDENTIFIED BY 'slavepass';

See Section 12.5.1.3, “GRANT Syntax”, for more information on the GRANT statement.

You may wish to create a different user for each slave, or use the same user for each slave that needs to connect. As long as each user that you want to use for the replication process has the REPLICATION SLAVE privilege you can create as many users as you require.

16.1.1.2. Setting the Replication Master Configuration

For replication to work you must enable binary logging on the master. If binary logging is not enabled, replication will not be possible as it is the binary log that is used to exchange data between the master and slaves.

Each server within a replication group must be configured with a unique server-id value. The server ID is used to identify individual servers within the group, and must be positive integer between 1 and (2³²)–1. How you organize and select the numbers is entirely up to you.

To configure the binary log and server ID options, you will need to shut down your MySQL server and edit the configuration of the my.cnf or my.ini file.

You will need to add the following options to the configuration file within the [mysqld] section. If these options already exist, but are commented out, uncomment the options and alter them according to your needs. For example, to enable binary logging, using a log file name prefix of mysql-bin, and setting a server ID of 1:

[mysqld]
log-bin=mysql-bin
server-id=1

Note

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 master my.cnf file.

Note

Ensure that the skip-networking option has not been enabled on your replication master. If networking has been disabled, then your slave will not able to communicate with the master and replication will fail.

16.1.1.3. Setting the Replication Slave Configuration

The only option you must configure on the slave is to set the unique server ID. If this option is not already set, or the current value conflicts with the value that you have chosen for the master server, then you should shut down your slave server, and edit the configuration to specify the server ID. For example:

[mysqld]
server-id=2

If you are setting up multiple slaves, each one must have a unique server-id value that differs from that of the master and from each of the other slaves. Think of server-id values as something similar to IP addresses: These IDs uniquely identify each server instance in the community of replication partners.

If you do not specify a server-id value, it defaults to 0.

Note

If you omit server-id (or set it explicitly to 0), a master refuses connections from all slaves, and a slave refuses to connect to a master. Thus, omitting server-id is good only for backup with a binary log.

You do not have to enable binary logging on the slave for replication to be enabled. However, if you enable binary logging on the slave then you can use the binary log for data backups and crash recovery on the slave, and also use the slave as part of a more complex replication topology (for example, where the slave acts as a master to other slaves).

16.1.1.4. Obtaining the Master Replication Information

To configure replication on the slave you must determine the master's current point within the master binary log. You will need this information so that when the slave starts the replication process, it is able to start processing events from the binary log at the correct point.

If you have existing data on your master that you want to synchronize on your slaves before starting the replication process, then you must stop processing statements on the master, obtain the current position, and then dump the data, before allowing the master to continue executing statements. If you do not stop the execution of statements, the data dump and the master status information that you use will not match and you will end up with inconsistent or corrupted databases on the slaves.

To get the master status information, follow these steps:

1. Start 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;

   For InnoDB tables, note that FLUSH TABLES WITH READ LOCK also blocks COMMIT operations.

   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. Use the SHOW MASTER STATUS statement to determine the current binary log file name and offset on the master:

   mysql > SHOW MASTER STATUS;
   +---------------+----------+--------------+------------------+
   | File          | Position | Binlog_Do_DB | Binlog_Ignore_DB |
   +---------------+----------+--------------+------------------+
   | mysql-bin.003 | 73       | test         | manual,mysql     |
   +---------------+----------+--------------+------------------+
   

   The File column shows the name of the log file and Position shows the offset within the file. In this example, the binary log file is mysql-bin.003 and the offset 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 without binary logging enabled, the log 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.

If you have existing data that needs be to synchronized with the slave before you start replication, leave the client running so that the lock remains in place and then proceed to Section 16.1.1.5, “Creating a Data Snapshot Using mysqldump”, or Section 16.1.1.6, “Creating a Data Snapshot Using Raw Data Files”.

If you are setting up a brand new master and slave replication group, then you can exit the client and release the locks.

16.1.1.5. Creating a Data Snapshot Using mysqldump

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

To obtain a snapshot of the data using mysqldump:

• If you haven't already locked the tables on the server to prevent statements that update data from executing:

  Start 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;

  Remember to use SHOW MASTER STATUS and record the binary log details for use when starting up the slave. The point in time of your snapshot and the binary log position must match. See Section 16.1.1.4, “Obtaining the Master Replication Information”.

• In another session, use mysqldump to create a dump either of all the databases you want to replicate, or of selected individual databases. For example:

  shell> mysqldump --all-databases --lock-all-tables >dbdump.db

• An alternative to using a bare dump, is to use 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

• In the client where you acquired the read lock, release the lock:

  mysql> UNLOCK TABLES;
  

When choosing databases to include in the dump, remember that you will need to filter out databases on each slave that you do not want to include in the replication process.

You will need either to copy the dump file to the slave, or to use the file from the master when connecting remotely to the slave to import the data.

16.1.1.6. Creating a Data Snapshot Using Raw Data Files

If your database is particularly large, copying the raw data files may be more efficient than using mysqldump and importing the file on each slave.

However, using this method with tables in storage engines with complex caching or logging algorithms may not give you a perfect “in time” snapshot as cache information and logging updates may not have been applied, even if you have acquired a global read lock. How the storage engine responds to this depends on its crash recovery abilities.

In addition, this method 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 fulltext indexes.

If you are using InnoDB tables, you should use the InnoDB Hot Backup tool to obtain a consistent snapshot. This tool records the log name and offset corresponding to the snapshot to be later used on the slave. Hot Backup is a non-free (commercial) tool that is not included in the standard MySQL distribution. See the InnoDB Hot Backup home page at http://www.innodb.com/hot-backup for detailed information.

Otherwise, you can obtain a reliable binary snapshot of InnoDB tables only after shutting down the MySQL Server.

To create a raw data snapshot of MyISAM tables you can use standard 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, providing that your MySQL data files exist on a single file system. If you are replicating only certain databases then make sure you copy only those files that related to those tables. (For InnoDB, all tables in all databases are stored in a single file unless you have the innodb_file_per_table option enabled.)

You may want to specifically exclude the following files from your archive:

• Files relating to the mysql database.

• The master.info file.

• The master's binary log files.

• Any relay log files.

To get the most consistent results with a raw data snapshot you should shut down the server during the process, as below:

1. Acquire a read lock and get the master's status. See Section 16.1.1.4, “Obtaining the Master Replication Information”.

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

   shell> mysqladmin shutdown
   

3. Make a copy of the MySQL data files. Examples are shown below for 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. Start up the MySQL instance on the master.

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 16.1.1.4, “Obtaining the Master Replication Information”.

2. Take a copy of the MySQL data files. Examples are shown below for common solutions - you need to choose only one of these solutions:

   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, you will need to copy the files to each slave before starting the slave replication process.

16.1.1.7. Setting Up Replication with New Master and Slaves

Setting up replication with a new master and slaves (that is, with no existing data) is the easiest and most straightforward method for setting up replication.

You can also use this method 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 will be automatically replicated to the slaves.

To set up replication between a new master and slave:

1. Configure the MySQL master with the necessary configuration properties. See Section 16.1.1.2, “Setting the Replication Master Configuration”.

2. Start up the MySQL master.

3. Set up a user. See Section 16.1.1.1, “Creating a User for Replication”.

4. Obtain the master status information. See Section 16.1.1.4, “Obtaining the Master Replication Information”.

5. On the master, release the read lock:

   mysql> UNLOCK TABLES;

6. On the slave, edit the MySQL configuration. See Section 16.1.1.3, “Setting the Replication Slave Configuration”.

7. Start up the MySQL slave.

8. Execute the CHANGE MASTER TO statement to set the master replication server configuration.

Perform the slave setup steps on each slave.

Because there is no data to load or exchange on a new server configuration you do not need to copy or import any information.

