<varname>search_path</varname> was not reported by releases
    before 18.)
    Note that
    <varname>server_version</varname>,
    <varname>server_encoding</varname> and
    <varname>integer_datetimes</varname>
    are pseudo-parameters that cannot change after startup.
    This set might change in the future, or even become configurable.
    Accordingly, a frontend should simply ignore ParameterStatus for
    parameters that it does not understand or care about.
   </para>

   <para>
    If a frontend issues a <command>LISTEN</command> command, then the
    backend will send a NotificationResponse message (not to be
    confused with NoticeResponse!)  whenever a
    <command>NOTIFY</command> command is executed for the same
    channel name.
   </para>

   <note>
    <para>
     At present, NotificationResponse can only be sent outside a
     transaction, and thus it will not occur in the middle of a
     command-response series, though it might occur just before ReadyForQuery.
     It is unwise to design frontend logic that assumes that, however.
     Good practice is to be able to accept NotificationResponse at any
     point in the protocol.
    </para>
   </note>
  </sect2>

  <sect2 id="protocol-flow-canceling-requests">
   <title>Canceling Requests in Progress</title>

   <para>
    During the processing of a query, the frontend might request
    cancellation of the query.  The cancel request is not sent
    directly on the open connection to the backend for reasons of
    implementation efficiency: we don't want to have the backend
    constantly checking for new input from the frontend during query
    processing.  Cancel requests should be relatively infrequent, so
    we make them slightly cumbersome in order to avoid a penalty in
    the normal case.
   </para>

   <para>
    To issue a cancel request, the frontend opens a new connection to
    the server and sends a CancelRequest message, rather than the
    StartupMessage message that would ordinarily be sent across a new
    connection.  The server will process this request and then close
    the connection.  For security reasons, no direct reply is made to
    the cancel request message.
   </para>

   <para>
    A CancelRequest message will be ignored unless it contains the
    same key data (PID and secret key) passed to the frontend during
    connection start-up.  If the request matches the PID and secret
    key for a currently executing backend, the processing of the
    current query is aborted.  (In the existing implementation, this is
    done by sending a special signal to the backend process that is
    processing the query.)
   </para>

   <para>
    The cancellation signal might or might not have any effect &mdash; for
    example, if it arrives after the backend has finished processing
    the query, then it will have no effect.  If the cancellation is
    effective, it results in the current command being terminated
    early with an error message.
   </para>

   <para>
    The upshot of all this is that for reasons of both security and
    efficiency, the frontend has no direct way to tell whether a
    cancel request has succeeded.  It must continue to wait for the
    backend to respond to the query.  Issuing a cancel simply improves
    the odds that the current query will finish soon, and improves the
    odds that it will fail with an error message instead of
    succeeding.
   </para>

   <para>
    Since the cancel request is sent across a new connection to the
    server and not across the regular frontend/backend communication
    link, it is possible for the cancel request to be issued by any
    process, not just the frontend whose query is to be canceled.
    This might provide additional flexibility when building
    multiple-process applications.  It also introduces a security
    risk, in that unauthorized persons might try to cancel queries.
    The security risk is addressed by requiring a dynamically
    generated secret key