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, $closed ) = @_;

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

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

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

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

 $loop->add( $sequencer );

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

When used as a server:

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

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

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

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

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

    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\n";
    },
 );

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:

  • A pure client

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

  • 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.

  • 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, $closed )

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 = "\x0d\x0a"; # More portable than \r\n

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

    $buffref =~ s/^(.*)$CRLF// and
       $self->incoming_request( $1 ), return 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, $closed ) = @_;

       print STDERR "Spurious input: $$buffref\n";
       $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, $closed ) = @_;

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

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

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

TODO

  • 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?

  • 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>