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

shell> mysql -h master < fulldb.dump

16.1.1.8. Setting Up Replication with Existing Data

When setting up replication with existing data, you will need to decide how best to get the data from the master to the slave before starting the replication service.

The basic process for setting up replication with existing data is as follows:

1. If you have not already configured the server-id and binary logging, you will need to shut down your master to configure these options. See Section 16.1.1.2, “Setting the Replication Master Configuration”.

   If you have to shut down your master server, this is a good opportunity to take a snapshot of its databases. You should obtain the master status (see Section 16.1.1.4, “Obtaining the Master Replication Information”) before taking down the master, updating the configuration and taking a snapshot. For information on how to create a snapshot using raw data files, see Section 16.1.1.6, “Creating a Data Snapshot Using Raw Data Files”.

2. If your server is already correctly configured, obtain the master status (see Section 16.1.1.4, “Obtaining the Master Replication Information”) and then use mysqldump to take a snapshot (see Section 16.1.1.5, “Creating a Data Snapshot Using mysqldump”) or take a raw snapshot of the live server using the guide in Section 16.1.1.6, “Creating a Data Snapshot Using Raw Data Files”.

3. With the MySQL master running, create a user to be used by the slave when connecting to the master during replication. See Section 16.1.1.1, “Creating a User for Replication”.

4. Update the configuration of the slave. See Section 16.1.1.3, “Setting the Replication Slave Configuration”.

5. The next step depends on how you created the snapshot of data on the master.

   If you used mysqldump:

   1. Start the slave, skipping replication by using the --skip-slave option.

   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 to match the configuration of your slave.

   2. Start the slave, skipping replication by using the --skip-slave option.

6. Configure the slave with the master status information. This will tell the slave the binary log file and position within the file where replication needs to start, and configure the login credentials and host name of the master. For more information on the statement required, see Section 16.1.1.10, “Setting the Master Configuration on the Slave”.

7. Start the slave threads:

   mysql> START SLAVE;
   

After you have performed this procedure, the slave should connect to the master and catch up on any updates that have occurred since the snapshot was taken.

If you have forgotten to set the server-id option for the master, slaves cannot connect to it.

If you have forgotten to set the server-id option for the slave, you get the following error in the slave's error log:

Warning: You should set server-id to a non-0 value if master_host
is set; we will force server id to 2, but this MySQL server will
not act as a slave.

You also find error messages in the slave's error log if it is not able to replicate for any other reason.

Once a slave is replicating, you can find in its data directory one file named master.info and another named relay-log.info. The slave uses these two files to keep track of how much of the master's binary log it has processed. Do not remove or edit these files 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 will use the values specified in the statement to update the status files automatically.

Note

The content of master.info overrides some of the server options specified on the command line or in my.cnf. See Section 16.1.3, “Replication and Binary Logging Options and Variables”, for more details.

Once you have a snapshot of the master, you can use it to set up other slaves by following the slave portion of the procedure just described. You do not need to take another snapshot of the master; you can use the same one for each slave.

16.1.1.9. Introducing Additional Slaves to an Existing Replication Environment

If you want to add another slave to the existing replication configuration then you can do so without stopping the master. Instead, you duplicate the settings on the slaves by making a copy of one of the slaves.

To duplicate the slave:

1. Shut down the existing slave:

   shell> mysqladmin shutdown

2. Copy the data directory from the existing slave to the new slave. 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. Ensure that you also copy the log files and relay log files.

   Note

   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 is due to the fact that, if the --relay-log option is not specified, 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 16.1.3, “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 feasible, then 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 a STOP SLAVE on the new slave.

      If you have already started the existing slave again, issue a 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.

    

3. Copy the master.info and relay-log.info files from the existing slave to the new slave. These files hold the current log positions.

4. Start the existing slave.

5. On the new slave, edit the configuration and the give the new slave a new unique server-id.

6. Start the new slave; the master.info file options will be used to start the replication process.

16.1.1.10. Setting the Master Configuration on the Slave

To set up the slave to communicate with the master for replication, you must tell the slave 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 following table shows the maximum allowable length for the string-valued options.

Option	Maximum Length
MASTER_HOST	60
MASTER_USER	16
MASTER_PASSWORD	32
MASTER_LOG_FILE	255

16.1.2. 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 the type of event being recorded. 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:

• Replication capabilities in MySQL originally were based on propagation of SQL statements from master to slave. This is called statement-based replication (often abbreviated as SBR), which corresponds to the standard statement-based binary logging format. In MySQL 5.1.4 and earlier, binary logging and replication used this format exclusively.

• Row-based binary logging logs changes in individual table rows. When used with MySQL replication, this is known as row-based replication (often abbreviated as RBR). In row-based replication, the master writes messages known as events to the binary log that indicate how individual table rows are changed.

• As of MySQL 5.1.8, the binary logging format can be changed in real time according to the event being logged using mixed-format logging.

  When the mixed format is in effect, statement-based logging is used by default, but automatically switches to row-based logging in particular cases as described below. Replication using the mixed format is often referred to as mixed-based replication or mixed-format replication. For more information, see Section 5.2.4.3, “Mixed Binary Logging Format”.

From MySQL 5.1.12 to MySQL 5.1.28 (inclusive), the mixed format is the default for MySQL replication. Beginning with MySQL 5.1.29, statement-based format is the default.

Note

MySQL Cluster.  The default binary logging format in all MySQL Cluster NDB 6.1, 6.2, 6.3, and later 6.x releases is ROW. MySQL Cluster Replication always uses row-based replication, and the NDBCLUSTER storage engine is incompatible with statement-based replication. Using NDBCLUSTER sets row-based logging format automatically.

See Section 17.9.2, “MySQL Cluster Replication — Assumptions and General Requirements”, for more information.

Starting with MySQL 5.1.20, 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 5.2.4.3, “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 requires a restart of the server in order to take effect. For more information, see Section 12.5.4, “SET Syntax”.

You must have the SUPER privilege to set the binary logging format on the global level. Starting with MySQL 5.1.29, you must also have the SUPER privilege to set the binary logging format for the current session. (Bug#39106)

The statement-based and row-based replication formats have different issues and limitations. For a comparison of their relative advantages and disadvantages, see Section 16.1.2.1, “Comparison 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 19.6, “Binary Logging of Stored Programs”.

If you build MySQL from source, row-based replication is available by default unless you invoke configure with the --without-row-based-replication option.

For MySQL 5.1.20 and later (and MySQL 5.0.46 for backward compatibility), the following session variables are written to the binary log and honored by the replication slave when parsing the binary log:

• sql_mode

• foreign_key_checks

• unique_checks

• character_set_client

• collation_connection

• collation_database

• collation_server

• sql_auto_is_null

Important

Even though session variables relating to character sets and collations are written to the binary log, replication between different character sets is not supported.

16.1.2.1. Comparison 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, then 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 that has existed in MySQL since 3.23.

• 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.

• Tables replicated using statement-based replication are not required to have explicit primary keys.

Disadvantages of statement-based replication:

• Statements which are unsafe for SBR.  Not all statements which modify data (such as INSERT DELETE, UPDATE, and REPLACE statements) can be replicated using statement-based replication. Any non-deterministic behavior is difficult to replicate when using statement-based replication. Examples of such DML (Data Modification Language) statements include the following:

  • A statement that depends on a UDF or stored program that is non-deterministic, since the value returned by such a UDF or stored program or depends on factors other than the parameters supplied to it. (Row-based replication, however, simply replicates the value returned by the UDF or stored program, so its effect on table rows and data is the same on both the master and slave.) See Section 16.3.1.7, “Replication of Invoked Features”, for more information.

  • DELETE and UPDATE statements that use a LIMIT clause without an ORDER BY are also non-deterministic. See Section 16.3.1.11, “Replication and LIMIT”.

