NOTIFY

NOTIFY

7

SQL - Language Statements

NOTIFY

generate a notification

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 MDASH 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 with identical
payload strings within the same transaction, only one instance of the
notification event is delivered to listeners. 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
[???](#functions-info) for more information.

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

pg\_notify
----------

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
========
