IO::Async::Sequencer(3pm)				User Contributed Perl Documentation				 IO::Async::Sequencer(3pm)

NAME

       "IO::Async::Sequencer" - handle a serial pipeline of requests / responses (EXPERIMENTAL)

SYNOPSIS

       When used as a client:

	use IO::Async::Loop;
	my $loop = IO::Async::Loop->new;

	my $sock = ...

	my $sequencer = IO::Async::Sequencer->new(
	   handle => $sock,

	   on_read => sub {
	      my ( $self, $buffref, $eof ) = @_;

	      return 0 unless $$buffref =~ s/^(.*)
//;
	      my $line = $1;

	      $line =~ m/^RESPONSE (.*)$/ and
		 $self->incoming_response( $1 ), return 1;

	      print STDERR "An error - didn't recognise the line $line
";
	   },

	   marshall_request => sub {
	      my ( $self, $request ) = @_;
	      return "REQUEST $request
";
	   },
	);

	$loop->add( $sequencer );

	$sequencer->request(
	   request     => "hello",
	   on_response => sub {
	      my ( $response ) = @_;
	      print "The response is: $response
";
	   },
	);

       When used as a server:

	my $sequencer = IO::Async::Sequencer->new(
	   handle => $sock,

	   on_read => sub {
	      my ( $self, $buffref, $eof ) = @_;

	      return 0 unless $$buffref =~ s/^(.*)
//;
	      my $line = $1;

	      $line =~ m/^REQUEST (.*)$/ and
		 $self->incoming_request( $1 ), return 1;

	      print STDERR "An error - didn't recognise the line $line
";
	   },

	   on_request => sub {
	      my ( $self, $token, $request ) = @_;

	      # Now to invoke the application logic, whatever it may be
	      solve_request(
		 request      => $request,
		 on_completed => sub {
		    my ( $response ) = @_;
		    $self->respond( $token, $response );
		 }
	      );
	   },

	   marshall_response => sub {
	      my ( $self, $response ) = @_;
	      return "RESPONSE $response
";
	   },
	);

DESCRIPTION

       This module provides an EXPERIMENTAL subclass of "IO::Async::Stream" which may be helpful in implementing serial pipeline-based network
       protocols of requests and responses. It deals with low-level details such as pairing up responses to requests in ordered protocols, and
       allows a convenient location to store the line stream marshalling and demarshalling code.

       The intention is that individual network protocols may be implemented as a subclass of this class, providing the marshalling and
       demarshalling code itself, providing a clean implementation to be used by the using code. An example protocol that would be easy to
       implement in this way would be HTTP.

       Objects in this class can operate in any of three ways:

       o   A pure client

	   The object is asked to send requests by the containing code and invokes response handling code when responses arrive.

       o   A pure server

	   The object receives requests from the filehandle, processes them, and sends responses back, but does not initiate any traffic of its
	   own.

       o   Mixed

	   The object behaves as a mixture of the two; initiating requests, as well as responding to those of its peer connection.

       The exact mode of operation of any object is not declared explicitly, but instead is an artefact of the set of callbacks provided to the
       constructor or methods defined by the subclass. Certain callbacks or methods only make sense for one mode or the other.

       The various operations required can each be provided as callback functions given in keys to the constructor, or as object methods on a
       subclass of this class. Keys passed to the constructor will take precidence over defined methods.

       As it is still EXPERIMENTAL, any details of this class are liable to change in future versions. It shouldn't yet be relied upon as a stable
       interface.

PARAMETERS

       The following named parameters may be passed to "new" or "configure":

       on_read => CODE
	       As for IO::Async::Stream. The code here should invoke the "incoming_request" or "incoming_response" methods when appropriate, after
	       having parsed the incoming stream. See the SYNOPSIS or EXAMPLES sections for more detail.

	       Each request can optionally provide its own handler for reading its response, using the "on_read" key to the "request" method. The
	       handler provided to the constructor is only used if this is not provided.

       on_request => CODE
	       A callback that is invoked when the "incoming_request" method is called (i.e. when operating in server mode). It is passed the
	       request, and a token to identify it when sending a response.

		$on_request->( $self, $token, $request );

	       The token should be considered as an opaque value - passed into the "respond" method when a response is ready, but not otherwise
	       used or modified.

       marshall_request => CODE
       marshall_response => CODE
	       Callbacks that the "request" or "respond" methods will use, respectively, to stream a request or response object into a string of
	       bytes to write to the underlying file handle.

		$string = $marshall_request->( $self, $request );

		$string = $marshall_response->( $self, $response );

	       These are used respectively by the client and server modes.

       pipeline => BOOL
	       Optional. Controls whether requests will be pipelined; that is, all requests will be sent by the client before responses are
	       received. If this option is disabled, only the first request will be sent. Other requests will be queued internally, and each will
	       be sent when the response to the previous has been received. Defaults enabled; supply a defined but false value to disable.

