Introduction to Read Replicas

What Is a Read Replica?

In read-intensive scenarios, a single DB instance may be unable to handle the read pressure and workloads may be affected. To offload read pressure from a primary instance, you can create one or more read replicas in the same region as the primary instance. These read replicas can process a large number of read requests and increase application throughput.

Read replicas replicate data from their primary instance with a slight delay but the data on read replicas will eventually be consistent with that on the primary instance. Use them for workloads where this delay is acceptable.

Billing

Read replicas are billed on a pay-per-use basis.

Functions

• Read replica specifications can be different from primary DB instance specifications. It is recommended that the read replica specifications be greater than or equal to the primary DB instance specifications to prevent long delay and high load.
• Pay-per-use billing is supported. You only pay for what you use.
• Read replicas support system performance monitoring.

  RDS provides up to 20 monitoring metrics, including storage space, IOPS, database connections, CPU usage, and network traffic. You can view these metrics to learn about the load on read replicas.

Constraints

• A maximum of five read replicas can be created for a DB instance. To create more read replicas, submit a service ticket to request permissions. You can create up to 10 read replicas for each DB instance.
• Read replicas cannot be purchased separately. You can purchase them only for your created instance.
• All databases and tables in the primary instance are synchronized to read replicas. Data of the primary instance, standby instance, and read replicas is consistent.
• Read replicas do not support automated backups or manual backups.
• Read replicas do not support restoration from backups to new, existing, or original read replicas.
• Data cannot be migrated to read replicas.
• Read replicas do not support database creation and deletion.
• Read replicas do not support database account creation. Create database accounts on the primary DB instance. For details, see Creating a Database Account.
• Read replicas cannot be recycled after they are deleted.