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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 has a group membership service that defines which servers 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 servers are the members participating actively in the group at a given moment in time.

Group members must agree not only on transaction commits, but also on which is the current view. If existing members agree that a new server should become part of the group, the group is reconfigured to integrate that server in it, which triggers a view change. If a server leaves the group, either voluntarily or not, the group dynamically rearranges its configuration and a view change is triggered.

In the case where a member leaves the group voluntarily, it first initiates a dynamic group reconfiguration, during which all members have to agree on a new view without the leaving server. However, if a member leaves the group involuntarily, for example because it has stopped unexpectedly or the network connection is down, it cannot initiate the reconfiguration. In this situation, Group Replication's failure detection mechanism recognizes after a short period of time that the member has left, and a reconfiguration of the group without the failed member is proposed. As with a member that leaves voluntarily, the reconfiguration requires agreement from the majority of servers in the group. However, if the group is not able to reach agreement, for example because it partitioned in such a way that there is no majority of servers online, the system is not able to dynamically change the configuration, and blocks to prevent a split-brain situation. This situation requires intervention from an administrator.

It is possible for a member to go offline for a short time, then attempt to rejoin the group again before the failure detection mechanism has detected its failure, and before the group has been reconfigured to remove the member. In this situation, the rejoining member forgets its previous state, but if other members send it messages that are intended for its pre-crash state, this can cause issues including possible data inconsistency. If a member in this situation participates in XCom's consensus protocol, it could potentially cause XCom to deliver different values for the same consensus round, by making a different decision before and after failure.

To counter this possibility, from MySQL 5.7.22 and in MySQL 8.0, servers are given a unique identifier when they join a group. This enables Group Replication to be aware of the situation where a new incarnation of the same server (with the same address but a new identifier) is trying to join the group while its old incarnation is still listed as a member. The new incarnation is blocked from joining the group until the old incarnation can be removed by a reconfiguration. If the group_replication_member_expel_timeout system variable has been set to allow additional time for members to return to the group before they are expelled, a member under suspicion can rejoin as long as it has not actually failed. If Group Replication is stopped and restarted on the server, the member becomes a new incarnation and cannot rejoin until the suspicion times out.