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. 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 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 and RDS for MariaDB: semi-synchronous RDS for PostgreSQL: asynchronous RDS for SQL Server: synchronous. SQL Server 2017 Enterprise Edition and 2019 Enterprise Edition use AlwaysOn 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 or HA architecture	Read replicas include single read replicas and HA read replicas. Single read replicas: If you choose single read replicas, you are advised to buy more than one single read replica and enable database proxy. That way, if one read replica fails, the database proxy can route traffic to other read replicas. HA read replicas: If the physical server where a primary read replica is deployed fails, the standby read replica automatically takes over the workloads. When you purchase a read replica, select the same value for Table Name as the DB instance. If the replication between a read replica (single 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).
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.