NAME

       Message::Passing::Manual::Concepts - The concepts behind the framework

DESCRIPTION

       This framework tries to be a simplifying layer over message passing, allowing you to
       easily make the networking parts of message passing become just configuration. This allows
       you to concentrate on the hard parts (i.e. your application, not the message passing).

COMPONENTS AND CHAINS

       There are only a few core concepts to grasp to use the framework. We'll start with the
       component types, and then move onto chains.

       There are only three types of components:

   OUTPUTS
       An output is simply a class with a "consume" method. This will be called with a message as
       its only parameter, like this:

           $output->consume($message);

       Outputs are expected to compose Message::Passing::Role::Output.

   INPUTS
       An input is simply a class with an "output_to" attribute. Your code just calls the consume
       method on its output, like this:

           $self->output_to->consume($message);

       Inputs are expected to compose Message::Passing::Role::Input which provides this
       attribute, and use the "BUILD" method from Moo to do any work needed to start listening
       for events.

   FILTER
       A filter is just a combination of an output and input. Some (or all) of the messages
       consumed by the input are sent on to the output.

       An optional Message::Passing::Role::Filter is supplied, allowing you to provide a simple
       filter method:

           with 'Message::Passing::Role::Filter';

           sub filter {
               my ($self, $message) = @_;
               return $message; # Or return undef to drop it
           }

       However, you can write a filter manually as:

           with qw/
               Message::Passing::Role::Input
               Message::Passing::Role::Output
           /;

           sub consume {
               my ($self, $message) = @_;
               # Do something to $message here
               $self->output_to->consume($message);
           }

       As you've hopefully guessed by now, a "chain" is just an input, outputting to zero or more
       filters, which output to an output.

DSL

       So, this is all pretty easy, and you already know enough to pick up some components and
       use them! For example:

           use Message::Passing::Input::FileTail;
           use Message::Passing::Output::STDOUT;

           Message::Passing::Input::FileTail->new(
               filename => $ARGV[0],
               output_to => Message::Passing::Output::STDOUT->new,
           );
           AnyEvent->condvar->recv; # Enter event loop

       There you go - you're tailing a file to screen - however you could just as easily by
       sending it over the network with ZeroMQ or any other output.

       This is, however, a bit ugly! If you're building a chain of several filters, or you have
       several inputs being multiplexed into one output, then the code gets ugly fast.

       To make it easy to build chains of processing, and your own scripts, a simple DSL is
       provided.  The example above becomes:

           use Message::Passing::DSL;

           run_message_server message_chain {
               input file => (
                   class => 'FileTail',
                   output_to => 'stdout',
               );
               output stdout => (
                   class => 'STDOUT',
               );
           };

Event loop

       AnyEvent has been mentioned, and it's expected that scripts will use a supported event
       loop. This implies that your code is asynchronous, which is generally fine - however it
       should be noted that doing any long operation (non trivial database queries) will block
       the entire server - meaning no events will be processed.

       In cases such as these, running a pool of worker processes to distribute the blocking jobs
       is more appropriate, and easy to wire up (on one or more hosts). This is documented more
       fully in Message::Passing::Manual::Workers

ZeroMQ

       ZeroMQ is the recommended transport for messages, and Message::Passing::Output::ZeroMQ is
       designed to work inside a traditional synchronous application. This means that you can
       emit messages into ZeroMQ without blocking your application, or having to use or run the
       AnyEvent event loop.

SEE ALSO

   Message::Passing::Manual::Cookbook
       Recipies for achieving common tasks

AUTHOR, COPYRIGHT & LICENSE

       See Message::Passing.