DB Instance Types

Single-node

It uses a standalone architecture, which is more cost-effective than mainstream primary/standby instances.

If a fault occurs on a single-node instance, the instance cannot recover in a timely manner. Do not use single-node instances in any production environment.

Primary/Standby

It uses an HA architecture. The primary and standby instances can be deployed in different AZs. The primary and standby instances share the same IP address.

• Standby instances improve instance reliability. When you create a primary instance, a standby instance is automatically created alongside it. Once created, the standby instance is invisible on the console.
• If the primary instance fails, a failover is automatically triggered, during which clients may experience a brief interruption. If there is a replication delay between the primary and standby instances, the failover takes an extended period of time. Clients need to be able to reconnect to the instance.
• The default replication mode between the primary and standby instances is as follows:
  • RDS for MySQL: semi-synchronous
  • RDS for PostgreSQL: asynchronous
  • RDS for SQL Server uses asynchronous replication between the primary instance and read replicas.

Read replica

It uses a standalone architecture (without a standby node).

• Read replicas help reduce read pressure on single-node or primary/standby instances, but they cannot be purchased separately. For a single-node read replica, if the physical server hosting it fails or if replication with its primary instance is abnormal, you need to rebuild and restore the read replica. The recovery time will vary depending on the data volume.
• Read replicas extend read capabilities. To reduce read pressure on a primary instance, you can create one or more read replicas in the same region as the primary instance. They can handle a large number of read requests, thereby increasing the throughput of your application.

Cluster

The Microsoft Always On architecture. There is one primary node, one standby node, and up to five read-only nodes. It features higher availability, reliability, and scalability.

Only for RDS for SQL Server.