<function>TerminateBackgroundWorker(<parameter>BackgroundWorkerHandle *</parameter>)</function>.
   <function>GetBackgroundWorkerPid</function> can be used to poll the status of the
   worker: a return value of <literal>BGWH_NOT_YET_STARTED</literal> indicates that
   the worker has not yet been started by the postmaster;
   <literal>BGWH_STOPPED</literal> indicates that it has been started but is
   no longer running; and <literal>BGWH_STARTED</literal> indicates that it is
   currently running.  In this last case, the PID will also be returned via the
   second argument.
   <function>TerminateBackgroundWorker</function> causes the postmaster to send
   <literal>SIGTERM</literal> to the worker if it is running, and to unregister it
   as soon as it is not.
  </para>

  <para>
   In some cases, a process which registers a background worker may wish to
   wait for the worker to start up.  This can be accomplished by initializing
   <structfield>bgw_notify_pid</structfield> to <literal>MyProcPid</literal> and
   then passing the <type>BackgroundWorkerHandle *</type> obtained at
   registration time to
   <function>WaitForBackgroundWorkerStartup(<parameter>BackgroundWorkerHandle
   *handle</parameter>, <parameter>pid_t *</parameter>)</function> function.
   This function will block until the postmaster has attempted to start the
   background worker, or until the postmaster dies.  If the background worker
   is running, the return value will be <literal>BGWH_STARTED</literal>, and
   the PID will be written to the provided address.  Otherwise, the return
   value will be <literal>BGWH_STOPPED</literal> or
   <literal>BGWH_POSTMASTER_DIED</literal>.
  </para>

  <para>
   A process can also wait for a background worker to shut down, by using the
   <function>WaitForBackgroundWorkerShutdown(<parameter>BackgroundWorkerHandle
   *handle</parameter>)</function> function and passing the
   <type>BackgroundWorkerHandle *</type> obtained at registration. This
   function will block until the background worker exits, or postmaster dies.
   When the background worker exits, the return value is
   <literal>BGWH_STOPPED</literal>, if postmaster dies it will return
   <literal>BGWH_POSTMASTER_DIED</literal>.
  </para>

  <para>
   Background workers can send asynchronous notification messages, either by
   using the <command>NOTIFY</command> command via <acronym>SPI</acronym>,
   or directly via <function>Async_Notify()</function>.  Such notifications
   will be sent at transaction commit.
   Background workers should not register to receive asynchronous
   notifications with the <command>LISTEN</command> command, as there is no
   infrastructure for a worker to consume such notifications.
  </para>

  <para>
   The <filename>src/test/modules/worker_spi</filename> module
   contains a working example,
   which demonstrates some useful techniques.
  </para>

  <para>
   The maximum number of registered background workers is limited by
   <xref linkend="guc-max-worker-processes"/>.
  </para>
</chapter>