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	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. Do not use single-node instances in the production environment.
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. 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	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). Database proxy is recommended for read-intensive workloads. Before using database proxy, ensure that you have purchased more than one read replica. If a single read replica is faulty, database proxy can distribute traffic to other read replicas.
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.

You can use RDS to create and manage DB instances running various DB engines.

For details about differences and function comparison between different instance types, see RDS for MySQL Product Series, RDS for PostgreSQL Product Series, and RDS for SQL Server Product Series.

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