RDS Read Replicas

• Multiple database engines have built-in replication functionality to create a special type of database instance called a read replica from a source database instance.

• The source DB instance becomes the primary database instance.

• Updates made to the primary database instance are asynchronously copied to the read replica.

• You can reduce the load on your primary database instance by routing read queries from your applications to the read replica.

• You can promote a read replica into a standalone database instance.

  • When you promote a read replica, the database instance is rebooted before it becomes available.

• The following are where read-replicas are useful:

  • Scaling beyond the compute or I/O capacity of a single DB instance for read-heavy database workloads.

    • You can direct this excess read traffic to one or more read replicas.

  • Serving read traffic while the source database instance is unavailable.

    • In these cases, you can direct read traffic to your read replicas.

      • For this use case, keep in mind that the data on the read replica might be "stale" because the source database instance is unavailable.

  • Business reporting or data warehousing scenarios.

    • You might want business reporting queries to run against a read replica, rather than your production database instance.

  • Implementing disaster recovery.

    • You can promote a read replica to a standalone instance as a disaster recovery solution if the primary database instance fails.

• Data goes from 1 AZ to another = Data Charges.

• Data stays in the same AZ = Free.