NAME

       NOTIFY - generate a notification

SYNOPSIS

       NOTIFY channel [ , payload ]

DESCRIPTION

       The NOTIFY command sends a notification event together with an optional “payload” string
       to each client application that has previously executed LISTEN channel for the specified
       channel name in the current database. Notifications are visible to all users.

       NOTIFY provides a simple interprocess communication mechanism for a collection of
       processes accessing the same PostgreSQL database. A payload string can be sent along with
       the notification, and higher-level mechanisms for passing structured data can be built by
       using tables in the database to pass additional data from notifier to listener(s).

       The information passed to the client for a notification event includes the notification
       channel name, the notifying session's server process PID, and the payload string, which is
       an empty string if it has not been specified.

       It is up to the database designer to define the channel names that will be used in a given
       database and what each one means. 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”. But no such association is enforced by the NOTIFY
       and LISTEN commands. For example, a database designer could use several different channel
       names to signal different sorts of changes to a single table. Alternatively, the payload
       string could be used to differentiate various cases.

       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 statement trigger 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.

       NOTIFY interacts with SQL transactions in some important ways. Firstly, if a NOTIFY is
       executed inside a transaction, the notify events are not delivered until and unless the
       transaction is committed. This is appropriate, since if the transaction is aborted, all
       the commands within it have had no effect, including NOTIFY. But it can be disconcerting
       if one is expecting the notification events to be delivered immediately. Secondly, if a
       listening session receives a notification signal while it is within a transaction, the
       notification event will not be delivered to its connected client until just after the
       transaction is completed (either committed or aborted). Again, the reasoning is that if a
       notification were delivered within a transaction that was later aborted, one would want
       the notification to be undone somehow — but the server cannot “take back” a notification
       once it has sent it to the client. So notification events are only delivered between
       transactions. The upshot of this is that applications using NOTIFY for real-time signaling
       should try to keep their transactions short.

       If the same channel name is signaled multiple times from the same transaction with
       identical payload strings, the database server can decide to deliver a single notification
       only. On the other hand, notifications with distinct payload strings will always be
       delivered as distinct notifications. Similarly, notifications from different transactions
       will never get folded into one notification. Except for dropping later instances of
       duplicate notifications, NOTIFY guarantees that notifications from the same transaction
       get delivered in the order they were sent. It is also guaranteed that messages from
       different transactions are delivered in the order in which the transactions committed.

       It is common for a client that executes NOTIFY to be listening on the same notification
       channel itself. In that case it will get back a notification event, just like all the
       other listening sessions. Depending on the application logic, this could result in useless
       work, for example, reading a database table to find the same updates that that session
       just wrote out. It is possible to avoid such extra work by noticing whether the notifying
       session's server process PID (supplied in the notification event message) is the same as
       one's own session's PID (available from libpq). When they are the same, the notification
       event is one's own work bouncing back, and can be ignored.

PARAMETERS

       channel
           Name of the notification channel to be signaled (any identifier).

       payload
           The “payload” string to be communicated along with the notification. This must be
           specified as a simple string literal. In the default configuration it must be shorter
           than 8000 bytes. (If binary data or large amounts of information need to be
           communicated, it's best to put it in a database table and send the key of the record.)

NOTES

       There is a queue that holds notifications that have been sent but not yet processed by all
       listening sessions. If this queue becomes full, transactions calling NOTIFY will fail at
       commit. The queue is quite large (8GB in a standard installation) and should be
       sufficiently sized for almost every use case. However, no cleanup can take place if a
       session executes LISTEN and then enters a transaction for a very long time. Once the queue
       is half full you will see warnings in the log file pointing you to the session that is
       preventing cleanup. In this case you should make sure that this session ends its current
       transaction so that cleanup can proceed.

       The function pg_notification_queue_usage returns the fraction of the queue that is
       currently occupied by pending notifications. See Section 9.25 for more information.

       A transaction that has executed NOTIFY cannot be prepared for two-phase commit.

   pg_notify
       To send a notification you can also use the function pg_notify(text, text). The function
       takes the channel name as the first argument and the payload as the second. The function
       is much easier to use than the NOTIFY command if you need to work with non-constant
       channel names and payloads.

EXAMPLES

       Configure and execute a listen/notify sequence from psql:

           LISTEN virtual;
           NOTIFY virtual;
           Asynchronous notification "virtual" received from server process with PID 8448.
           NOTIFY virtual, 'This is the payload';
           Asynchronous notification "virtual" with payload "This is the payload" received from server process with PID 8448.

           LISTEN foo;
           SELECT pg_notify('fo' || 'o', 'pay' || 'load');
           Asynchronous notification "foo" with payload "payload" received from server process with PID 14728.

COMPATIBILITY

       There is no NOTIFY statement in the SQL standard.

SEE ALSO

       LISTEN(7), UNLISTEN(7)