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Introducing Read Replicas

Updated at: May 26, 2020 GMT+08:00

Introduction

Currently, RDS for MySQL 5.6 and 5.7 support read replicas and read/write splitting.

In read-intensive scenarios, a single DB instance may be unable to handle the read pressure and service performance may be affected. To expand the DB instance read ability to offload read pressure on the database, you can create one or more read replicas in a region. These read replicas can process a large number of read requests and increase application throughput.

You can enable read/write splitting after read replicas are created. Write requests are automatically routed to the primary DB instance and read requests are routed to each read replica by user-defined weights.

If read/write splitting is disabled, you need to separately configure connection addresses of the primary DB instance and each read replica on your applications so that all read requests can be sent to read replicas and write requests to the primary DB instance.

A read replica uses the architecture of a single physical node (without a slave node). Changes to the primary DB instance are also automatically synchronized to all associated read replicas through the native replication function of MySQL. The synchronization is not affected by network latency. Read replicas and the primary DB instance must be in the same region but can be in different AZs.

Billing Standards

Read replicas are additionally billed on the pay-per-use basis.

Functions

  • Specifications of read replicas can be different from those of the primary DB instance, and can be changed at any time to facilitate flexible scaling.
  • You do not need to maintain accounts or databases. Both of them are synchronized from the primary DB instance.
  • Read replicas support system performance monitoring.

    RDS provides up to 20 monitoring metrics, including storage space, IOPS, number of database connections, CPU usage, and network traffic. You can view these metrics to understand the load of DB instances.

Constraints

  • A maximum of five read replicas can be created for a primary DB instance. RDS for MySQL supports a maximum of 10 read replicas. You need to contact customer service to apply for the permission to create up to 10 read replicas.
  • Read replicas do not support backup settings or temporary backups.
  • Read replicas do not support using backup files to restore data to the original DB instance or create a DB instance.
  • Data cannot be migrated to read replicas.
  • Read replicas do not support database creation and deletion.
  • Read replicas do not support account creation. You can create accounts only on primary DB instances.
  • Ultra-high performance (advanced) DB instances support the creation of ultra-high performance (advanced) read replicas only. General-enhanced DB instances support the creation of general-enhanced read replicas only.

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