Embedding Cramp in a Rails App

So I have an application that would work great with websockets because I don't want to be doing client polling. Rails doesn't deal with this right now, but Cramp does (cramp is a young web framework built on top of EventMachine). I know, I know, Node.js right? But I've already deployed on Heroku and they aren't taking new participants for their Node.js beta, so I'm going to have to make it work.

Now, Cramp is indeed a Rack-based framework (mostly, sometimes not compliant), so theoretically I should be able to do this, and if it works out you'll have this post to help you along the way (using Rails 3.0, BTW).

First, I added Cramp to my gemfile (gem "cramp") and tried to bundle install it. Failure. The current cramp gem depends on arel (= 0.3.3), and Rails 3 of course need arel 1.1.0.

To fix that, I tried loading edge cramp into my app by using the git repo in my gemfile:

gem "cramp",:git=>'git://github.com/lifo/cramp.git'

So far so good, at least the bundle installed correctly.

Before going any farther, I also added "thin" to my gemfile to use as my webserver in development, because in the Cramp documentation they point out that Cramp only works with Thin or Rainbows!.

Now, to get a cramp application running. I saw a great Screencast by Ryan Bates about Routing with Rack, so that's the approach I'm going to take using the "Hello World" example from this blogpost (IMPORTANT: Edge cramp does not use "Cramp::Controller::Action" anymore like in that blog post, Action is directly under Cramp now). To clarify, I'm dropping the following class into the lib directory:

class ChatAction < Cramp::Action
  on_start :send_hello_world

  def send_hello_world
    render "Hello World"
    finish
  end
end

And then tried to route a path to it (routes.rb):

MyApp::Application.routes.draw do
  match "/chat"=>ChatAction
end

Starting up the server, I got a failure due to an uninitialized constant (It doesn't know about my ChatAction yet). I had to think about the right place to load it, because I haven't really traced the rails startup sequence before. Looking in "application.rb", Bundler loads all it's gems after loading up rails, so I decided to drop in my requirement right after that (load all bundler gems, than load my Cramp application, then start with the rails app config):

require File.expand_path('../boot', __FILE__)

require 'rails/all'

Bundler.require(:default, Rails.env) if defined?(Bundler)
require "lib/chat_action"

module MyApp
  class Application < Rails::Application
    config.autoload_paths += %W( #{config.root}/extras )
    
    config.encoding = "utf-8"

    config.filter_parameters += [:password]
  end
end

That seemed to work; at least everything booted without any more errors when I did this from the application home directory:

rails server thin

and when I traveled to "localhost:3000/chat", I saw my "Hello World" render as expected.

So there you have it, how to get a cramp application running in your Rails 3 application (in the patented "Stumble your way through" methodology). Now to actually make it do something useful!