20.5.4.3 Configuring Distributed Recovery

Several aspects of Group Replication's distributed recovery process can be configured to suit your system.

Number of Connection Attempts

For state transfer from the binary log, Group Replication limits the number of attempts a joining member makes when trying to connect to a donor from the pool of donors. If the connection retry limit is reached without a successful connection, the distributed recovery procedure terminates with an error. Note that this limit specifies the total number of attempts that the joining member makes to connect to a donor. For example, if 2 group members are suitable donors, and the connection retry limit is set to 4, the joining member makes 2 attempts to connect to each of the donors before reaching the limit.

The default connection retry limit is 10. You can configure this setting using the group_replication_recovery_retry_count system variable. The following command sets the maximum number of attempts to connect to a donor to 5:

mysql> SET GLOBAL group_replication_recovery_retry_count= 5;

For remote cloning operations, this limit does not apply. Group Replication makes only one connection attempt to each suitable donor for cloning, before starting to attempt state transfer from the binary log.

Sleep Interval for Connection Attempts

For state transfer from the binary log, the group_replication_recovery_reconnect_interval system variable defines how much time the distributed recovery process should sleep between donor connection attempts. Note that distributed recovery does not sleep after every donor connection attempt. As the joining member is connecting to different servers and not to the same one repeatedly, it can assume that the problem that affects server A does not affect server B. Distributed recovery therefore suspends only when it has gone through all the possible donors. Once the server joining the group has made one attempt to connect to each of the suitable donors in the group, the distributed recovery process sleeps for the number of seconds configured by the group_replication_recovery_reconnect_interval system variable. For example, if 2 group members are suitable donors, and the connection retry limit is set to 4, the joining member makes one attempt to connect to each of the donors, then sleeps for the connection retry interval, then makes one further attempt to connect to each of the donors before reaching the limit.

The default connection retry interval is 60 seconds, and you can change this value dynamically. The following command sets the distributed recovery donor connection retry interval to 120 seconds:

mysql> SET GLOBAL group_replication_recovery_reconnect_interval= 120;

For remote cloning operations, this interval does not apply. Group Replication makes only one connection attempt to each suitable donor for cloning, before starting to attempt state transfer from the binary log.

Marking the Joining Member Online

When distributed recovery has successfully completed state transfer from the donor to the joining member, the joining member can be marked as online in the group and ready to participate. By default, this is done after the joining member has received and applied all the transactions that it was missing. Optionally, you can allow a joining member to be marked as online when it has received and certified (that is, completed conflict detection for) all the transactions that it was missing, but before it has applied them. If you want to do this, use the group_replication_recovery_complete_at system variable to specify the alternative setting TRANSACTIONS_CERTIFIED.