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 create and modify DB instances using the management console or 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	Uses a single-node architecture. More cost-effective than primary/standby DB instances.	If a fault occurs on a single instance, the instance cannot recover in a timely manner.
Primary/Standby	Uses an HA architecture. A pair of primary and standby instances 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 for a short period of time. You need to reconnect the client to the instance.
Read replica	Uses 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.