16.3.9.1 Semisynchronous Replication Administrative Interface

The administrative interface to semisynchronous replication has several components:

Two plugins implement semisynchronous capability. There is one plugin for the source side and one for the replica side.
System variables control plugin behavior. Some examples:
- rpl_semi_sync_master_enabled
  
  Controls whether semisynchronous replication is enabled on the source. To enable or disable the plugin, set this variable to 1 or 0, respectively. The default is 0 (off).
- rpl_semi_sync_master_timeout
  
  A value in milliseconds that controls how long the source waits on a commit for acknowledgment from a replica before timing out and reverting to asynchronous replication. The default value is 10000 (10 seconds).
- rpl_semi_sync_slave_enabled
  
  Similar to rpl_semi_sync_master_enabled, but controls the replica plugin.
All rpl_semi_sync_xxx system variables are described at Section 16.1.6.2, “Replication Source Options and Variables” and Section 16.1.6.3, “Replica Server Options and Variables”.
From MySQL 5.7.33, you can improve the performance of semisynchronous replication by enabling the system variables replication_sender_observe_commit_only, which limits callbacks, and replication_optimize_for_static_plugin_config, which adds shared locks and avoids unnecessary lock acquisitions. These settings help as the number of replicas increases, because contention for locks can slow down performance. Semisynchronous replication source servers can also get performance benefits from enabling these system variables, because they use the same locking mechanisms as the replicas.
Status variables enable semisynchronous replication monitoring. Some examples:
- Rpl_semi_sync_master_clients
  
  The number of semisynchronous replicas.
- Rpl_semi_sync_master_status
  
  Whether semisynchronous replication currently is operational on the source. The value is 1 if the plugin has been enabled and a commit acknowledgment has not occurred. It is 0 if the plugin is not enabled or the source has fallen back to asynchronous replication due to commit acknowledgment timeout.
- Rpl_semi_sync_master_no_tx
  
  The number of commits that were not acknowledged successfully by a replica.
- Rpl_semi_sync_master_yes_tx
  
  The number of commits that were acknowledged successfully by a replica.
- Rpl_semi_sync_slave_status
  
  Whether semisynchronous replication currently is operational on the replica. This is 1 if the plugin has been enabled and the replication I/O thread is running, 0 otherwise.
All Rpl_semi_sync_xxx status variables are described at Section 5.1.9, “Server Status Variables”.

The system and status variables are available only if the appropriate source or replica plugin has been installed with INSTALL PLUGIN.