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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 previously executed LISTEN name for the
specified notification name in the current database.
NOTIFY provides a simple form of signal or interprocess com-
munication mechanism for a collection 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 notification 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
different 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 application programmer can't acciden-
tally 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
SQL - Language StatemLast change: 2005-11-05 1
SQL Commands NOTIFY()
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.
NOTIFY behaves like Unix signals in one important respect:
if the same notification name is signaled multiple times in
quick succession, recipients may 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 atten-
tion 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 listen-
ing 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 data-
base table to find the same updates that that session just
wrote out. It is possible to avoid such extra work by notic-
ing whether the notifying session's server process PID (sup-
plied 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. Post-
greSQL keeps self-notifications separate from notifications
arriving from other sessions, so you cannot miss an outside
notification by ignoring your own notifications.)
PARAMETERS
name Name of the notification to be signaled (any identif-
ier).
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.
SQL - Language StatemLast change: 2005-11-05 2
SQL Commands NOTIFY()
SEE ALSO
LISTEN [listen(5)], UNLISTEN [unlisten(l)]
SQL - Language StatemLast change: 2005-11-05 3
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This page was generated on Wed Sep 12 11:27:54 GMT 2007
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