7.5.5 Configuring Automatic Rejoin of Instances

Instances running MySQL 8.0.16 and later support the Group Replication automatic rejoin functionality, which enables you to configure instances to automatically rejoin the cluster after being expelled. See Responses to Failure Detection and Network Partitioning for background information. AdminAPI provides the autoRejoinTries option to configure the number of tries instances make to rejoin the cluster after being expelled. By default instances do not automatically rejoin the cluster. You can configure the autoRejoinTries option at either the cluster level or for an individual instance using the following commands:

• dba.createCluster()

• Cluster.addInstance()

• Cluster.setOption()

• Cluster.setInstanceOption()

The autoRejoinTries option accepts positive integer values between 0 and 2016 and the default value is 3. When using the automatic rejoin functionality, your cluster is more tolerant to faults, especially temporary ones such as unreliable networks. But if quorum has been lost, you should not expect members to automatically rejoin the cluster, because majority is required to rejoin instances.

Instances running MySQL version 8.0.12 and later have the group_replication_exit_state_action variable, which you can configure using the AdminAPI exitStateAction option. This controls what instances do in the event of leaving the cluster unexpectedly. By default the exitStateAction option is READ_ONLY, which means that instances which leave the cluster unexpectedly become read-only. If exitStateAction is set to OFFLINE_MODE, instances which leave the cluster unexpectedly become read-only and also enter offline mode, where they disconnect existing clients and do not accept new connections (except from clients with administrator privileges). If exitStateAction is set to ABORT_SERVER then in the event of leaving the cluster unexpectedly, the instance shuts down MySQL, and it has to be started again before it can rejoin the cluster. Note that when you are using the automatic rejoin functionality, the action configured by the exitStateAction option only happens in the event that all attempts to rejoin the cluster fail.

There is a chance you might connect to an instance and try to configure it using the AdminAPI, but at that moment the instance could be rejoining the cluster. This could happen whenever you use any of these operations:

• Cluster.status()

• dba.getCluster()

• Cluster.rejoinInstance()

• Cluster.addInstance()

• Cluster.removeInstance()

• Cluster.rescan()

These operations might provide extra information while the instance is automatically rejoining the cluster. In addition, when you are using Cluster.removeInstance(), if the target instance is automatically rejoining the cluster the operation aborts unless you pass in force:true.