What is the difference between an RDS read replica and a Multi-AZ standby instance, and when should you use each?

A read replica is an asynchronously replicated copy of your database that can serve read only traffic, helping to offload read heavy workloads from your primary database and improve overall read performance, while a Multi-AZ standby instance exists purely for high availability and failover purposes and cannot serve read traffic directly, meaning you would use read replicas specifically to scale read capacity and Multi-AZ specifically to protect against an availability failure, and many production databases actually use both together for different purposes.

aws rds create-db-instance-read-replica --db-instance-identifier my-read-replica --source-db-instance-identifier my-database