Replication with Amazon Aurora PostgreSQL
Following, you can find information about replication with Amazon Aurora PostgreSQL, including how to monitor 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 AWS 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 AWS Regions, using Aurora global database. For more information, see Using Amazon Aurora global databases.
Note
With the improved 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:
15.2 and higher 15 versions
14.7 and higher 14 versions
13.10 and higher 13 versions
12.14 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 improved availability of Aurora Replicas, the DB cluster
maintains read availability during these restarts, which reduces the impact of changes to the writer instance. 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_statusfunction is updated to return the most up-to-date information for the reader instance when it is still connected. The last update time stamp inaurora_replica_statusis 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 the following error message in the database error log:
Canceling statement due to conflict with recovery.
Limitations
The following limitations apply to Aurora Replicas with improved availability:
Global DB Aurora Replicas in the secondary AWS Regions aren't supported.
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.
