Replication with Amazon Aurora PostgreSQL - Amazon Aurora
Aurora ReplicasImproving the read availability of Aurora ReplicasMonitoring replication
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Replication with Amazon Aurora PostgreSQL

Following, you can find information about replication with Amazon Aurora PostgreSQL, including how to monitor and use logical replication.

Topics

Using Aurora Replicas

An Aurora Replica is an independent endpoint in an Aurora DB cluster, best used for scaling read operations and increasing availability. An Aurora DB cluster can include up to 15 Aurora Replicas located throughout the Availability Zones of the Aurora DB cluster's Amazon Region.

The DB cluster volume is made up of multiple copies of the data for the DB cluster. However, the data in the cluster volume is represented as a single, logical volume to the primary writer DB instance and to Aurora Replicas in the DB cluster. For more information about Aurora Replicas, see Aurora Replicas.

Aurora Replicas work well for read scaling because they're fully dedicated to read operations on your cluster volume. The writer DB instance manages write operations. The cluster volume is shared among all instances in your Aurora PostgreSQL DB cluster. Thus, no extra work is needed to replicate a copy of the data for each Aurora Replica.

With Aurora PostgreSQL, when an Aurora Replica is deleted, its instance endpoint is removed immediately, and the Aurora Replica is removed from the reader endpoint. If there are statements running on the Aurora Replica that is being deleted, there is a three minute grace period. Existing statements can finish gracefully during the grace period. When the grace period ends, the Aurora Replica is shut down and deleted.

Aurora PostgreSQL DB clusters support Aurora Replicas in different Amazon Regions, using Aurora global database. For more information, see Using Amazon Aurora Global Database.

Note

With the read availability feature, if you want to reboot the Aurora Replicas in the DB cluster, you have to perform it manually. For the DB clusters created prior to this feature rebooting the writer DB instance automatically reboots the Aurora Replicas. The automatic reboot re-establishes an entry point that guarantees read/write consistency across the DB cluster.

Improving the read availability of Aurora Replicas

Aurora PostgreSQL improves the read availability in the DB cluster by continuously serving the read requests when the writer DB instance restarts or when the Aurora Replica is unable to keep up with the write traffic.

The read availability feature is available by default on the following versions of Aurora PostgreSQL:

  • 16.1 and all higher versions

  • 15.2 and higher 15 versions

  • 14.7 and higher 14 versions

  • 13.10 and higher 13 versions

  • 12.14 and higher 12 versions

The read availability feature is supported by Aurora global database in the following versions:

  • 16.1 and all higher versions

  • 15.4 and higher 15 versions

  • 14.9 and higher 14 versions

  • 13.12 and higher 13 versions

  • 12.16 and higher 12 versions

To use the read availability feature for a DB cluster created on one of these versions prior to this launch, restart the writer instance of the DB cluster.

When you modify static parameters of your Aurora PostgreSQL DB cluster, you must restart the writer instance so that the parameter changes take effect. For example, you must restart the writer instance when you set the value of shared_buffers. With the read availability feature of Aurora Replicas, the DB cluster maintains improved availability, reducing the impact on it when the writer instance restarts. The reader instances don't restart and continue to respond to the read requests. To apply static parameter changes, reboot each individual reader instance.

An Aurora PostgreSQL DB cluster's Aurora Replica can recover from replication errors such as writer restarts, failover, slow replication, and network issues by quickly recovering to in-memory database state after it reconnects with the writer. This approach allows Aurora Replica instances to reach consistency with the latest storage updates while the client database is still available.

The in-progress transactions that conflict with replication recovery might receive an error but the client can retry these transactions, after the readers catch up with the writer.

Monitoring Aurora Replicas

You can monitor the Aurora Replicas when recovering from a writer disconnect. Use the metrics below to check for the latest information about the reader instance and to track in-process read-only transactions.

  • The aurora_replica_status function is updated to return the most up-to-date information for the reader instance when it is still connected. The last update time stamp in aurora_replica_status is always empty for the row corresponding to the DB instance that the query is executed on. This indicates that the reader instance has the latest data.

  • When the Aurora replica disconnects from the writer instance and reconnects back, the following database event is emitted:

    Read replica has been disconnected from the writer instance and reconnected.

  • When a read-only query is canceled due to a recovery conflict, you might see one or more of the following error messages in the database error log:

    Canceling statement due to conflict with recovery.

    User query may not have access to page data to replica disconnect.

    User query might have tried to access a file that no longer exists.

    When the replica reconnects, you will be able to repeat your command.

Limitations

The following limitations apply to Aurora Replicas with the read availability feature:

  • Aurora Replicas of secondary DB cluster can restart if the data can't be streamed from the writer instance during replication recovery.

  • Aurora Replicas don't support online replication recovery if one is already in progress and will restart.

  • Aurora Replicas will restart when your DB instance is nearing the transaction ID wraparound. For more information on transaction ID wraparound, see Preventing Transaction ID Wraparound Failures.

  • Aurora Replicas can restart when the replication process is blocked under certain circumstances.

Monitoring Aurora PostgreSQL replication

Read scaling and high availability depend on minimal lag time. You can monitor how far an Aurora Replica is lagging behind the writer DB instance of your Aurora PostgreSQL DB cluster by monitoring the Amazon CloudWatch ReplicaLag metric. Because Aurora Replicas read from the same cluster volume as the writer DB instance, the ReplicaLag metric has a different meaning for an Aurora PostgreSQL DB cluster. The ReplicaLag metric for an Aurora Replica indicates the lag for the page cache of the Aurora Replica compared to that of the writer DB instance.

For more information on monitoring RDS instances and CloudWatch metrics, see Monitoring metrics in an Amazon Aurora cluster.