  • Statements using any of the following functions cannot be replicated properly using statement-based replication:

    • LOAD_FILE()

    • UUID(), UUID_SHORT()

    • USER()

    • FOUND_ROWS()

    • SYSDATE() (unless the server is started with the --sysdate-is-now option)

    However, all other functions are replicated correctly using statement-based replication, including RAND(), NOW(), LOAD DATA INFILE, and so forth.

    For more information, see Section 16.3.1.10, “Replication and System Functions”.

  When using statement-based replication, statements that cannot be replicated correctly using statement-based mode are logged with a warning like the one shown here:

  090213 16:58:54 [Warning] Statement is not safe to log in statement format.
  

  A similar warning is also issued to the client in such cases.

• INSERT ... SELECT requires a greater number of row-level locks than with row-based replication.

• UPDATE statements that require a table scan (because no index is used in the WHERE clause) must lock a greater number of rows than with row-based replication.

• For InnoDB: An INSERT statement that uses AUTO_INCREMENT blocks other non-conflicting INSERT statements.

• For complex statements, the statement must be evaluated and executed on the slave before the rows are updated or inserted. With row-based replication, the slave only has to run the statement to apply the differences, not the full statement.

• Stored functions execute with the same NOW() value as the calling statement. However, this is not true of stored procedures.

• Deterministic UDFs must be applied on the slaves.

• If there is an error in evaluation on the slave, particularly when executing complex statements, then using statement-based replication may slowly increase the margin of error across the affected rows over time. See Section 16.3.1.20, “Slave Errors during Replication”.

• Tables must be (nearly) identical on master and slave. See Section 16.3.1.4, “Replication with Differing Tables on Master and Slave”, for more information.

Advantages of row-based replication:

• All changes can be replicated. This is the safest form of replication.

  For MySQL versions earlier than 5.1.14, DDL (Data Definition Language) statements such as CREATE TABLE are replicated using statement-based replication, while DML statements, as well as GRANT and REVOKE statements, are replicated using row-based-replication.

  In MySQL 5.1.14 and later, the mysql database is not replicated. The mysql database is instead seen as a node-specific database. Row-based replication is not supported on tables in this database. Instead, statements that would normally update this information — 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 like CREATE ... SELECT, a CREATE statement is generated from the table definition and replicated using the statement-based format, while the row insertions are replicated using the row-based format.

• The technology is the same as in most other database management systems; knowledge about other systems transfers to MySQL.

• Fewer locks are needed (and thus higher concurrency) on the master for the following types of statements:

  • INSERT ... SELECT

  • INSERT statements with AUTO_INCREMENT

  • UPDATE or DELETE statements with WHERE clauses that don't use keys or don't change most of the examined rows.

• Fewer locks are required on the slave for any INSERT, UPDATE, or DELETE statement.

Disadvantages of row-based replication:

• RBR tends to generate more data that must be logged. This is because, when using row-based replication to replicate a DML statement (such as an UPDATE or DELETE statement), each changed row must be written to the binary log. (When using statement-based replication, only the statement is written to the binary log.) This means that, 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 taking and restoring from 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.

• All tables replicated using row-based replication must have explicit primary keys.

• Deterministic UDFs that generate large BLOB values take longer to replicate with row-based replication than with statement-based replication. This is because, when using row-based replication, the BLOB column data is itself logged, rather than the statement generating the data.

• You cannot examine the logs to see what statements were executed, nor can you see on the slave what statements were received from the master and executed.

  However, beginning with MySQL 5.1.29, you can see what data was changed using mysqlbinlog with the options --base64-output=DECODE-ROWS and --verbose.

• When performing a bulk operation that includes non-transactional storage engines, changes are applied as the statement executes. With row-based replication logging, this means that the binary log is written while the statement is running. On the master, this does not cause problems with concurrency, because tables are locked until the bulk operation terminates. On the slave server, however, tables are not locked while the slave applies changes, because the slave does not know that those changes are part of a bulk operation.

  In such cases, if you retrieve data from a table on the master (for example, using SELECT * FROM table_name), the server waits for the bulk operation to complete before executing the SELECT statement, because the table is read-locked. On the slave, the server does not wait (because there is no lock). This means that, until the bulk operation on the slave has completed, you obtain different results for the same SELECT query on the master and on the slave.

  This behavior is expected to change in a future MySQL release; however, until it changes, you may prefer to use statement-based replication when your application requires concurrent large bulk inserts and selects.

16.1.2.2. Usage of Row-based Logging and Row-Based Replication

Using row-based logging or replication, rather than statement-based logging or replication, can result in major changes in the replication environment and in the behavior of applications. This section describes a number of issues known to exist when using row-based logging or row-based replication, and discusses some best practices for taking advantage of row-based logging (RBL) and row-based replication (RBR).

For additional information, see Section 16.1.2.1, “Comparison of Statement-Based and Row-Based Replication”, and Section 16.1.2, “Replication Formats”.

• RBR and primary keys.  Currently, row-based replication may fail if any table to be replicated does not have an explicit primary key. This is a known issue which we are working to fix in a future MySQL release.

  Note

  Beginning with MySQL 5.1.7, it is possible to replicate NDBCLUSTER tables without explicit primary keys. However, in the event that a MySQL Cluster data node fails, MySQL Cluster Replication can also fail for tables without explicit primary keys. For this reason, it is highly recommended that all NDBCLUSTER tables being replicated have explicit primary keys. For more information, see Section 17.9.3, “Known Issues in MySQL Cluster Replication”.

• RBL, RBR, and temporary tables.  As noted elsewhere in this chapter (see Section 16.3.1.22, “Replication and Temporary Tables”), temporary tables are not replicated when using the row-based format. However, you can use the mixed format; when mixed format is in effect, “safe” statements involving temporary tables are logged using the statement-based format. For more information, see Section 16.1.2.1, “Comparison of Statement-Based and Row-Based Replication”.

  Note

  There is actually no need to replicate temporary tables when using RBR. In addition, since 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 mode.

• RBL and the BLACKHOLE storage engine.  Prior to MySQL 5.1.29, DELETE and UPDATE statements did not work with RBL and BLACKHOLE tables. (Bug#38360)

• RBL and synchronization of non-transactional tables.  When using row-based replication of a MyISAM or other non-transactional table, changed rows are written to the transaction cache. Often, when many rows are affected, the set of changes are 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, then the rows are applied in batch mode. (This may or may not be effective, depending on the engine used for the slave's copy of the table).

• Latency and binary log size.  Because RBL writes changes for each row to the binary log, the size of the binary log can grow quite rapidly. When used in a replication environment, this can significantly increase the time required for making the 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.  With the --base64-output=DECODE-ROWS and --verbose options, mysqlbinlog is able to format the contents of the binary log in a manner that is easily human-readable, in case you want to read or recover from a replication or database failure using the contents of the binary log. For more information, see Section 4.6.7.2, “mysqlbinlog Row Event Display”. Before MySQL 5.1.28, this was not possible (Bug#31455).

• Binary log execution errors and slave_exec_mode.  If you use slave_exec_mode=IDEMPOTENT, a failure to apply changes from RBL because the original row cannot be found does not trigger an error, and does not 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 non-transactional tables when using slave_exec_mode=IDEMPOTENT and RBR can cause the master and slave to diverge even further. For more information about slave_exec_mode, see Section 5.1.3, “Server System Variables”.

  Note

  slave_exec_mode=IDEMPOTENT is generally useful only for circular replication or multi-master replication with MySQL Cluster (see Section 17.9, “MySQL Cluster Replication”). For other scenarios, the default value (slave_exec_mode=STRICT) is normally sufficient.

• Lack of binary log checksums.  No checksums are used for RBL. This means that network, disk, and other errors may not be identified when processing the binary log. To ensure that data is transmitted without network corruption, you may want to consider using SSL, which adds another layer of checksumming, for replication connections. See Section 5.5.7, “Using SSL for Secure Connections”, for more information about setting up MySQL with SSL.

