Question: What causes PostgreSQL replication to go out of sync?

Answer

Replication in PostgreSQL is a process used to create a copy of a database on another server, ensuring high availability and backup. However, there are scenarios where the replication might go out of sync, leading to data inconsistency between the primary and replica databases.

Causes of Replication Going Out of Sync

1. Network Issues: Interruptions in network connectivity can cause the replica to lag behind the primary database, potentially leading to out-of-sync issues if not resolved promptly.

2. Hardware Failures: Disk errors or hardware failures on the replica server can disrupt the replication process, causing data mismatches.

3. Manual Changes: Any manual changes made directly on the replica (in case of logical replication) can lead to inconsistencies since these changes are not replicated back to the primary.

4. Write-Ahead Logging (WAL) Configuration Issues: Improper configuration of WAL settings can lead to missing or unapplied log segments on the replica, resulting in data divergence.

5. Replication Slot Issues: In PostgreSQL, replication slots track the progress of streaming replication. If a replication slot is not used or incorrectly configured, it could lead to missed updates.

6. Software Bugs or Version Mismatches: Bugs in PostgreSQL or using different versions of PostgreSQL for the primary and replica can sometimes lead to unexpected replication issues.

Detecting and Fixing Replication Out of Sync

To detect replication lag, you can use the pg_stat_replication view on the primary server to monitor the delay of each replica. On the replica, pg_last_wal_receive_lsn() and pg_last_wal_replay_lsn() functions can be used to check the WAL positions.

-- On the primary
SELECT * FROM pg_stat_replication;

-- On the replica
SELECT pg_last_wal_receive_lsn(), pg_last_wal_replay_lsn();

If you find that your replication is out of sync, here are general steps to resolve it:

1. Identify and Resolve the Root Cause: First, identify what caused the replication to go out of sync. It could be one of the reasons mentioned above or something else specific to your environment.

2. Resynchronize Data: Depending on the cause and extent of divergence, you may need to resynchronize the data. For minor discrepancies, tools like pg_rewind can help synchronize a replica with the primary without a full base backup. However, in more severe cases, setting up a new replica from scratch might be necessary.

3. Prevent Future Issues: Once replication is back in sync, take steps to prevent future issues. This could include improving network reliability, updating hardware, reviewing and adjusting WAL configurations, and ensuring all manual interventions on replicas are done with caution.

Remember, regular monitoring and proactive management of your PostgreSQL replication setup can significantly reduce the risk of it going out of sync.