RDS for SQL Server Product Series

RDS for SQL Server DB instances are classified into the following types:

**Table 1** DB instance types
DB Instance Type	Description	Notes	Scenarios
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.	Personal learning Microsites Development and testing environment of small- and medium-sized enterprises
Primary/Standby	An HA architecture. A pair of primary and standby DB instances shares the same IP address and 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 the primary instance fails, a failover occurs, during which database connection is interrupted. If there is a replication delay between the primary and standby instances, the failover takes an extended period of time. The client needs to be able to reconnect to the instance.	Production databases of large and medium enterprises Applications for the Internet, Internet of Things (IoT), retail e-commerce sales, logistics, gaming, and other industries
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.	Finance industry Internet industry Hotel industry Online education

Single DB instances: support the creation of read replicas and support the queries of error logs and slow query logs. Different from primary/standby DB instances that have two database nodes, a single DB instance has only one node, reducing the price to half of primary/standby DB instances. If the node fails, the restoration will take a long time. Therefore, single DB instances are not recommended for sensitive services that have high requirements on database availability.
Primary/Standby DB instances: use the standby database node only for failover and restoration. The standby database node does not provide services. The performance of single DB instances is similar to or even higher than primary/standby DB instances because standby nodes cause extra performance overhead.
Cluster instances: use the Microsoft Always On architecture with one primary node, one standby node, and up to five read-only nodes. It features higher availability, reliability, and scalability.

**Table 2** Function comparisons
Function	Single	Primary/Standby	Cluster
Number of nodes	1	2	2
Specifications	vCPUs: a maximum of 64 Memory: a maximum of 512 GB Storage: a maximum of 4,000 GB	vCPUs: a maximum of 64 Memory: a maximum of 512 GB Storage: a maximum of 4,000 GB	vCPUs: a maximum of 64 Memory: a maximum of 512 GB Storage: a maximum of 4 TB
Monitoring and alarms	Supported	Supported	Supported
Security group	Supported	Supported	Supported
Backup and restoration	Supported	Supported	Supported
Recycle bin	Supported	Supported	Supported
Parameter settings	Supported	Supported	Supported
SSL	Supported	Supported	Supported
Log management	Supported	Supported	Supported
Read replicas (which need to be created)	Not supported	Not supported	Supported
High-frequency monitoring	Supported	Supported	Supported
Failover	Not supported	Supported	Supported
Standby DB instance migration	Not supported	Supported	Supported
Manual primary/standby switchover	Not supported	Supported	Supported
Instance class change	Supported	Supported	Supported