1NOTIFY(7)                PostgreSQL 11.6 Documentation               NOTIFY(7)
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NAME

6       NOTIFY - generate a notification
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SYNOPSIS

9       NOTIFY channel [ , payload ]
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DESCRIPTION

12       The NOTIFY command sends a notification event together with an optional
13       “payload” string to each client application that has previously
14       executed LISTEN channel for the specified channel name in the current
15       database. Notifications are visible to all users.
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17       NOTIFY provides a simple interprocess communication mechanism for a
18       collection of processes accessing the same PostgreSQL database. A
19       payload string can be sent along with the notification, and
20       higher-level mechanisms for passing structured data can be built by
21       using tables in the database to pass additional data from notifier to
22       listener(s).
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24       The information passed to the client for a notification event includes
25       the notification channel name, the notifying session's server process
26       PID, and the payload string, which is an empty string if it has not
27       been specified.
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29       It is up to the database designer to define the channel names that will
30       be used in a given database and what each one means. Commonly, the
31       channel name is the same as the name of some table in the database, and
32       the notify event essentially means, “I changed this table, take a look
33       at it to see what's new”. But no such association is enforced by the
34       NOTIFY and LISTEN commands. For example, a database designer could use
35       several different channel names to signal different sorts of changes to
36       a single table. Alternatively, the payload string could be used to
37       differentiate various cases.
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39       When NOTIFY is used to signal the occurrence of changes to a particular
40       table, a useful programming technique is to put the NOTIFY in a
41       statement trigger that is triggered by table updates. In this way,
42       notification happens automatically when the table is changed, and the
43       application programmer cannot accidentally forget to do it.
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45       NOTIFY interacts with SQL transactions in some important ways. Firstly,
46       if a NOTIFY is executed inside a transaction, the notify events are not
47       delivered until and unless the transaction is committed. This is
48       appropriate, since if the transaction is aborted, all the commands
49       within it have had no effect, including NOTIFY. But it can be
50       disconcerting if one is expecting the notification events to be
51       delivered immediately. Secondly, if a listening session receives a
52       notification signal while it is within a transaction, the notification
53       event will not be delivered to its connected client until just after
54       the transaction is completed (either committed or aborted). Again, the
55       reasoning is that if a notification were delivered within a transaction
56       that was later aborted, one would want the notification to be undone
57       somehow — but the server cannot “take back” a notification once it has
58       sent it to the client. So notification events are only delivered
59       between transactions. The upshot of this is that applications using
60       NOTIFY for real-time signaling should try to keep their transactions
61       short.
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63       If the same channel name is signaled multiple times from the same
64       transaction with identical payload strings, the database server can
65       decide to deliver a single notification only. On the other hand,
66       notifications with distinct payload strings will always be delivered as
67       distinct notifications. Similarly, notifications from different
68       transactions will never get folded into one notification. Except for
69       dropping later instances of duplicate notifications, NOTIFY guarantees
70       that notifications from the same transaction get delivered in the order
71       they were sent. It is also guaranteed that messages from different
72       transactions are delivered in the order in which the transactions
73       committed.
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75       It is common for a client that executes NOTIFY to be listening on the
76       same notification channel itself. In that case it will get back a
77       notification event, just like all the other listening sessions.
78       Depending on the application logic, this could result in useless work,
79       for example, reading a database table to find the same updates that
80       that session just wrote out. It is possible to avoid such extra work by
81       noticing whether the notifying session's server process PID (supplied
82       in the notification event message) is the same as one's own session's
83       PID (available from libpq). When they are the same, the notification
84       event is one's own work bouncing back, and can be ignored.
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PARAMETERS

87       channel
88           Name of the notification channel to be signaled (any identifier).
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90       payload
91           The “payload” string to be communicated along with the
92           notification. This must be specified as a simple string literal. In
93           the default configuration it must be shorter than 8000 bytes. (If
94           binary data or large amounts of information need to be
95           communicated, it's best to put it in a database table and send the
96           key of the record.)
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NOTES

99       There is a queue that holds notifications that have been sent but not
100       yet processed by all listening sessions. If this queue becomes full,
101       transactions calling NOTIFY will fail at commit. The queue is quite
102       large (8GB in a standard installation) and should be sufficiently sized
103       for almost every use case. However, no cleanup can take place if a
104       session executes LISTEN and then enters a transaction for a very long
105       time. Once the queue is half full you will see warnings in the log file
106       pointing you to the session that is preventing cleanup. In this case
107       you should make sure that this session ends its current transaction so
108       that cleanup can proceed.
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110       The function pg_notification_queue_usage returns the fraction of the
111       queue that is currently occupied by pending notifications. See
112       Section 9.25 for more information.
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114       A transaction that has executed NOTIFY cannot be prepared for two-phase
115       commit.
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117   pg_notify
118       To send a notification you can also use the function pg_notify(text,
119       text). The function takes the channel name as the first argument and
120       the payload as the second. The function is much easier to use than the
121       NOTIFY command if you need to work with non-constant channel names and
122       payloads.
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EXAMPLES

125       Configure and execute a listen/notify sequence from psql:
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127           LISTEN virtual;
128           NOTIFY virtual;
129           Asynchronous notification "virtual" received from server process with PID 8448.
130           NOTIFY virtual, 'This is the payload';
131           Asynchronous notification "virtual" with payload "This is the payload" received from server process with PID 8448.
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133           LISTEN foo;
134           SELECT pg_notify('fo' || 'o', 'pay' || 'load');
135           Asynchronous notification "foo" with payload "payload" received from server process with PID 14728.
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COMPATIBILITY

138       There is no NOTIFY statement in the SQL standard.
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SEE ALSO

141       LISTEN(7), UNLISTEN(7)
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145PostgreSQL 11.6                      2019                            NOTIFY(7)
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