NOTIFY(7)						  PostgreSQL 9.2.7 Documentation						 NOTIFY(7)

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.

       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
       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.

       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.

       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)

PostgreSQL 9.2.7						    2014-02-17								 NOTIFY(7)