A random donor is selected from the existing online members in the group. This way there is a good chance that the same server is not selected more than once when multiple members enter the group.

If the connection to the selected donor fails, a new connection is automatically attempted to a new candidate donor. Once the connection retry limit is reached the recovery procedure terminates with an error.

The other main point of concern in recovery as a whole is to make sure that it copes with failures. Hence, Group Replication provides robust error detection mechanisms. In earlier versions of Group Replication, when reaching out to a donor, recovery could only detect connection errors due to authentication issues or some other problem. The reaction to such problematic scenarios was to switch over to a new donor thus a new connection attempt was made to a different member.

This behavior was extended to also cover other failure scenarios:

The recovery data transfer relies on the binary log and existing MySQL replication framework, therefore it is possible that some transient errors could cause errors in the receiver or applier threads. In such cases, the donor switch over process has retry functionality, similar to that found in regular replication.

The number of attempts a server joining the group makes when trying to connect to a donor from the pool of donors is 10. This is configured through the group_replication_recovery_retry_count plugin variable . The following command sets the maximum number of attempts to connect to a donor to 10.

mysql> SET GLOBAL group_replication_recovery_retry_count= 10;

Note that this accounts for the global number of attempts that the server joining the group makes connecting to each one of the suitable donors.

The group_replication_recovery_reconnect_interval plugin variable defines how much time the recovery process should sleep between donor connection attempts. This variable has its default set to 60 seconds and you can change this value dynamically. The following command sets the recovery donor connection retry interval to 120 seconds.

mysql> SET GLOBAL group_replication_recovery_reconnect_interval= 120;

Note, however, that recovery does not sleep after every donor connection attempt. As the server joining the group is connecting to different servers and not to the same one over and over again, it can assume that the problem that affects server A does not affect server B. As such, recovery suspends only when it has gone through all the possible donors. Once the server joining the group has tried to connect to all the suitable donors in the group and none remains, the recovery process sleeps for the number of seconds configured by the group_replication_recovery_reconnect_interval variable.