NOTIFY - generate a notification
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 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).
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
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
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 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.)
name Name of the notification to be signaled (any identifier).
Configure and execute a listen/notify sequence from psql:
Asynchronous notification "virtual" received from server process with PID 8448.
There is no NOTIFY statement in the SQL standard.
LISTEN [listen(7)], UNLISTEN [unlisten(7)]