RDS

Amazon RDS Multi-AZ is designed to improve database availability and resilience.

RDS maintains a standby database in a different Availability Zone and synchronously replicates changes from the primary. If the primary becomes unavailable, RDS can automatically fail over to the standby.

Simple mental model

Multi-AZ → "What if my database fails?"
Read Replica → "What if I have too many reads?"

With RDS Multi-AZ, you generally don't switch between primary and standby in the application. RDS handles the failover behind the endpoint, and the application reconnects to the same endpoint.

Amazon RDS Read Replicas are read-only copies of an RDS database that help handle read-heavy workloads.

The primary database handles writes, while changes are asynchronously replicated to one or more read replicas. Applications can send read queries to the replicas, reducing the load on the primary.

Simple mental model

Read Replica → "I need more read capacity."
Multi-AZ → "I need higher database availability."

If the requirement says read traffic is increasing or the database is overloaded by reads → think Read Replicas.

If the requirement is automatic failover when the primary database or AZ fails → think Multi-AZ.

One important detail: because replication is asynchronous, a Read Replica can temporarily have replication lag and be slightly behind the primary.

For RDS Read Replicas, the application generally decides where to send reads and writes. The application might have separate database connections:

write_db  → primary
read_db   → read replica

But Multi-AZ is different

With RDS Multi-AZ, you generally don't switch between primary and standby in the application.

RDS handles the failover behind the endpoint, and the application reconnects to the same endpoint.

Simple distinction:

Read Replica → application chooses where reads/writes go.
Multi-AZ → RDS handles primary/standby failover.