Listen/Notify

Overview

Postgres has a system of asynchronous messages and notifications, implemented by listen and notify keywords.

• This means that as soon as a connection is established with PostgreSQL, the server can send messages to the client even when the client is idle.
  • This method of communication is also carried out with the COPY command.

Commonly, the channel name is the same as the name of some table in the database, and the notify event essentially means, "I changed this table, take a look at it to see what's new"

Notify

notify provides a simple interprocess communication (IPC) mechanism for a collection of processes accessing the same PostgreSQL database.

When notify is used to signal the occurrence of changes to a particular table, a useful programming technique is to put the notify in a rule that is triggered by table updates.

• In this way, notification happens automatically when the table is changed, and the application programmer cannot accidentally forget to do it.

In a transaction, notify events are not delivered until the surrounding transaction has been committed.

Listen

When we execute listen, we are registering the current postgres session as a listener on the specified notification channel.

• when the specified channel is notified (either by the current session, or another one connected to the same db), all sessions subscribed to that channel will receive the message, and will in turn pass it on to the client whom it is connecting.

The payload passed to the client includes 3 things:

1. Notification channel name
2. Session server's PID
3. Payload string ('' if unspecified)

A session's listen registrations are automatically cleared when the session ends.

The method a client application must use to detect notification events depends on which PostgreSQL API it uses (ie. libpq, libpgtcl)

pg_notify(channel_name text, payload text)

We can also send a notification using the pg_notify function

• The function is much easier to use than the notify command if you need to work with non-constant channel names and payloads.

Example

psql## listen my_notification_channel;
LISTEN

psql## notify my_notification_channel, 'foo';
NOTIFY
Asynchronous notification "my_notification_channel" with payload "foo"  ⏎
received from server process with PID 40430.

UE Resources

Dmitiri Fontaine on Listen-Notify