[Answered] How do you restart replication in PostgreSQL?

Answer

In PostgreSQL, replication is a method of copying and maintaining the same database objects across multiple PostgreSQL servers. Restarting replication might be necessary after a failure, configuration change, or for maintenance purposes. The steps involved can vary depending on the setup (streaming replication, logical replication, etc.), but here's a general guide for restarting streaming replication:

Stop the Replication Slave (Standby): Ensure the standby server is not running to prevent any conflicts during the restart process.
```
pg_ctl stop -D /path/to/standby/data
```
Verify Replication Status: Before proceeding, it's crucial to verify the status of replication and identify the last replicated transaction. You can use the pg_stat_replication view on the primary server to check if the replication slot used by the standby is still active.
Backup Master Data Directory (optional but recommended): It's a good practice to create a backup of your master (primary) server's data directory before making changes.
Copy Necessary WAL Files: If the replication delay is minimal, you might need to copy only the latest WAL files from the primary to the standby server. This step depends significantly on how far behind the standby is and the specifics of your replication setup.
Update Standby Configuration (if needed): Review and update the recovery.conf or equivalent settings in the postgresql.conf file (depending on your PostgreSQL version) on the standby server to ensure they reflect the desired state of replication.
Restart the Standby Server: Once everything is set up and verified, start the standby server:
```
pg_ctl start -D /path/to/standby/data
```
Monitor Replication: After restarting the standby server, closely monitor the replication status to ensure it catches up without issues. You can use tools like pg_stat_replication on the primary and pg_stat_wal_receiver on the standby to observe the process.

Notes:

In some cases, particularly after a significant failure or if the standby is too far behind, you may need to reinitialize the standby using a fresh base backup from the primary.
Always refer to the specific documentation for your PostgreSQL version, as commands and configurations can vary.

This process assumes a relatively healthy replication setup needing a restart due to manageable issues. In more complex scenarios, such as major failures or significant configuration changes, additional steps might be necessary.