Multi-Primary and Single-Primary Modes •If the existing primary leaves the group, whether voluntarily or unexpectedly, a new primary is elected automatically. •You can appoint a specific member as the new primary using the group_replication_set_as_primary()  function. •If you use the  group_replication_switch_to_single_primary_mode()  function to change a group that was running in multi-primary mode to run in single-primary mode, a new primary is elected automatically, or you can appoint the new primary by specifying it with the function. The functions can only be used when all group members are running MySQL 8.0.13 or higher. When a new primary server is elected automatically or appointed manually, it is automatically set to read-write, and the other group members remain as secondaries, and as such, read-only. Figure 18.4, “New Primary Election”  shows this process. Figure 18.4 New Primary Election When a new primary is elected or appointed, it might have a backlog of changes that had been applied on the old primary but have not yet been applied on this server. In this situation, until the new primary catches up with the old primary, read-write transactions might result in conflicts and be rolled back, and read-only transactions might result in stale reads. Group Replication’s flow control mechanism, which minimizes the difference between fast and slow members, reduces the chances of this happening if it is activated and properly tuned. For more information on flow control, see Section 18.7.2, “Flow Control” . From MySQL 8.0.14, you can also use the  group_replication_consistency  system variable to configure the group’s level of transaction consistency to prevent this issue. The setting BEFORE_ON_PRIMARY_FAILOVER  (or any higher consistency level) holds new transactions on a newly elected primary until the backlog has been applied. For more information on transaction consistency, see Section 18.5.2, “Transaction Consistency Guarantees” . If flow control and transaction consistency guarantees are not used for a group, it is a good practice to wait for the new primary to apply its replication- related relay log before re-routing client applications to it. The automatic primary member election process involves each member looking at the new view of the group, ordering the potential new primary members, and choosing the member that qualifies as the most suitable. Each member makes its own decision locally, following the primary election algorithm in its MySQL Server release. Because all members must reach the same decision, members adapt their primary election algorithm if other group members are running lower MySQL Server versions, so that they have the same behavior as the member with the lowest MySQL Server version in the group. Primary Election Algorithm 3694