Listen to Database Changes Through the Postgres WAL
12 days ago
- #Database
- #Postgres
- #CDC
- Postgres WAL (Write-Ahead Log) is a log of every change to the database, essential for replication, backups, and change data capture (CDC).
- NOTIFY/pg_notify can be used to react to database changes but has limitations like a single queue bottleneck, size limits, and no retry mechanism.
- The WAL allows Postgres to delay writing data pages to disk, improving performance and supporting features like replication and point-in-time recovery.
- To listen to WAL changes, configure Postgres with wal_level='logical' and create a publication and replication slot.
- Replication slots can be temporary (auto-cleaned on disconnect) or persistent (retains messages for offline listeners).
- The Postgrex.ReplicationConnection in Elixir can be used to listen to WAL changes, decode messages, and handle heartbeats.
- WAL messages include BEGIN, RELATION (schema info), INSERT/UPDATE/DELETE (data changes), and COMMIT (transaction end).
- Handling duplicate messages is crucial as Postgres may resend unacknowledged messages after a crash.
- Further tools like Walex, Cainophile, and Supabase Realtime can help implement CDC using the WAL.