postgres-impl-listen-notify

Quick Reference :

LISTEN / NOTIFY is PostgreSQL's built-in publish-subscribe channel. A session runs LISTEN channel to subscribe; any session runs NOTIFY channel, 'payload' (or pg_notify('channel', 'payload')) to broadcast. The server delivers the payload asynchronously to every listening session.

It is a signalling mechanism, not a message queue. Three facts decide every design :

1. Delivery is at COMMIT. A NOTIFY inside a transaction is queued and sent only when that transaction commits; a rollback sends nothing.
2. There is no replay. A LISTEN is per-connection. A client that is not connected and listening at the moment of the NOTIFY misses the event forever. Notifications are never stored or re-sent.
3. The payload is small. A payload must be a string literal under 8000 bytes. Send a row id, not the row.