3.10.3 Semisynchronous Replication Monitoring

The plugins for semisynchronous replication expose a number of status variables that enable you to monitor their operation. To check the current values of the status variables, use SHOW STATUS:

mysql> SHOW STATUS LIKE 'Rpl_semi_sync%';

From MySQL 8.0.26, new versions of the source and replica plugins are supplied, which replace the terms “master” and “slave” with “source” and “replica” in system variables and status variables. If you install the new rpl_semi_sync_source and rpl_semi_sync_replica plugins, the new system variables and status variables are available but the old ones are not. If you install the old rpl_semi_sync_master and rpl_semi_sync_slave plugins, the old system variables and status variables are available but the new ones are not. You cannot have both the new and the old version of the relevant plugin installed on an instance.

All Rpl_semi_sync_xxx status variables are described at Server Status Variables. Some examples are:

• Rpl_semi_sync_source_clients or Rpl_semi_sync_master_clients

  The number of semisynchronous replicas that are connected to the source server.

• Rpl_semi_sync_source_status or Rpl_semi_sync_master_status

  Whether semisynchronous replication currently is operational on the source server. 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_source_no_tx or Rpl_semi_sync_master_no_tx

  The number of commits that were not acknowledged successfully by a replica.

• Rpl_semi_sync_source_yes_tx or Rpl_semi_sync_master_yes_tx

  The number of commits that were acknowledged successfully by a replica.

• Rpl_semi_sync_replica_status or 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 (receiver) thread is running, 0 otherwise.

When the source switches between asynchronous or semisynchronous replication due to commit-blocking timeout or a replica catching up, it sets the value of the Rpl_semi_sync_source_status or Rpl_semi_sync_master_status status variable appropriately. Automatic fallback from semisynchronous to asynchronous replication on the source means that it is possible for the rpl_semi_sync_source_enabled or rpl_semi_sync_master_enabled system variable to have a value of 1 on the source side even when semisynchronous replication is in fact not operational at the moment. You can monitor the Rpl_semi_sync_source_status or Rpl_semi_sync_master_status status variable to determine whether the source currently is using asynchronous or semisynchronous replication.