• Filtering based on server ID not supported.  A common practice is to filter out changes on some slaves by using a WHERE clause that includes the relation @server_id <> server-id clause with UPDATE and DELETE statements, a simple example of such a clause being WHERE @server_id <> 1. However, this does not work correctly with row-based logging. If you must use the server_id system variable for statement filtering, then you must also use --binlog_format=STATEMENT.

• Database-level replication options.  The effects of the options --replicate-do-db, --replicate-ignore-db, and --replicate-rewrite-db differ considerably depending on whether row-based or statement-based logging is in use. Because of this, we recommend that you avoid the database-level options and use the table-level options such as --replicate-do-table and --replicate-ignore-table instead. For more information about these options and the impact that your choice of replication format has on how they operate, see Section 16.1.3, “Replication and Binary Logging Options and Variables”.

16.1.3. Replication and Binary Logging Options and Variables

The next few 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. A set of quick-reference tables providing basic information about these options and variables is also included (in the next section following this one).

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

Command Line Format	--server-id=#
Config File Format	server-id
Option Sets Variable	Yes, server_id
Variable Name	server_id
Variable Scope	Global
Dynamic Variable	Yes
Value Set	Type numeric Default 0 Range 0-4294967295

This option is common to both master and slave replication servers, and is used in replication to enable master and slave servers to identify themselves uniquely. For additional information, see Section 16.1.3.2, “Replication Master Options and Variables”, and Section 16.1.3.3, “Replication Slave Options and Variables”.

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; by “unique”, we mean that each ID must be different from every other ID in use by any other replication master or slave. Example: server-id=3.

If you omit --server-id, it assumes the default value 0, in which case a master refuses connections from all slaves, and a slave refuses to connect to a master. See Section 16.1.1.3, “Setting the Replication Slave Configuration”, for more information.

16.1.3.1. Replication and Binary Logging Option and Variable Reference

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

Table 16.1. mysqld Replication Option/Variable Summary

Name	Cmd-Line	Option file	System Var	Status Var	Var Scope	Dynamic
abort-slave-event-count	Yes	Yes
Com_change_master				Yes	Both	No
Com_show_master_status				Yes	Both	No
Com_show_new_master				Yes	Both	No
Com_show_slave_hosts				Yes	Both	No
Com_show_slave_status				Yes	Both	No
Com_slave_start				Yes	Both	No
Com_slave_stop				Yes	Both	No
disconnect-slave-event-count	Yes	Yes
have_row_based_replication			Yes		Global	No
init_slave	Yes	Yes	Yes		Global	Yes
log-slave-updates	Yes	Yes			Global	No
- Variable: log_slave_updates			Yes		Global	No
master-bind	Yes	Yes	Yes			No
master-connect-retry	Yes	Yes
master-host	Yes	Yes
master-info-file	Yes	Yes
master-password	Yes	Yes
master-port	Yes	Yes
master-retry-count	Yes	Yes
master-ssl	Yes	Yes
master-ssl-ca	Yes	Yes
master-ssl-capath	Yes	Yes
master-ssl-cert	Yes	Yes
master-ssl-cipher	Yes	Yes
master-ssl-key	Yes	Yes
master-user	Yes	Yes
relay-log	Yes	Yes
relay-log-index	Yes	Yes
- Variable: relay_log_index
relay_log_purge	Yes	Yes	Yes		Global	Yes
replicate-do-db	Yes	Yes
replicate-do-table	Yes	Yes
replicate-ignore-db	Yes	Yes
replicate-ignore-table	Yes	Yes
replicate-rewrite-db	Yes	Yes
replicate-same-server-id	Yes	Yes
replicate-wild-do-table	Yes	Yes
replicate-wild-ignore-table	Yes	Yes
report-host	Yes	Yes			Global	No
- Variable: report_host			Yes		Global	No
report-password	Yes	Yes			Global	No
- Variable: report_password			Yes		Global	No
report-port	Yes	Yes			Global	No
- Variable: report_port			Yes		Global	No
report-user	Yes	Yes			Global	No
- Variable: report_user			Yes		Global	No
rpl_recovery_rank			Yes		Global	Yes
Rpl_status				Yes	Global	No
show-slave-auth-info	Yes	Yes
skip-slave-start	Yes	Yes
slave_compressed_protocol	Yes	Yes	Yes		Global	Yes
slave_exec_mode			Yes		Global	Yes
Slave_heartbeat_period				Yes	Global	No
slave-load-tmpdir	Yes	Yes			Global	No
- Variable: slave_load_tmpdir			Yes		Global	No
slave-net-timeout	Yes	Yes			Global	Yes
- Variable: slave_net_timeout			Yes		Global	Yes
Slave_open_temp_tables				Yes	Global	No
Slave_received_heartbeats				Yes	Global	No
Slave_retried_transactions				Yes	Global	No
Slave_running				Yes	Global	No
slave-skip-errors	Yes	Yes			Global	No
- Variable: slave_skip_errors			Yes		Global	No
slave_transaction_retries	Yes	Yes	Yes		Global	Yes
sql_slave_skip_counter			Yes		Global	Yes

Section 16.1.3.2, “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 Section 16.1.3.3, “Replication Slave Options and Variables”.

Table 16.2. mysqld Binary Logging Option/Variable Summary

Name	Cmd-Line	Option file	System Var	Status Var	Var Scope	Dynamic
Binlog_cache_disk_use				Yes	Global	No
binlog_cache_size	Yes	Yes	Yes		Global	Yes
Binlog_cache_use				Yes	Global	No
binlog-do-db	Yes	Yes
binlog-format	Yes	Yes			Both	Yes
- Variable: binlog_format			Yes		Both	Yes
binlog-ignore-db	Yes	Yes
binlog-row-event-max-size	Yes	Yes
Com_show_binlog_events				Yes	Both	No
Com_show_binlogs				Yes	Both	No
max_binlog_cache_size	Yes	Yes	Yes		Global	Yes
max-binlog-dump-events	Yes	Yes
max_binlog_size	Yes	Yes	Yes		Global	Yes
sporadic-binlog-dump-fail	Yes	Yes

Section 16.1.3.4, “Binary Log 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 5.2.4, “The Binary Log”.

For a table showing all command-line options, system and status variables used with mysqld, see Section 5.1.1, “Server Option and Variable Reference”.

16.1.3.2. 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 16.1.3.4, “Binary Log Options and Variables”.

• auto_increment_increment

  Command Line Format	--auto_increment_increment[=#]
  Config File Format	auto_increment_increment
  Option Sets Variable	Yes, auto_increment_increment
  Variable Name	auto_increment_increment
  Variable Scope	Both
  Dynamic Variable	Yes
  Value Set	Type numeric Default 1 Range 1-65535

  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 non-integer value gives rise to an error, and the actual value of the variable remains unchanged.

  Note

  auto_increment_increment is supported for use with NDB tables beginning with MySQL 5.1.20, MySQL Cluster NDB 6.2.5, and MySQL Cluster NDB 6.3.2. Previously, setting it when using MySQL Cluster tables or MySQL Cluster Replication produced unpredictable results.

  These two variables 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)
    

    (Note how SHOW VARIABLES is used here to obtain the current values for these variables.)

  • 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)
    

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

  Should one or both of these variables be 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. In other words, the series is calculated like so:

  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 greatest 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 confine the effects of these two variables to a single table, and thus they do not take the place of the sequences offered by some other database management systems; 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 16.3.1.1, “Replication and AUTO_INCREMENT”.

• auto_increment_offset

  Command Line Format	--auto_increment_offset[=#]
  Config File Format	auto_increment_offset
  Option Sets Variable	Yes, auto_increment_offset
  Variable Name	auto_increment_offset
  Variable Scope	Both
  Dynamic Variable	Yes
  Value Set	Type numeric Default 1 Range 1-65535

  This variable has a default value of 1. For particulars, see the description for auto_increment_increment.

