Using cascading read replicas with RDS for PostgreSQL

As of version 14.1, RDS for PostgreSQL supports cascading read replicas. With cascading read replicas, you can scale reads without adding overhead to your source RDS for PostgreSQL DB instance. Updates to the WAL log aren't sent by the source DB instance to each read replica. Instead, each read replica in a cascading series sends WAL log updates to the next read replica in the series. This reduces the burden on the source DB instance.

With cascading read replicas, your RDS for PostgreSQL DB instance sends WAL data to the first read replica in the chain. That read replica then sends WAL data to the second replica in the chain, and so on. The end result is that all read replicas in the chain have the changes from the RDS for PostgreSQL DB instance, but without the overhead solely on the source DB instance.

You can create a series of up to three read replicas in a chain from a source RDS for PostgreSQL DB instance. For example, suppose that you have an RDS for PostgreSQL 14.1 DB instance, rpg-db-main. You can do the following:

• Starting with rpg-db-main, create the first read replica in the chain, read-replica-1.

• Next, from read-replica-1, create the next read replica in the chain, read-replica-2.

• Finally, from read-replica-2, create the third read replica in the chain, read-replica-3.

You can't create another read replica beyond this third cascading read replica in the series for rpg-db-main. A complete series of instances from an RDS for PostgreSQL source DB instance through to the end of a series of cascading read replicas can consist of at most four DB instances.

For cascading read replicas to work, turn on automatic backups on your RDS for PostgreSQL. Create the read replica first and then turn on automatic backups on the RDS for PostgreSQL DB instance. The process is the same as for other Amazon RDS DB engines. For more information, see Creating a read replica.

As with any read replica, you can promote a read replica that's part of a cascade. Promoting a read replica from within a chain of read replicas removes that replica from the chain. For example, suppose that you want to move some of the workload off of your rpg-db-main DB instance to a new instance for use by the accounting department only. Assuming the chain of three read replicas from the example, you decide to promote read-replica-2. The chain is affected as follows:

• Promoting read-replica-2 removes it from the replication chain.

  • It is now a full read/write DB instance.

  • It continues replicating to read-replica-3, just as it was doing before promotion.

• Your rpg-db-main continues replicating to read-replica-1.

For more information about promoting read replicas, see Promoting a read replica to be a standalone DB instance.

Note

• RDS for PostgreSQL doesn't support major version upgrades for cascading replicas. Before performing a major version upgrade, you need to remove cascading replicas. You can recreate them after completing the upgrade on your source DB instance and first-level replicas.

• For cascading read replicas, RDS for PostgreSQL supports 15 read replicas for each source DB instance at first level of replication, and 5 read replicas for each source DB instance at the second and third level of replication.