18.1.4.1 Group Membership

In MySQL Group Replication, a set of servers forms a replication group. A group has a name, which takes the form of a UUID. The group is dynamic and servers can leave (either voluntarily or involuntarily) and join it at any time. The group adjusts itself whenever servers join or leave.

If a server joins the group, it automatically brings itself up to date by fetching the missing state from an existing server. If a server leaves the group, for instance it was taken down for maintenance, the remaining servers notice that it has left and reconfigure the group automatically.

Group Replication provides a group membership service built into the plugin, which defines the servers that are online and participating in the group. The list of online servers is referred to as a view. Every server in the group has a consistent view of which are the members participating actively in the group at a given moment in time.

Servers have to agree not only on transaction commits, but also which is the current view. Therefore, if servers agree that a new server becomes part of the group, then the group itself is reconfigured to integrate that server in it, triggering a view change. The opposite also happens, if a server leaves the group, voluntarily or not, then the group dynamically rearranges its configuration and a view change is triggered.

Note though that when a member leaves voluntarily, it first initiates a dynamic group reconfiguration. This triggers a procedure, where all members have to agree on the new view without the leaving server. However, if a member leaves involuntarily (for example it has stopped unexpectedly or the network connection is down) then the failure detection mechanism realizes this fact and a reconfiguration of the group is proposed, this one without the failed member. As mentioned this requires agreement from the majority of servers in the group. If the group is not able to reach agreement (for example it partitioned in such a way that there is no majority of servers online), then the system is not be able to dynamically change the configuration and as such, blocks to prevent a split-brain situation. Ultimately, this means that the administrator needs to step in and fix this.