Question: How do you check the replication status in PostgreSQL?

Answer

In PostgreSQL, replication is a method to copy and synchronize data from one database server (the primary) to one or more other servers (the replicas or standbys). Monitoring the replication status is crucial for ensuring data consistency and availability. Here's how you can check the replication status in PostgreSQL:

Using the pg_stat_replication View

The pg_stat_replication view provides information about the current replication connections to a PostgreSQL primary server. You can query this view to check the status of your replication setup:

SELECT * FROM pg_stat_replication;

This command will display various columns such as pid, usesysid, usename, application_name, client_addr, state, sync_state, and more. The state column shows whether the replication is active, and the sync_state indicates if the replica is synchronous or asynchronous.

Checking Replication Lag

One important aspect of monitoring replication is to check for lag, which represents how far a replica is behind the primary. You can use the following query on the replica to see the replication lag in bytes:

SELECT pg_wal_lsn_diff(pg_current_wal_lsn(), replay_lsn) AS replication_lag_bytes
FROM pg_stat_replication
WHERE application_name = 'your_replica_application_name';

Replace 'your_replica_application_name' with the actual application name of your replica.

Using the pg_stat_wal_receiver View

On a standby server, you can query the pg_stat_wal_receiver view to get information about the WAL (Write-Ahead Logging) receiving process. This view can help you monitor the connection and replication state from the perspective of the replica:

SELECT * FROM pg_stat_wal_receiver;

This command provides details such as conninfo, state, received_lsn, and more, which can help you assess the health of the replication connection from the replica side.

Conclusion

By regularly monitoring the replication status using these views and queries, you can ensure that your PostgreSQL replication setup operates smoothly and efficiently. Remember to have proper access rights to view these statistics, as they may require superuser privileges.