Oracle starts maintenance of read replicas along with the source DB system.

• Standalone DB systems: The read replicas are upgraded in parallel with the DB system, and hence the source DB system and the read replicas go offline for the upgrade at the same time. See Maintenance of a Standalone DB System.

  Any read replica with a MySQL version lower than the source DB system is automatically set to NEEDS_ATTENTION state, and you must delete the read replica and create a new one. You cannot access a read replica in NEEDS_ATTENTION state from the read replica load balancer, but you can access it directly using the read replica endpoint.

• High availability DB systems: The read replicas are upgraded one by one using the rolling maintenance process. Hence only one read replica goes offline at a time. Once all read replicas are upgraded successfully, the source DB system is upgraded. See Maintenance of a High Availability DB System.

  If the upgrade of any read replica fails, the rolling maintenance process of the high availability DB system stops immediately to prevent having replicas with a MySQL version lower than the source DB system.