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= Writing remote ruote participants with daemon-kit

daemon-kit is an ideal housing for remote ruote participants, providing a lot
of convenience in terms of receiving and sending workitems, delegating work to
pseudo-participant classes, handling configuration of the communication channel
between ruote and the remote participant, and much more.

== What is ruote?

Ruote is a Ruby workflow engine. It is a powerful tool for defining, running and
orchestrating business processes.

* http://openwferu.rubyforge.org/
* http://www.opensourcery.co.za/2009/03/04/ruote-in-20-minutes/
* http://www.opensourcery.co.za/2009/07/06/driving-business-processes-in-ruby/

== What are remote participants?

Remote participants are participants that perform their work in a different
Ruby processes from the one running the engine. This is useful in two cases,
possibily many more, that involves autonomous participants.

* Autonomous participants located on remote servers, driven by identity
* Clustering autonomous participants to process workitems from a queue

To learn more about the differences between local and remote participants
please see http://openwferu.rubyforge.org/part.html

Currently on the AMQP components are in place in daemon-kit, with XMPP coming
soon.

== Creating a remote participant with daemon-kit

Generate your daemon using the 'ruote' generator:

  $ daemon_kit partd -i ruote

Make sure you have the JSON gem install, and the AMQP gem as well.

== Configuring the daemon

You need to review +config/ruote.yml+ to specify the AMQP queues that the daemon
will subscribe to for receiving workitems. You'll also need to configure the
AMQP gem be updating +config/amqp.yml+

The generated daemon in +libexec/+ already defaults to using AMQP as a transport
for workitems.

== Writing pseudo-participants

Pseudo-participants in daemon-kit are pure Ruby classes. Implement your classes
in +lib/+ and require them from +lib/<daemon_name>.rb+.

Register your classes as pseudo-participants by registering them in the daemon
file in +libexec+, just as the Sample class is registered in the generated
code. Your class will be instantiated upon registration, and will be re-used
for every incoming workitem passed to it.

All your public methods in the pseudo-participant classes should be accept
a single parameter, which is a ruote workitem in pure Hash form.

== Wiring up the remote participant in ruote

See the complete code here: http://gist.github.com/144861

A sample process definition might look something like this:

  class QuoteProcess < OpenWFE::ProcessDefinition
    sequence do
      kit :command => '/sample/quote', :queue => 'work1'

      console
    end
  end

+kit+ in the above process definition is registered with the ruote engine as an
AMQPParticipant. The AMQPParticipant delivers workitems to the specified AMQP
queue.

Based on the values in +config/ruote.yml+, your daemon will be subscribed to
those queues.

The second part is delegating the workitem inside the daemon to the correct
pseudo-participant. This is handled by the +:command+ parameter in the process
definition. DaemonKit::Workitem looks at the command parameter of the incoming
workitem, and then finds a registered pseudo-participant instance and calls the
requested method on that class.

The +:command+ parameter follows the following convention (stolen shamelessly
from Nanite):

  :command => '/class_name/method_name'

When classes are registered, the name of the class is downcased and camel-case
words are separated by underscores. Method names are not changed, but methods
are required to be public.

== Processing workitems and replying to the engine

The methods called in the pseudo-participants receive a single parameter, a
ruote workitem as a hash. The participant is then free to analyze the hash
and perform the appropriate actions required. The return value of the method
is discarded, and the workitem is returned back to the engine. If the method
modified the workitem, these changes will be sent along as well.

== Random other notes

Apart from configuring the AMPQ client (or XMPP in future) and the ruote.yml
file, daemon developers don't need to worry about anything related to receiving
workitems or sending replies.

Our aim is to allow you to swap between participants on both sides of the
transport without changing any of your code.
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