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'\" t
.\" Title: NOTIFY
.\" Author: The PostgreSQL Global Development Group
.\" Generator: DocBook XSL Stylesheets vsnapshot <http://docbook.sf.net/>
.\" Date: 2024
.\" Manual: PostgreSQL 16.2 Documentation
.\" Source: PostgreSQL 16.2
.\" Language: English
.\"
.TH "NOTIFY" "7" "2024" "PostgreSQL 16.2" "PostgreSQL 16.2 Documentation"
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.SH "NAME"
NOTIFY \- generate a notification
.SH "SYNOPSIS"
.sp
.nf
NOTIFY \fIchannel\fR [ , \fIpayload\fR ]
.fi
.SH "DESCRIPTION"
.PP
The
\fBNOTIFY\fR
command sends a notification event together with an optional
\(lqpayload\(rq
string to each client application that has previously executed
\fBLISTEN \fR\fB\fIchannel\fR\fR
for the specified channel name in the current database\&. Notifications are visible to all users\&.
.PP
\fBNOTIFY\fR
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)\&.
.PP
The information passed to the client for a notification event includes the notification channel name, the notifying session\*(Aqs server process
PID, and the payload string, which is an empty string if it has not been specified\&.
.PP
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,
\(lqI changed this table, take a look at it to see what\*(Aqs new\(rq\&. But no such association is enforced by the
\fBNOTIFY\fR
and
\fBLISTEN\fR
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\&.
.PP
When
\fBNOTIFY\fR
is used to signal the occurrence of changes to a particular table, a useful programming technique is to put the
\fBNOTIFY\fR
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\&.
.PP
\fBNOTIFY\fR
interacts with SQL transactions in some important ways\&. Firstly, if a
\fBNOTIFY\fR
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
\fBNOTIFY\fR\&. 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 \(em but the server cannot
\(lqtake back\(rq
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
\fBNOTIFY\fR
for real\-time signaling should try to keep their transactions short\&.
.PP
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,
\fBNOTIFY\fR
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\&.
.PP
It is common for a client that executes
\fBNOTIFY\fR
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\*(Aqs server process
PID
(supplied in the notification event message) is the same as one\*(Aqs own session\*(Aqs
PID
(available from
libpq)\&. When they are the same, the notification event is one\*(Aqs own work bouncing back, and can be ignored\&.
.SH "PARAMETERS"
.PP
\fIchannel\fR
.RS 4
Name of the notification channel to be signaled (any identifier)\&.
.RE
.PP
\fIpayload\fR
.RS 4
The
\(lqpayload\(rq
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\*(Aqs best to put it in a database table and send the key of the record\&.)
.RE
.SH "NOTES"
.PP
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
\fBNOTIFY\fR
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
\fBLISTEN\fR
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\&.
.PP
The function
\fBpg_notification_queue_usage\fR
returns the fraction of the queue that is currently occupied by pending notifications\&. See
Section\ \&9.26
for more information\&.
.PP
A transaction that has executed
\fBNOTIFY\fR
cannot be prepared for two\-phase commit\&.
.SS "pg_notify"
.PP
To send a notification you can also use the function
\fBpg_notify\fR(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
\fBNOTIFY\fR
command if you need to work with non\-constant channel names and payloads\&.
.SH "EXAMPLES"
.PP
Configure and execute a listen/notify sequence from
psql:
.sp
.if n \{\
.RS 4
.\}
.nf
LISTEN virtual;
NOTIFY virtual;
Asynchronous notification "virtual" received from server process with PID 8448\&.
NOTIFY virtual, \*(AqThis is the payload\*(Aq;
Asynchronous notification "virtual" with payload "This is the payload" received from server process with PID 8448\&.
LISTEN foo;
SELECT pg_notify(\*(Aqfo\*(Aq || \*(Aqo\*(Aq, \*(Aqpay\*(Aq || \*(Aqload\*(Aq);
Asynchronous notification "foo" with payload "payload" received from server process with PID 14728\&.
.fi
.if n \{\
.RE
.\}
.SH "COMPATIBILITY"
.PP
There is no
\fBNOTIFY\fR
statement in the SQL standard\&.
.SH "SEE ALSO"
\fBLISTEN\fR(7), \fBUNLISTEN\fR(7)
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