  Note

  auto_increment_offset is supported for use with NDB tables beginning with MySQL 5.1.20, MySQL Cluster NDB 6.2.5, and MySQL Cluster NDB 6.3.2. Previously, setting it when using MySQL Cluster tables or MySQL Cluster Replication produced unpredictable results.

16.1.3.3. Replication Slave Options and Variables

This section describes the server options and system variables that you can use on slave replication servers. You can specify the options either on the command line or in an option file. Many of the options can be reset while the server is running by using the CHANGE MASTER TO statement. You can 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. 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. Example: server-id=3.

Important

Certain options are handled in a special way in order to ensure that the active replication configuration is not inadvertently altered or affected:

• In MySQL 5.1.16 and earlier, these options are ignored if the master.info file exists (i.e. when the MySQL server has already previously been configured for replication). If the file exists and these options are present in the my.cnf or as options on the command line to mysqld, they are silently ignored and the information in master.info used instead.

• Options deprecated.  Beginning with MySQL 5.1.17, these options are deprecated. As of MySQL 6.0, they have been removed. In MySQL 5.1.17 and later versions of MySQL 5.1, these options have no effect when mysqld is started and an appropriate warning is written to the error log. To set the replication parameters associated with these you must use the CHANGE MASTER TO ... statement (see Section 12.6.2.1, “CHANGE MASTER TO Syntax”).

 

The options affected are shown in this list:

• --master-host

• --master-user

• --master-password

• --master-port

• --master-connect-retry

• --master-ssl

• --master-ssl-ca

• --master-ssl-capath

• --master-ssl-cert

• --master-ssl-cipher

• --master-ssl-key

 

The master.info file format in MySQL 5.1 includes values corresponding to the SSL options. In addition, the file format includes as its first line the number of lines in the file. (See Section 16.4.2, “Replication Relay and Status Files”.) If you upgrade an older server (before MySQL 4.1.1) to a newer version, the new server upgrades the master.info file to the new format automatically when it starts. However, if you downgrade a newer server to an older version, you should remove the first line manually before starting the older server for the first time.

If no master.info file exists when the slave server starts, it uses the values for those options that are specified in option files or on the command line. This occurs when you start the server as a replication slave for the very first time, or when you have run RESET SLAVE and then have shut down and restarted the slave.

If the master.info file exists when the slave server starts, the server uses its contents and ignores any options that correspond to the values listed in the file. Thus, if you start the slave server with different values of the startup options that correspond to values in the master.info file, the different values have no effect, because the server continues to use the master.info file. To use different values, you must either restart after removing the master.info file or (preferably) use the CHANGE MASTER TO statement to reset the values while the slave is running.

Suppose that you specify this option in your my.cnf file:

[mysqld]
master-host=some_host

The first time you start the server as a replication slave, it reads and uses that option from the my.cnf file. The server then records the value in the master.info file. The next time you start the server, it reads the master host value from the master.info file only and ignores the value in the option file. If you modify the my.cnf file to specify a different master host of some_other_host, the change still has no effect. You should use CHANGE MASTER TO instead.

This example shows a more extensive use of startup options to configure a pre-5.1.17 slave server:

[mysqld]
server-id=2
master-host=db-master.mycompany.com
master-port=3306
master-user=pertinax
master-password=freitag
master-connect-retry=60
report-host=db-slave.mycompany.com

Note

Because the server gives an existing master.info file precedence over the startup options just described, you might prefer not to use startup options for these values at all, and instead to specify them by using the CHANGE MASTER TO statement. Beginning with MySQL 5.1.17, you must use CHANGE MASTER TO to set the values corresponding to the deprecated options listed earlier in this section.

Startup options for replication slaves.  The following list describes startup options for controlling replication slaves. Many of these options can be reset 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

  Normally, a slave does not log to its own binary log any updates that are received from a master server. This option tells the slave to log the updates performed by its SQL thread to its own binary log. For this option to have any effect, the slave must also be started with the --log-bin option to enable binary logging. --log-slave-updates is used when you want to chain replication servers. 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. You must start both A and B with --log-bin to enable binary logging, and B with the --log-slave-updates option so that updates received from A are logged by B to its binary log.

  When using MySQL Cluster Replication prior to MySQL Cluster NDB 6.2.16 and MySQL Cluster NDB 6.3.13, records for “empty” epochs — that is, epochs in which no changes to NDBCLUSTER data or tables took place — were inserted into the ndb_apply_status and ndb_binlog_index tables on the slave even when --log-slave-updates was disabled (Bug#37472). Beginning with MySQL Cluster NDB 6.3.21 and MySQL Cluster NDB 6.4.1, it is possible to re-enable the older behavior by using the --ndb-log-empty-epochs option.

• --log-slow-slave-statements

  Version Introduced	5.1.21
  Command Line Format	--log-slow-slave-statements
  Config File Format	log-slow-slave-statements
  Value Set	Type boolean Default off

  When the slow query log is enabled, this option enables logging for queries that have taken more than long_query_time seconds to execute on the slave.

  This option was added in MySQL 5.1.21.

• --log-warnings[=level]

  This option causes a server to print more messages to the error log about what it is doing. With respect to replication, the server generates warnings that it succeeded in reconnecting after a network/connection failure, and informs you as to how each slave thread started. This option is enabled by default; to disable it, use --skip-log-warnings. Aborted connections are not logged to the error log unless the value is greater than 1.

  Note that the effects of this option are not limited to replication. It produces warnings across a spectrum of server activities.

• --master-connect-retry=seconds

  The number of seconds that the slave thread sleeps before trying to reconnect to the master in case the master goes down or the connection is lost. The value in the master.info file takes precedence if it can be read. If not set, the default is 60. Connection retries are not invoked until the slave times out reading data from the master according to the value of --slave-net-timeout. The number of reconnection attempts is limited by the --master-retry-count option.

  This option is deprecated as of MySQL 5.1.17, and removed as of MySQL 6.0.

• --master-host=host_name

  The host name or IP number of the master replication server. The value in master.info takes precedence if it can be read. If no master host is specified, the slave thread does not start.

  This option is deprecated as of MySQL 5.1.17, and removed as of MySQL 6.0.

• --master-info-file=file_name

  The name to use for the file in which the slave records information about the master. The default name is master.info in the data directory.

• --master-password=password

  The password of the account that the slave thread uses for authentication when it connects to the master. The value in the master.info file takes precedence if it can be read. If not set, an empty password is assumed.

  This option is deprecated as of MySQL 5.1.17, and removed as of MySQL 6.0.

• --master-port=port_number

  The TCP/IP port number that the master is listening on. The value in the master.info file takes precedence if it can be read. If not set, the compiled-in setting is assumed (normally 3306).

  This option is deprecated as of MySQL 5.1.17, and removed as of MySQL 6.0.

• --master-retry-count=count

  The number of times that the slave tries to connect to the master before giving up. Reconnects are attempted at intervals set by --master-connect-retry and reconnects are triggered when data reads by the slave time out according to the --slave-net-timeout option. The default value is 86400.

  You can also set the retry count by using the MASTER_CONNECT_RETRY option for the CHANGE MASTER TO statement.

• --master-ssl, --master-ssl-ca=file_name, --master-ssl-capath=directory_name, --master-ssl-cert=file_name, --master-ssl-cipher=cipher_list, --master-ssl-key=file_name

  These options are used for setting up a secure replication connection to the master server using SSL. Their meanings are the same as the corresponding --ssl, --ssl-ca, --ssl-capath, --ssl-cert, --ssl-cipher, --ssl-key options that are described in Section 5.5.7.3, “SSL Command Options”. The values in the master.info file take precedence if they can be read.

  These options are deprecated as of MySQL 5.1.17, and removed as of MySQL 6.0.

• --master-user=user_name

  The user name of the account that the slave thread uses for authentication when it connects to the master. This account must have the REPLICATION SLAVE privilege. The value in the master.info file takes precedence if it can be read. If the master user name is not set, the name test is assumed.

