Use the ReplicaSet.setPrimaryInstance() operation to safely perform a change of the primary of a ReplicaSet to another instance. The current primary is demoted to a secondary and made read-only, while the promoted instance becomes the new primary and is made read-write. All other secondary instances are updated to replicate from the new primary. MySQL Router instances, which have been bootstrapped against the ReplicaSet automatically start redirecting read-write clients to the new primary.
For a safe change of the primary to be possible, all ReplicaSet instances must be reachable by MySQL Shell and have consistent GTID_EXECUTED sets. If the primary is not available, and there is no way to restore it, a forced failover might be the only option instead, see Section 9.7, “Forcing a New Primary Instance”.
During a change of primary instance, the promoted instance is synchronized with the old primary, ensuring that all transactions present on the primary are applied before the topology change is committed. If this synchronization step takes too long or is not possible on any of the secondary instances, the operation is aborted. In such a situation, the secondary instances must be repaired or removed from the ReplicaSet for the failover to be possible.
