'\" t .\" Title: NOTIFY .\" Author: The PostgreSQL Global Development Group .\" Generator: DocBook XSL Stylesheets vsnapshot .\" 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)