Semisynchronous replication is implemented using plugins, so the plugins must be installed into the server to make them available. After a plugin has been installed, you control it by means of the system variables associated with it. These system variables are unavailable until the associated plugin has been installed.

This section describes how to install the semisynchronous replication plugins. For general information about installing plugins, see Section 5.5.2, “Installing and Uninstalling Plugins”.

To use semisynchronous replication, the following requirements must be satisfied:

To set up semisynchronous replication, use the following instructions. The INSTALL PLUGIN, SET GLOBAL, STOP SLAVE, and START SLAVE statements mentioned here require the SUPER privilege.

MySQL distributions include semisynchronous replication plugin files for the master side and the slave side.

To be usable by a master or slave server, the appropriate plugin library file must be located in the MySQL plugin directory (the directory named by the plugin_dir system variable). If necessary, set the value of plugin_dir at server startup to tell the server the plugin directory location.

The plugin library file base names are semisync_master and semisync_slave. The file name suffix differs per platform (for example, .so for Unix and Unix-like systems, .dll for Windows).

The master plugin library file must be present in the plugin directory of the master server. The slave plugin library file must be present in the plugin directory of each slave server.

To load the plugins, use the INSTALL PLUGIN statement on the master and on each slave that is to be semisynchronous (adjust the .so suffix for your platform as necessary).

On the master:

INSTALL PLUGIN rpl_semi_sync_master SONAME 'semisync_master.so';

On each slave:

INSTALL PLUGIN rpl_semi_sync_slave SONAME 'semisync_slave.so';

If an attempt to install a plugin results in an error on Linux similar to that shown here, you must install libimf:

mysql> INSTALL PLUGIN rpl_semi_sync_master SONAME 'semisync_master.so';
ERROR 1126 (HY000): Can't open shared library
'/usr/local/mysql/lib/plugin/semisync_master.so'
(errno: 22 libimf.so: cannot open shared object file:
No such file or directory)

You can obtain libimf from http://dev.mysql.com/downloads/os-linux.html.

To see which plugins are installed, use the SHOW PLUGINS statement, or query the INFORMATION_SCHEMA.PLUGINS table.

To verify plugin installation, examine the INFORMATION_SCHEMA.PLUGINS table or use the SHOW PLUGINS statement (see Section 5.5.3, “Obtaining Server Plugin Information”). For example:

mysql> SELECT PLUGIN_NAME, PLUGIN_STATUS
       FROM INFORMATION_SCHEMA.PLUGINS
       WHERE PLUGIN_NAME LIKE '%semi%';
+----------------------+---------------+
| PLUGIN_NAME          | PLUGIN_STATUS |
+----------------------+---------------+
| rpl_semi_sync_master | ACTIVE        |
+----------------------+---------------+

If the plugin fails to initialize, check the server error log for diagnostic messages.

After a semisynchronous replication plugin has been installed, it is disabled by default. The plugins must be enabled both on the master side and the slave side to enable semisynchronous replication. If only one side is enabled, replication will be asynchronous.

To control whether an installed plugin is enabled, set the appropriate system variables. You can set these variables at runtime using SET GLOBAL, or at server startup on the command line or in an option file.

At runtime, these master-side system variables are available:

SET GLOBAL rpl_semi_sync_master_enabled = {0|1};
SET GLOBAL rpl_semi_sync_master_timeout = N;

On the slave side, this system variable is available:

SET GLOBAL rpl_semi_sync_slave_enabled = {0|1};

For rpl_semi_sync_master_enabled or rpl_semi_sync_slave_enabled, the value should be 1 to enable semisynchronous replication or 0 to disable it. By default, these variables are set to 0.

For rpl_semi_sync_master_timeout, the value N is given in milliseconds. The default value is 10000 (10 seconds).

If you enable semisynchronous replication on a slave at runtime, you must also start the slave I/O thread (stopping it first if it is already running) to cause the slave to connect to the master and register as a semisynchronous slave:

STOP SLAVE IO_THREAD;
START SLAVE IO_THREAD;

If the I/O thread is already running and you do not restart it, the slave continues to use asynchronous replication.

At server startup, the variables that control semisynchronous replication can be set as command-line options or in an option file. A setting listed in an option file takes effect each time the server starts. For example, you can set the variables in my.cnf files on the master and slave sides as follows.

On the master:

[mysqld]
rpl_semi_sync_master_enabled=1
rpl_semi_sync_master_timeout=1000 # 1 second

On each slave:

[mysqld]
rpl_semi_sync_slave_enabled=1