DB Instance Types

The smallest management unit of RDS is the 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 DB instance identifier.

DB instances are classified into the following types.

**Table 1** DB instance types
DB Instance Type	Description	Notes
Single	A single-node architecture is more cost-effective than a primary/standby DB pair.	If a fault occurs on a single 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.	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 briefly. You need to reconnect the client to the instance.
Read replica	A single-node architecture (without a standby node)	A read replica is a single-node instance. If the physical server hosting a read replica is faulty or database replication between the read replica and its primary instance is abnormal, it takes a long time to rebuild and restore the read replica (depending on the data volume).