Scenario answers

Multi-AZ load balancer with one-AZ targets

Distribute healthy targets and sufficient surviving capacity across AZs. The load balancer alone does not make the application highly available.

Read replica versus Multi-AZ

Use Multi-AZ for managed in-Region availability. Use replicas primarily for read scaling and explicit promotion or DR.

Replication versus backup

Replication can copy unwanted writes and deletes. Keep protected backups and point-in-time recovery.

DNS failover

Move traffic only after data, capacity, application version, secrets, certificates, dependencies, and observability are ready.