Does RDS Support Cross-AZ High Availability?

Yes. When you buy a DB instance, you can select Primary/Standby for DB Instance Type and then select different AZs for Primary AZ and Standby AZ.

RDS does not support 3-AZ deployment.

An AZ is a physical region where resources have independent power supplies and networks. AZs are physically isolated but interconnected through an internal network. You can deploy your instance across AZs in some regions.

To achieve higher reliability, if you deploy the primary and standby instances in the same AZ, RDS will automatically deploy the primary and standby instances in different physical servers. If you attempt to deploy your primary and standby instances in the same AZ in a Dedicated Computing Cluster (DCC) and there is only one physical server available, the creation will fail.

RDS allows you to deploy primary/standby DB instances in an AZ or across AZs. You can determine whether the standby AZ is the same as the primary AZ.

• If they are different (default setting), the primary and standby instances are deployed in different AZs to ensure failover support and high availability.
• If they are the same, the primary and standby instances are deployed in the same AZ. If an AZ failure occurs, high availability cannot be ensured.

Figure 1 Cross-AZ high availability