  This option is deprecated as of MySQL 5.1.17, and removed as of MySQL 6.0.

• --max-relay-log-size=size

  The size at which the server rotates relay log files automatically. For more information, see Section 16.4.2, “Replication Relay and Status Files”. The default size is 1GB.

• --read-only

  Cause the slave to allow no updates except from slave threads or from users having the SUPER privilege. On a slave server, this can be useful to ensure that the slave accepts updates only from its master server and not from clients. This variable does not apply to TEMPORARY tables.

• --relay-log=file_name

  The basename for the relay log. The default basename is host_name-relay-bin. The server creates relay log files in sequence by adding a numeric suffix to the basename.

  Due to the manner in which MySQL parses server options, if you specify this option, you must supply a value; the default basename 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 4.2.3, “Specifying Program Options”.

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

  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.

• --relay-log-index=file_name

  The name to use for the relay log index file. The default name is host_name-relay-bin.index in the data directory, where host_name is the name of the slave server.

  Due to the manner in which MySQL parses server options, if you specify this option, you must supply a value; the default basename 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 4.2.3, “Specifying Program Options”.

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

• --relay-log-info-file=file_name

  The name to use for the file in which the slave records information about the relay logs. The default name is relay-log.info in the data directory.

• --relay-log-purge={0|1}

  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.

• --relay-log-space-limit=size

  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

  The effects of this option depend on whether statement-based or row-based replication is in use.

  Statement-based replication.  Tell the slave 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 will be 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 to restrict replication to database db_name. Only tables belonging to db_name are changed; the current database has no effect on this. For example, 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 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 current 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 16.4.3, “How Servers Evaluate Replication Rules”.

  Note

  This option effects replication in the same manner that --binlog-do-db affects binary logging, and the affects 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.

• --replicate-ignore-db=db_name

  As with --replicate-do-db, the effects of this option depend on whether statement-based or row-based replication is in use.

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

  Row-based replication.  Tells the slave not to update any tables in the database db_name. The current 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.

  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.

  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 16.4.3, “How Servers Evaluate Replication Rules”.

  If you need cross-database updates to work, use --replicate-wild-ignore-table=db_name.% instead. See Section 16.4.3, “How Servers Evaluate Replication Rules”.

  Note

  This option effects replication in the same manner that --binlog-ignore-db affects binary logging, and the affects 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.

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

  Tells the slave thread to restrict replication to the specified table. To specify more than one table, use this option multiple times, once for each table. This works for cross-database updates, in contrast to --replicate-do-db. See Section 16.4.3, “How Servers Evaluate Replication Rules”.

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

  Tells the slave thread to not 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 16.4.3, “How Servers Evaluate Replication Rules”.

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

  Tells the slave to translate 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. This does not work for cross-database updates. 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.

  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"
  

• --replicate-same-server-id

  To be used on slave servers. Usually you should use the default setting of 0, to prevent infinite loops caused by circular replication. If set to 1, the slave does not skip events having its own server ID. Normally, this is useful only in rare configurations. Cannot be set to 1 if --log-slave-updates is used. 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

  Tells 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 16.4.3, “How Servers Evaluate Replication Rules”.

  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're using 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

  Tells the slave thread not to replicate a statement where 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 16.4.3, “How Servers Evaluate Replication Rules”.

  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

  The host name or IP number 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 that it is not sufficient for the master to simply read the IP number 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

  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 --show-slave-auth-info option is given.

• --report-port=slave_port_num

  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 non-default 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.

• --report-user=user_name

  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 --show-slave-auth-info option is given.

• --show-slave-auth-info

  Display slave user names and passwords in the output of SHOW SLAVE HOSTS on the master server for slaves started with the --report-user and --report-password options.

• --skip-slave-start

  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_compressed_protocol={0|1}

  If this option is set to 1, use compression for the slave/master protocol if both the slave and the master support it. The default is 0 (no compression).

• --slave-load-tmpdir=file_name

  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. When the slave SQL thread replicates a LOAD DATA INFILE 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) because the temporary files used to replicate LOAD DATA INFILE must survive machine restarts. The directory also should not be one that is cleared by the operating system during the system startup process.

• --slave-net-timeout=seconds

  The number of seconds to wait for more data from the master before the slave considers the connection broken, aborts the read, and tries to reconnect. 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 or --master-connect-retry option, and the number of reconnection attempts is limited by the --master-retry-count option. The default is 3600 seconds (one hour).

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

  Normally, replication stops when an error occurs on the slave. This gives you the opportunity to resolve the inconsistency in the data manually. This option tells 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.

  Note

  Prior to MySQL 5.1.35, this option had no effect with row-based logging. (Bug#39393)

  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.

  You can also (but should not) use the very non-recommended 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
  

• sql_slave_skip_counter

  Variable Name	sql_slave_skip_counter
  Variable Scope	Global
  Dynamic Variable	Yes
  Value Set	Type numeric

  The number of events from the master that a slave server should skip.

  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. See Section 12.6.2.6, “SET GLOBAL SQL_SLAVE_SKIP_COUNTER Syntax”, for more information.

• --abort-slave-event-count

  Command Line Format	--abort-slave-event-count=#
  Config File Format	abort-slave-event-count
  Value Set	Type numeric Default 0 Min 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 allowed 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.

  This option is used internally by the MySQL test suite for replication testing and debugging. It is not intended for use in a production setting.

• --disconnect-slave-event-count

  Command Line Format	--disconnect-slave-event-count=#
  Config File Format	disconnect-slave-event-count
  Value Set	Type numeric Default 0

  This option is used internally by the MySQL test suite for replication testing and debugging.

System variables used on replication slaves.  The following system variables are used for controlling replication slave servers. Those that can be set are specified using SET. Server options used with replication slaves are listed earlier in this section.

• init_slave

  Command Line Format	--init-slave=name
  Config File Format	init_slave
  Option Sets Variable	Yes, init_slave
  Variable Name	init_slave
  Variable Scope	Global
  Dynamic Variable	Yes
  Value Set	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.

  Note

  The SQL thread sends an acknowledgement to the client before init_slave is executed. Therefore, it is not guaranteed that init_slave has been executed when START SLAVE returns. See Section 12.6.2.7, “START SLAVE Syntax”, for more information.

• rpl_recovery_rank

  This variable is unused.

• slave_compressed_protocol

  Command Line Format	--slave_compressed_protocol
  Config File Format	slave_compressed_protocol
  Option Sets Variable	Yes, slave_compressed_protocol
  Variable Name	slave_compressed_protocol
  Variable Scope	Global
  Dynamic Variable	Yes
  Value Set	Type boolean Default FALSE

  Whether to use compression of the slave/master protocol if both the slave and the master support it.

• slave_exec_mode

  Version Introduced	5.1.24
  Variable Name	slave_exec_mode
  Variable Scope	Global
  Dynamic Variable	Yes
  Value Set	Type enumeration Default STRICT Valid Values IDEMPOTENT, STRICT

  Controls whether IDEMPOTENT or STRICT mode is used in replication conflict resolution and error checking. IDEMPOTENT mode causes suppression of duplicate-key and no-key-found errors. Beginning with MySQL 5.1.23-ndb-6.2.14 and MySQL 5.1.24, this mode should be employed in multi-master replication, circular replication, and some other special replication scenarios. STRICT mode is the default, and is suitable for most other cases.

• slave_load_tmpdir

  Command Line Format	--slave-load-tmpdir=name
  Config File Format	slave-load-tmpdir
  Option Sets Variable	Yes, slave_load_tmpdir
  Variable Name	slave_load_tmpdir
  Variable Scope	Global
  Dynamic Variable	No
  Value Set	Type filename Default /tmp

  The name of the directory where the slave creates temporary files for replicating LOAD DATA INFILE statements.

