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+'\" t
+.\"     Title: NOTIFY
+.\"    Author: The PostgreSQL Global Development Group
+.\" Generator: DocBook XSL Stylesheets v1.79.1 <http://docbook.sf.net/>
+.\"      Date: 2021
+.\"    Manual: PostgreSQL 13.4 Documentation
+.\"    Source: PostgreSQL 13.4
+.\"  Language: English
+.\"
+.TH "NOTIFY" "7" "2021" "PostgreSQL 13.4" "PostgreSQL 13.4 Documentation"
+.\" -----------------------------------------------------------------
+.\" * Define some portability stuff
+.\" -----------------------------------------------------------------
+.\" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+.\" http://bugs.debian.org/507673
+.\" http://lists.gnu.org/archive/html/groff/2009-02/msg00013.html
+.\" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+.ie \n(.g .ds Aq \(aq
+.el       .ds Aq '
+.\" -----------------------------------------------------------------
+.\" * set default formatting
+.\" -----------------------------------------------------------------
+.\" disable hyphenation
+.nh
+.\" disable justification (adjust text to left margin only)
+.ad l
+.\" -----------------------------------------------------------------
+.\" * MAIN CONTENT STARTS HERE *
+.\" -----------------------------------------------------------------
+.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)