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. Replication mode: RDS for SQL Server uses synchronous replication between the primary and standby instances. SQL Server 2017 Enterprise Edition and 2019 Enterprise Edition use Always On availability groups (AGs). Other editions use database mirroring. RDS for SQL Server uses asynchronous replication between the primary instance and read replicas.
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.
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 RDS for SQL Server 2022 Enterprise Edition, 2019 Enterprise Edition, and 2017 Enterprise Edition support cluster instances.