SUBCLASS METHODS

       This class is intended as a base class for building specific protocol handling code on top of. These methods are intended to be called by
       the specific subclass, rather than the containing application code.

   $sequencer->incoming_request( $request )
       To be called from "on_read".

       This method informs the sequencer that a new request has arrived. It will invoke "on_request", passing in a token to identify the request
       for stream ordering purposes, and the request itself.

   $sequencer->incoming_response( $response )
       To be called from "on_read".

       This method informs the sequencer that a response has arrived. It will invoke "on_response" that had been passed to the "request" method
       that sent the original request.

PUBLIC METHODS

       These methods are intended to be called by the application code using a subclass of this class.

   $sequencer->request( %params )
       Called in client mode, this method sends a request upstream, and awaits a response to it. Can be called in one of two ways; either giving a
       specific "on_read" handler to be used when the response to this request is expected, or by providing an "on_response" handler for when the
       default handler invokes "incoming_response".

       The %params hash takes the following arguments:

       request => SCALAR
	       The request value to pass to "marshall_request".

       on_response => CODE
	       A continuation to invoke when a response to this request arrives from the upstream server. It will be invoked as

		$on_response->( $response );

       on_read => CODE
	       A callback to use to parse the incoming stream while the response to this particular request is expected. It will be invoked the
	       same as for "IO::Async::Stream"; i.e.

		$on_read->( $self, $buffref, $eof )

	       This handler should return "undef" when it has finished handling the response, so that the next one queued can be invoked (or the
	       default if none exists). It MUST NOT call "incoming_response". Instead, the code should directly implement the behaviour for
	       receipt of a response.

       If the "on_read" key is used, it is intended that a specific subclass that implements a specific protocol would construct the callback code
       in a method it provides, intended for the using code to call.

   $sequencer->respond( $token, $response )
       Called in server mode, usually at the end of "on_request", or some continuation created within it, this method sends a response back
       downstream to a client that had earlier requested it.

       $token  The token that was passed into the "on_request". Used to ensure responses are sent in the right order.

       $response
	       The response value to pass to "marshall_response".

EXAMPLES

   A simple line-based server
       The following sequencer implements a simple server which takes and responds with CRLF-delimited lines.

	package LineSequencer;

	use base qw( IO::Async::Sequencer );

	my $CRLF = "x0dx0a"; # More portable than 


	sub on_read {
	   my ( $self, $buffref, $eof ) = @_;

	   while( $buffref =~ s/^(.*)$CRLF// ) {
	      $self->incoming_request( $1 );
	   }

	   return 0;
	}

	sub marshall_response {
	   my ( $self, $response ) = @_;
	   return $response . $CRLF;
	}

	1;

       The server could then be used, for example, as a simple echo server that replies whatever the client said, in uppercase. This would be done
       using an "on_request" like the following.

	my $linesequencer = LineSequencer->new(
	   handle => ...

	   on_request => sub {
	      my ( $self, $token, $request ) = @_;
	      $self->respond( $token, uc $request );
	   }
	);

       It is likely, however, that any real use of the server in a non-trivial way would perform much more work than this, and only call
       "$self->respond" in an eventual continuation at the end of performing its work. The $token is used to identify the request that the
       response responds to, so that it can be sent in the correct order.

   Per-request "on_read" handler
       If an "on_read" handler is provided to the "request" method in client mode, then that handler will be used when the response to that
       request is expected to arrive. This will be used instead of the "incoming_response" method and the "on_response" handler. If every request
       provides its own handler, then the one in the constructor would only be used for unrequested input from the server - perhaps to generate an
       error condition of some kind.

	my $sequencer = IO::Async::Sequencer->new(
	   ...

	   on_read => sub {
	      my ( $self, $buffref, $eof ) = @_;

	      print STDERR "Spurious input: $$buffref
";
	      $self->close;
	      return 0;
	   },

	   marshall_request => sub {
	      my ( $self, $request ) = @_;
	      return "GET $request" . $CRLF;
	   },
	);

	$sequencer->request(
	   request => "some key",
	   on_read => sub {
	      my ( $self, $buffref, $eof ) = @_;

	      return 0 unless $$buffref =~ s/^(.*)$CRLF//;
	      my $line = $1;

	      print STDERR "Got response: $1
" if $line =~ m/^HAVE (.*)$/;

	      return undef; # To indicate that this response is finished
	   }
	);

TODO

       o   Some consideration of streaming errors. How does the "on_read" signal to the containing object that a stream error has occured? Is it
	   fatal?  Can resynchronisation be attempted later?

       o   Support, either here or in a different class, for out-of-order protocols, such as IMAP, where responses can arrive in a different order
	   than the requests were sent.

AUTHOR

       Paul Evans <leonerd@leonerd.org.uk>

perl v5.14.2							    2012-10-24						 IO::Async::Sequencer(3pm)