In multi-primary mode (group_replication_single_primary_mode=OFF) no member has a special role. Any member that is compatible with the other group members is set to read-write mode when joining the group, and can process write transactions, even if they are issued concurrently.

If a member stops accepting write transactions, for example, in the event of an unexpected server exit, clients connected to it can be redirected, or failed over, to any other member that is in read-write mode. Group Replication does not handle client-side failover itself, so you need to arrange this using a middleware framework such as MySQL Router 8.0, a proxy, a connector, or the application itself. Figure 18.5, “Client Failover” shows how clients can reconnect to an alternative group member if a member leaves the group.

Group Replication is an eventual consistency system. This means that as soon as the incoming traffic slows down or stops, all group members have the same data content. While traffic is flowing, transactions can be externalized on some members before the others, especially if some members have less write throughput than others, creating the possibility of stale reads. In multi-primary mode, slower members can also build up an excessive backlog of transactions to certify and apply, leading to a greater risk of conflicts and certification failure. To limit these issues, you can activate and tune Group Replication's flow control mechanism to minimize the difference between fast and slow members. For more information on flow control, see Section 18.7.2, “Flow Control”.

From MySQL 8.0.14, if you want to have a transaction consistency guarantee for every transaction in the group, you can do this using the group_replication_consistency system variable. You can choose a setting that suits the workload of your group and your priorities for data reads and writes, taking into account the performance impact of the synchronization required to increase consistency. You can also set the system variable for individual sessions to protect particularly concurrency-sensitive transactions. For more information on transaction consistency, see Section 18.5.2, “Transaction Consistency Guarantees”.