| NOTIFY(7) - phpMan
NOTIFY(7) PostgreSQL 12.3 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. Notifications are visible to all users.
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 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.
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.
The function pg_notification_queue_usage returns the fraction of the queue that is
currently occupied by pending notifications. See Section 9.25 for more information.
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.
SEE ALSO
LISTEN(7), UNLISTEN(7)
PostgreSQL 12.3 2020 NOTIFY(7)
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