RDS Read Replicas vs Multi-AZ Deployments

Read Replicas

• Asynchronous replication - Highly Scalable.

• Can be within a region or within an AZ or cross-region.

• Provide enhanced performance and durability for RDS database instances.

• They make it easy to elastically scale out beyond the capacity constraints of a single DB instance for read-heavy database workloads.

• Read replicas can also be promoted when needed to become standalone database instances.

Multi-AZ

• Synchronous replication - Highly durable.

• Always spans 2 AZs within a region.

• Provide enhanced availability and durability for RDS database instances.

• When you provision a Multi-AZ DB Instance, RDS automatically creates a primary DB Instance and synchronously replicates the data to a standby instance in a different Availability Zone.

• Each AZ runs on its own physically distinct, independent infrastructure, and is engineered to be highly reliable.

• In case of an infrastructure failure, RDS performs an automatic failover to the standby (or to a read replica in the case of Aurora), so that you can resume database operations as soon as the failover is complete.