DB Instance Types

The smallest management unit of RDS is DB instance. A DB instance is an isolated database environment on the cloud. Each DB instance can contain multiple user-created databases, and you can access a DB instance using the same tools and applications that you use with a stand-alone DB instance. You can easily create or modify DB instances using the management console or HTTPS-compliant application programming interfaces (APIs). RDS does not have limits on the number of running DB instances. Each DB instance has a unique identifier.

DB instances are classified into the following types.

**Table 1** DB instance types
DB Instance Type	Description	Notes
Single-node	A single-node architecture is less expensive than a primary/standby DB pair.	If a fault occurs on a single-node instance, the instance cannot recover in a timely manner.
Primary/Standby	An HA architecture. In a primary/standby pair, each instance has the same instance class. The primary and standby instances can be deployed in different AZs.	When a primary instance is being created, a standby instance is provisioned synchronously to provide data redundancy. The standby instance is invisible to you after being created. If a failover occurs due to a primary instance failure, your database client will be disconnected from the instance briefly and then reconnects to the instance. The default replication mode between the primary and standby instances is semi-synchronous.
Read replica	A single-node or HA architecture	If the replication between a read replica (single-node or HA) and the DB instance is abnormal, it can take a long time to rebuild and restore the read replica (depending on the data volume). If the physical server where the primary read replica is deployed fails, the standby read replica automatically takes over workloads. When you purchase a read replica, select the same value for Table Name as that of the DB instance.