An Aurora cluster already stores its data across Availability Zones, and the only database instance is the writer in one zone. Loss of that zone would leave no instance to promote. What should be added?
Select an answer to reveal the explanation.
Short Explanation
Aurora already keeps the data itself in more than one zone, like a book stored in several libraries. You still need a reader in another zone so failover has an instance to promote. A second writer in the writer's zone is not how this cluster provides a standby.
Full Explanation
Aurora already replicates the cluster volume across Availability Zones, so the data is not pinned to the writer's zone. An Aurora cluster has one writer. Failover promotes a reader, and with no reader in another zone there is no instance ready to promote if that zone is lost. Adding an Aurora reader in another Availability Zone gives failover a target. The storage layer does not need another copy for this step. A second writer in the same zone is not how this cluster is built, and another instance in the writer's zone would not survive the loss of that zone. A single-AZ disk copy is a new failure domain of one zone, and failover does not mount that disk in place of an Aurora reader. A reader placed only beside the writer does not start storage replication. That replication is already in place, and a same-zone reader fails with the writer. Caveat: the reader is a failover target, not a second writer. The cluster still has one primary after you add it. Operational check: confirm the cluster shows a reader in a different Availability Zone from the writer and that the reader is available.