notify
NOTIFY() SQL Commands NOTIFY()
NAME
NOTIFY - generate a notification
SYNOPSIS
NOTIFY name
DESCRIPTION
The NOTIFY command sends a notification event to each client applica-
tion that has previously executed LISTEN name for the specified noti-
fication name in the current database.
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 associ-
ation 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.
NOTIFY provides a simple form of signal or interprocess communication
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).
When NOTIFY is used to signal the occurrence of changes to a particu-
lar 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 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 commit-
ted. 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 deliv-
ered immediately. Secondly, if a listening session receives a notifi-
cation 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 transac-
tion 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 applica-
tions 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 notifica-
tions 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 notifi-
cation 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. (Despite what was
said in the preceding paragraph, this is a safe technique. PostgreSQL
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 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 2008-01-03 NOTIFY()
