YoLinux.com: NOTIFY manpage

NOTIFY(7) PostgreSQL 9.2.24 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.

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