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NOTIFY(5)
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.

  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.

  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 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 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 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
  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.)

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(5)], UNLISTEN [unlisten(l)]

Index for Section 5

Alphabetical listing for N

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