• slave_net_timeout

  Command Line Format	--slave-net-timeout=#
  Config File Format	slave-net-timeout
  Option Sets Variable	Yes, slave_net_timeout
  Variable Name	slave_net_timeout
  Variable Scope	Global
  Dynamic Variable	Yes
  Value Set	Type numeric Default 3600 Min Value 1

  The number of seconds to wait for more data from a master/slave connection before aborting the read. This timeout applies only to TCP/IP connections, not to connections made via Unix socket files, named pipes, or shared memory.

• slave_skip_errors

  Command Line Format	--slave-skip-errors=name
  Config File Format	slave-skip-errors
  Option Sets Variable	Yes, slave_skip_errors
  Variable Name	slave_skip_errors
  Variable Scope	Global
  Dynamic Variable	No

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

• sql_slave_skip_counter

  Variable Name	sql_slave_skip_counter
  Variable Scope	Global
  Dynamic Variable	Yes
  Value Set	Type numeric

  The number of events from the master that a slave server should skip.

  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. See Section 12.6.2.6, “SET GLOBAL SQL_SLAVE_SKIP_COUNTER Syntax”, for more information.

• slave_transaction_retries

  Command Line Format	--slave_transaction_retries=#
  Config File Format	slave_transaction_retries
  Option Sets Variable	Yes, slave_transaction_retries
  Variable Name	slave_transaction_retries
  Variable Scope	Global
  Dynamic Variable	Yes
  Value Set	Platform Bit Size 32 Type numeric Default 10 Range 0-4294967295
  Value Set	Platform Bit Size 64 Type numeric Default 10 Range 0-18446744073709547520

  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 NDBCLUSTER's TransactionDeadlockDetectionTimeout or TransactionInactiveTimeout, it automatically retries slave_transaction_retries times before stopping with an error. The default value is 10.

16.1.3.4. Binary Log 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 5.2.4, “The Binary Log”. For additional information about using MySQL server options and system variables, see Section 5.1.2, “Server Command Options”, and Section 5.1.3, “Server System Variables”.

Startup options used with binary logging.  The following list describes startup options for enabling and configuring the binary log. Many of these options can be reset while the server is running by using the CHANGE MASTER TO statement. Others, can be set only when the slave server starts. System variables used with binary logging are discussed later in this section.

• --binlog-row-event-max-size=N

  Version Introduced	5.1.5
  Command Line Format	--binlog-row-event-max-size=#
  Config File Format	binlog-row-event-max-size
  Value Set	Platform Bit Size 32 Type numeric Default 1024 Range 256-4294967295
  Value Set	Platform Bit Size 64 Type numeric Default 1024 Range 256-18446744073709547520

  Specify the maximum size of a row-based binary log event, in bytes. Rows are grouped into events smaller than this size if possible. The value should be a multiple of 256. The default is 1024. See Section 16.1.2, “Replication Formats”. This option was added in MySQL 5.1.5.

• --log-bin[=base_name]

  Command Line Format	--log-bin
  Config File Format	log-bin
  Variable Name	log_bin
  Variable Scope	Global
  Dynamic Variable	No
  Value Set	Type filename Default OFF

  Enable binary logging. The server logs all statements that change data to the binary log, which is used for backup and replication. See Section 5.2.4, “The Binary Log”.

  The option value, if given, is the basename for the log sequence. The server creates binary log files in sequence by adding a numeric suffix to the basename. It is recommended that you specify a basename (see Section B.1.8.2, “Additional Known Issues”, for the reason). Otherwise, MySQL uses host_name-bin as the basename.

• --log-bin-index[=file_name]

  Command Line Format	--log-bin-index=name
  Config File Format	log-bin-index
  Value Set	Type filename Default OFF

  The index file for binary log file names. See Section 5.2.4, “The Binary Log”. If you omit the file name, and if you didn't specify one with --log-bin, MySQL uses host_name-bin.index as the file name.

• 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.

  • --binlog-do-db=db_name

    This option affects binary logging in the same manner 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.

    Statement-based logging.  Only those statements where the default database (that is, the one selected by USE) is db_name are written to the binary log. 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, if you supply a comma separated list then the list will be treated as the name of a single database.

    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.

    Row-based logging.  Logging is restricted to database db_name. Only changes to tables belonging to db_name are logged; the current database has no effect on this. For example, suppose that the server is started with --replicate-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 february table in the sales database is changed in accordance with the UPDATE statement; this occurs whether or not the USE statement was issued. However, the following statements are not logged when using the row-based logging format and --binlog-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 written to the binary log.

    Another important difference in how --binlog-do-db is handled when using the statement-based logging format as opposed to the row-based format occurs with regard to statements that refer to multiple databases. Suppose 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, then the updates to both tables are written to the binary log. However, when using the row-based format, only the changes to table1 logged; since table2 is in a different database, 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 would not be written to the binary log when using statement-based logging. However, if using row-based logging, the UPDATE would change table1, but not table2 — in other words, only tables in the database named by --binlog-do-db are changed, and the choice of current database has no effect on this behavior.

  • --binlog-ignore-db=db_name

    This option affects binary logging in the same manner 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.

    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.

    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, if you supply a comma separated list then the list will be treated as the name of a single database.

    You should not use this option if you are using cross-database updates and you do not want these updates to be logged.

  Additional server options that can be used to control logging also affect the binary log. For more information about these, see Section 5.1.2, “Server Command Options”. For more information about how the options in the previous list are applied, see Section 5.2.4, “The Binary Log”.

  There are also options for slave servers that control which statements received from the master should be executed or ignored. For details, see Section 16.1.3.3, “Replication Slave Options and Variables”.

• --log-bin-trust-function-creators[={0|1}]

  Command Line Format	--log-bin-trust-function-creators
  Config File Format	log-bin-trust-function-creators
  Option Sets Variable	Yes, log_bin_trust_function_creators
  Variable Name	log_bin_trust_function_creators
  Variable Scope	Global
  Dynamic Variable	Yes
  Value Set	Type boolean Default FALSE

  With no argument or an argument of 1, this option sets the log_bin_trust_function_creators system variable to 1. With an argument of 0, this option sets the system variable to 0. log_bin_trust_function_creators affects how MySQL enforces restrictions on stored function and trigger creation. See Section 19.6, “Binary Logging of Stored Programs”.

  Note

  Previously, this option was known as --log-bin-trust-routine-creators, which is now deprecated.

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

  Command Line Format	--max-binlog-dump-events=#
  Config File Format	max-binlog-dump-events
  Value Set	Type numeric Default 0

  This option is used internally by the MySQL test suite for replication testing and debugging.

• --sporadic-binlog-dump-fail

  Command Line Format	--sporadic-binlog-dump-fail
  Config File Format	sporadic-binlog-dump-fail
  Value Set	Type boolean Default FALSE

  This option is used internally by the MySQL test suite for replication testing and debugging.

System variables used with the binary log.  The following system variables are used for controlling replication slave servers. Those that can be set are specified using SET. Server options used with replication slaves are listed earlier in this section.

• binlog_cache_size

  Command Line Format	--binlog_cache_size=#
  Config File Format	binlog_cache_size
  Option Sets Variable	Yes, binlog_cache_size
  Variable Name	binlog_cache_size
  Variable Scope	Global
  Dynamic Variable	Yes
  Value Set	Platform Bit Size 32 Type numeric Default 32768 Range 4096-4294967295
  Value Set	Platform Bit Size 64 Type numeric Default 32768 Range 4096-18446744073709547520

  The size of the cache to hold the SQL statements for the binary log during a transaction. A binary log cache is allocated for each client if the server supports any transactional storage engines and if the server has the binary log enabled (--log-bin option). If you often use large, multiple-statement transactions, you can increase this cache size to get more performance. The Binlog_cache_use and Binlog_cache_disk_use status variables can be useful for tuning the size of this variable. See Section 5.2.4, “The Binary Log”.

