NOTIFY — generate a notification
NOTIFY command sends a notification event together
with an optional “payload” string to each client application that
has previously executed
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
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.
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
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 —
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
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.
Name of the notification channel to be signaled (any identifier).
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.)
There is a queue that holds notifications that have been sent but not
yet processed by all listening sessions. If this queue becomes full,
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.
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.
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.
There is no
NOTIFY statement in the SQL