Mastering PostgreSQL Replication with pg_standby

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What is pg_standby?

pg_standby is a utility that comes with PostgreSQL, designed for creating and managing a warm standby database server. A warm standby server is a backup system that is kept up-to-date with the primary server’s data changes and can be quickly promoted to a primary server in case of a failure, making it an essential component for high-availability (HA) setups. It’s a production-ready program that also serves as a customizable template for more specific needs.

How pg_standby Works

pg_standby is used in conjunction with PostgreSQL’s continuous archiving feature. It acts as a restore command within the recovery.conf file of the standby server, which is responsible for managing the retrieval and application of Write-Ahead Logging (WAL) files from the primary server’s archive location to the standby server.

Setting Up a Warm Standby with pg_standby

To set up a warm standby server using pg_standby, you will need to follow these general steps:

  1. Configure the primary server to archive completed WAL files by setting the archive_mode to on and defining an archive_command that copies the WAL files to a designated archive directory or to a shared storage accessible by the standby server.
  2. Prepare the standby server by creating a base backup of the primary server and copying it to the standby server.
  3. Create a recovery.conf file on the standby server with the restore_command using pg_standby to manage the retrieval of WAL files.

Example recovery.conf Entry

restore_command = 'pg_standby -l -d /path/to/archive %f %p %r'

In this command, %f is replaced with the filename of the required WAL file, %p with the path where the file must be created, and %r with the last valid restart point.

Use Cases for pg_standby

  • High Availability: pg_standby can be used to maintain a hot-standby database server that can take over in case the primary server fails.
  • Disaster Recovery: It can also be used for creating a standby server in a geographically different location to ensure business continuity in case of a site failure.
  • Read Scaling: While the standby server is in recovery, it can be used for read-only queries, which can help in load balancing read operations.

Common Mistakes and Issues

  • Incorrect File Paths: Ensure the file paths specified in the archive_command and restore_command are correct and accessible by the PostgreSQL process.
  • Permission Issues: The PostgreSQL user must have the necessary permissions to read from the archive location and write to the data directory.
  • Out-of-Date Standby: If the standby server falls too far behind, it may not be able to catch up with the primary server, which can lead to longer downtimes during failovers.

Troubleshooting Errors

  • WAL File Not Found: This can happen if the archive_command on the primary server is not working correctly, or if there is a network issue preventing access to the archive location.
  • Connection Issues: Ensure that the standby server can communicate with the primary server if you are using streaming replication alongside file-based log shipping.

Conclusion

pg_standby is a powerful tool for managing warm standby servers in PostgreSQL, providing an essential service for high availability and disaster recovery strategies. By carefully configuring and monitoring your pg_standby setup, you can ensure that your database system is resilient against failures and capable of maintaining continuous operations.

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