  MySQL Enterprise.  For recommendations on the optimum setting for binlog_cache_size subscribe to the MySQL Enterprise Monitor. For more information, see http://www.mysql.com/products/enterprise/advisors.html.

• binlog_format

  Version Introduced	5.1.5
  Command Line Format	--binlog-format
  Config File Format	binlog-format
  Option Sets Variable	Yes, binlog_format
  Variable Name	binlog_format
  Variable Scope	Both
  Dynamic Variable	Yes
  Value Set (>= 5.1.5, <= 5.1.7)	Type enumeration Default STATEMENT Valid Values ROW, STATEMENT
  Value Set (>= 5.1.8, <= 5.1.11)	Type enumeration Default STATEMENT Valid Values ROW, STATEMENT, MIXED
  Value Set (>= 5.1.12, <= 5.1.28)	Type enumeration Default MIXED Valid Values ROW, STATEMENT, MIXED
  Value Set (>= 5.1.29)	Type enumeration Default STATEMENT Valid Values ROW, STATEMENT, MIXED

  This variable sets the binary logging format, and can be any one of STATEMENT, ROW, or MIXED. binlog_format is set by the --binlog-format option at startup, or by the binlog_format variable at runtime.

  You must have the SUPER privilege to set this variable, which (unlike with most system variables) is true as of MySQL 5.1.29 even for the session value. See Section 16.1.2, “Replication Formats”.

  The startup variable was added in MySQL 5.1.5, and the runtime variable in MySQL 5.1.8. MIXED was added in MySQL 5.1.8.

  The rules governing when changing this variable takes effect and how long the effect lasts are the same as for other MySQL server system variables. See Section 12.5.4, “SET Syntax”, for more information.

  STATEMENT was used by default prior to MySQL 5.1.12; in MySQL 5.1.12, the default was changed to MIXED. In MySQL 5.1.29, the default was changed back to STATEMENT.

  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. An exception to this rule is that MIXED always uses statement-based replication for stored functions and triggers.

  As with other global variables, to set binlog_format globally, you must have the SUPER privilege. Starting with MySQL 5.1.29, you must also have the SUPER privilege to set binlog_format on the session level. (Bug#39106)

  There are exceptions when you cannot switch the replication format at runtime:

  • From within a stored function or a trigger.

  • If the NDBCLUSTER storage engine is enabled.

  • If the session is currently in row-based replication mode and has open temporary tables.

  Trying to switch the format in those cases results in an error.

  Before MySQL 5.1.8, switching to row-based replication format would implicitly set --log-bin-trust-function-creators=1 and --innodb_locks_unsafe_for_binlog. MySQL 5.1.8 and later no longer implicitly set these options when row-based replication is used.

  The binlog format affects the behavior of the following server options:

  • --replicate-do-db

  • --replicate-ignore-db

  • --binlog-do-db

  • --binlog-ignore-db

  These effects are discussed in detail in the descriptions of the individual options.

• max_binlog_cache_size

  Command Line Format	--max_binlog_cache_size=#
  Config File Format	max_binlog_cache_size
  Option Sets Variable	Yes, max_binlog_cache_size
  Variable Name	max_binlog_cache_size
  Variable Scope	Global
  Dynamic Variable	Yes
  Value Set	Platform Bit Size 32 Type numeric Default 4294967295 Range 4096-4294967295
  Value Set	Platform Bit Size 64 Type numeric Default 18446744073709547520 Range 4096-18446744073709547520

  If a multiple-statement 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 and default values are 4GB on 32-bit platforms and 16 PB (petabytes) on 64-bit platforms.

• max_binlog_size

  Command Line Format	--max_binlog_size=#
  Config File Format	max_binlog_size
  Option Sets Variable	Yes, max_binlog_size
  Variable Name	max_binlog_size
  Variable Scope	Global
  Dynamic Variable	Yes
  Value Set	Type numeric Default 1073741824 Min Value 4096

  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). You cannot set this variable to more than 1GB or to less than 4096 bytes. The default value is 1GB.

  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 logs larger than max_binlog_size.

  If max_relay_log_size is 0, the value of max_binlog_size applies to relay logs as well.

• sync_binlog

  Command Line Format	--sync-binlog=#
  Config File Format	sync-binlog
  Option Sets Variable	Yes, sync_binlog
  Variable Name	sync_binlog
  Variable Scope	Global
  Dynamic Variable	Yes
  Value Set	Platform Bit Size 32 Type numeric Default 0 Range 0-4294967295
  Value Set	Platform Bit Size 64 Type numeric Default 0 Range 0-18446744073709547520

  If the value of this variable is greater than 0, the MySQL server synchronizes its binary log to disk (using fdatasync()) after every sync_binlog writes to the binary log. There is one write to the binary log per statement if autocommit is enabled, and one write per transaction otherwise. The default value of sync_binlog is 0, which does no synchronizing to disk — in this case, the server relies on the operating system to flush the binary log's contents from 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 statement or transaction from the binary log. However, it is also the slowest choice (unless the disk has a battery-backed cache, which makes synchronization very fast).

16.1.4. Common Replication Administration Tasks

Once replication has been started it should execute without requiring much regular administration. Depending on your replication environment, you will want to check the replication status of each slave either periodically, daily, or even more frequently.

MySQL Enterprise.  For regular reports regarding the status of your slaves, subscribe to the MySQL Enterprise Monitor. For more information, see http://www.mysql.com/products/enterprise/advisors.html.

16.1.4.1. 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 primary statement for this is SHOW SLAVE STATUS which you must execute on each slave:

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
1 row in set (0.01 sec)

The key fields from the status report to examine are:

• Slave_IO_State — indicates the current status of the slave. See Section 7.5.6.5, “Replication Slave I/O Thread States”, and Section 7.5.6.6, “Replication Slave SQL Thread States”, for more information.

• Slave_IO_Running — shows whether the IO thread for the reading the master's binary log is running.

• Slave_SQL_Running — shows whether the SQL thread for the executing events in the relay log is running.

• Last_Error — shows the last error registered when processing the relay log. Ideally this should be blank, indicating no errors.

• Seconds_Behind_Master — shows 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 cope with the large number of statements from the master.

  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 peiod, you may see this value change back and forth repeatedly between 0 and a relatively large value.

On the master, you can check the status of slaves by examining the list of running processes. Slaves execute the Binlog Dump command:

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 core of the replication process, very little information is available in this report.

If you have used the --report-host option, then the SHOW SLAVE HOSTS statement will show basic information about connected slaves:

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)

The output includes the ID of the slave server, the value of the --report-host option, the connecting port, master ID and the priority of the slave for receiving binary log updates.

16.1.4.2. Pausing Replication on the Slave

You can stop and start the replication of statements on the slave using the STOP SLAVE and START SLAVE statements.

To stop execution of the binary log from the master, use STOP SLAVE:

mysql> STOP SLAVE;

When execution is stopped, the slave does not read the binary log from the master (the IO_THREAD) and stops processing events from the relay log that have not yet been executed (the SQL_THREAD). You can pause either the IO or SQL threads individually by specifying the thread type. For example:

mysql> STOP SLAVE IO_THREAD;

Stopping the SQL thread can be useful if you want to perform a backup or other task on a slave that only processes events from the master. The IO thread will continue to be read from the master, but not executed, which will make it easier for the slave to catch up when you start slave operations again.

Stopping the IO thread will allow the statements in the relay log to be executed up until the point where the relay log has ceased to receive new events. Using this option can be useful when you want to pause execution to allow the slave to catch up with events from the master, when you want to perform administration on the slave but also ensure you have the latest updates to a specific point. This method can also be used to pause execution on the slave while you conduct administration on the master while ensuring that there is not a massive backlog of events to be executed when replication is started again.

To start execution again, use the START SLAVE statement:

mysql> START SLAVE;

If necessary, you can start either the IO_THREAD or SQL_THREAD threads individually.