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NOTIFY() SQL Commands NOTIFY()
NAME
NOTIFY - generate a notification
SYNOPSIS
NOTIFY name
DESCRIPTION
The NOTIFY command sends a notification event to each client application that has previ-
ously executed LISTEN name for the specified notification name in the current database.
NOTIFY provides a simple form of signal or interprocess communication mechanism for a col-
lection of processes accessing the same PostgreSQL database. Higher-level mechanisms can
be built by using tables in the database to pass additional data (beyond a mere notifica-
tion name) from notifier to listener(s).
The information passed to the client for a notification event includes the notification
name and the notifying session's server process PID. It is up to the database designer to
define the notification names that will be used in a given database and what each one
means.
Commonly, the notification 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 notification names to signal dif-
ferent sorts of changes to a single table.
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 rule that is triggered by table updates.
In this way, notification happens automatically when the table is changed, and the appli-
cation 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 notifica-
tion 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.
NOTIFY behaves like Unix signals in one important respect: if the same notification name
is signaled multiple times in quick succession, recipients might get only one notification
event for several executions of NOTIFY. So it is a bad idea to depend on the number of
notifications received. Instead, use NOTIFY to wake up applications that need to pay
attention to something, and use a database object (such as a sequence) to keep track of
what happened or how many times it happened.
It is common for a client that executes NOTIFY to be listening on the same notification
name 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 ses-
sion'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. (Despite what was said in the
preceding paragraph, this is a safe technique. PostgreSQL keeps self-notifications sepa-
rate from notifications arriving from other sessions, so you cannot miss an outside noti-
fication by ignoring your own notifications.)
PARAMETERS
name Name of the notification to be signaled (any identifier).
EXAMPLES
Configure and execute a listen/notify sequence from psql:
LISTEN virtual;
NOTIFY virtual;
Asynchronous notification "virtual" received from server process with PID 8448.
COMPATIBILITY
There is no NOTIFY statement in the SQL standard.
SEE ALSO
LISTEN [listen(7)], UNLISTEN [unlisten(l)]
SQL - Language Statements 2011-09-22 NOTIFY()
Generated by $Id: phpMan.php,v 4.49 2006/02/26 13:18:18 chedong Exp $ Author: Che Dong
On Apache
Under GNU General Public License
2012-05-26 03:19 @38.107.179.237 Crawled by CCBot/1.0 (+http://www.commoncrawl.org/bot.html)