DatabasesTopology

RDS Multi-AZ with Read Replica

RDSMulti-AZRead ReplicaVPC

Use Case

High availability RDS setup with automatic failover via Multi-AZ Standby and read scaling via asynchronous Read Replica — two mechanisms often confused but serving distinct purposes.

Design Decisions

▸Primary DB handles all writes and synchronous replication to the Standby
▸Standby DB is passive — it only activates during a failover event
▸Read Replica receives async replication from Primary and serves read-only traffic
▸Application routes write traffic to Primary and read traffic to Read Replica

Trade-offs

–Multi-AZ Standby cannot serve read traffic — it exists purely for HA
–Read Replica has slight replication lag due to async nature — not suitable for reads requiring strong consistency
–Running both Multi-AZ and Read Replica increases RDS cost significantly