Have you ever considered using instant messages to communicate between programs? You can do this using Jabber's XMPP protocol, of course. But it's also worth taking a look at AMQP, a distributed messaging protocol first used at JPMorgan Chase. AMQP is fast, easy to use, and implemented by at least 4 open source servers.

To try it out, install the excellent Ruby AMQP bindings, and set up the RabbitMQ server (which is written in Erlang using Mnesia). On a Mac, you might do something like this:

sudo gem install amqp
sudo port install python25 rabbitmq-server
sudo rabbitmq-server

Once your server is running, save the following code as chat.rb:

require 'rubygems'
gem 'amqp'
require 'mq'

unless ARGV.length == 2
  STDERR.puts "Usage: #{$0} <channel> <nick>"
  exit 1
end
$channel, $nick = ARGV

AMQP.start(:host => 'localhost') do
  $chat = MQ.topic('chat')

  # Print any messages on our channel.
  queue = MQ.queue($nick)
  queue.bind('chat', :key => $channel)
  queue.subscribe do |msg|
    if msg.index("#{$nick}:") != 0
      puts msg
    end
  end

  # Forward console input to our channel.
  module KeyboardInput
    include EM::Protocols::LineText2
    def receive_line data
      $chat.publish("#{$nick}: #{data}",
                    :routing_key => $channel)
    end
  end
  EM.open_keyboard(KeyboardInput)
end

Now, run copies in two different terminals:

ruby chat.rb channel_1 sarah
ruby chat.rb channel_1 joe

Everything you type into one terminal will be relayed to the other.

How it works

The following line creates a topic exchange named "chat":

$chat = MQ.topic('chat')

A topic exchange allows many-to-many communication. Here, we bind a listener to our exchange, and ask to receive all messages tagged with our channel name:

queue.bind('chat', :key => $channel)

Note that :key may be hierarchical, and it may contain wildcards. To write data to our topic exchange, we use publish:

$chat.publish("#{$nick}: #{data}",
              :routing_key => $channel)

Our keyboard input is processed using EventMachine, a Ruby library for writing high-performance, multi-protocol servers. It's very similar to Python's Twisted library, though it has less documentation and support for fewer protocols.

We use EventMachine's EM.open_keyboard to create a asynchronous keyboard input channel, and we use EM::Protocols::LineText2 to treat the keyboard input as a line-oriented protocol.

Adding a Shoes GUI

Shoes is an eccentric, entertaining, and highly-portable GUI library by _why the lucky stiff. With a certain amount of grotesque kludging (and some pointers from "s1kx" on the #shoes IRC channel), I managed to get the Mac version of Shoes to talk to EventMachine. You may find that this code fails strangely on your computer. Honestly, I don't know anything about Shoes. And I'm doing some pretty bad things with threads.

First, the pretty pictures:

Next, the code:

Shoes.setup { gem 'amqp' }
require 'mq'

$app = Shoes.app(:width => 256) do
  background(gradient('#CFF', '#FFF'))
  @output = stack(:margin => 10)

  def nick str
    span(str, :stroke => red)
  end

  def display text
    @output.append do
      if text =~ /^([^:]+): (.*)$/
        para nick("#{$1}: "), $2
      else
        para text
      end
    end
  end
end

Thread.new do
  begin
    AMQP.start(:host => 'localhost') do
      MQ.topic('chat')
      queue = MQ.queue('shoes')
      queue.bind('chat')
      queue.subscribe do |msg|
        $app.display(msg)
      end
    end
  rescue => e
    # Try to report at least _some_ errors
    # where we'll be able to see them.
    $app.display(e.to_s)
  end
end

Note that the GUI client listens to all channels simultaneously, because it doesn't pass a :key to bind. And when writing code to run in a Shoes background thread, don't expect to see any error messages.

Learning more about AMQP

The Ruby AMQP documentation page has a good list of papers, magazine articles, and other background material on AMQP.