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 you might use to access a standalone database. 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 more cost-effective 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 along with it to provide data redundancy. The standby instance is invisible to you after being created. If a failover occurs due to a primary instance failure, there is a brief interruption between your database client and the instance. The client needs to be able to reconnect to the instance. RDS for PostgreSQL uses asynchronous replication by default.
Read replica	A single-node architecture (without a standby node)	To reduce read pressure on a primary DB instance, you can create one or more read replicas in the same region as the primary instance. These read replicas can handle a large number of read requests, thereby increasing the throughput of your application.