Function Comparison

Single DB instances use the single-node architecture. In contrast to the mainstream primary/standby DB instances, a single DB instance contains only one node and has no slave node for fault recovery.

Advantage Comparison

Single DB instances: supports the creation of read replicas (except for Microsoft SQL Server) and supports 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. If a 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: uses the slave database node only for failover and restoration. The slave database node does not provide services. The performance of single DB instances is similar to or even higher than the primary/standby DB instances.

**Table 1** Function comparisons
DB Engine	Single	Primary/Standby
MySQL	8.0 5.7 5.6	8.0 5.7 5.6
PostgreSQL	11 10 9.6 9.5	11 10 9.6 9.5
Number of nodes	1	2
Specifications	vCPUs: a maximum of 76 Memory: a maximum of 304 GB Storage space: a maximum of 4 TB	vCPUs: a maximum of 76 Memory: a maximum of 304 GB Storage space: a maximum of 4 TB
Monitoring and alarms	Supported	Supported
Security group	Supported	Supported
Backups and restorations	Supported	Supported
Parameter settings	Supported	Supported
SSL	Supported	Supported
Log management	Supported	Supported
Read replicas (need to be created)	Supported	Supported
High-frequency monitoring	Supported	Supported
Primary/standby switchover or failover	Not supported	Supported
Standby DB instance migration	Not supported	Supported