Rails on Rack

This guide covers Rails integration with Rack and interfacing with other Rack components. By referring to this guide, you will be able to:

  • Create Rails Metal applications

  • Use Rack Middlewares in your Rails applications

  • Understand Action Pack’s internal Middleware stack

  • Define custom internal Middleware stack

  • Understand the best practices for developing a middleware aimed at Rails applications

Note This guide assumes a working knowledge of Rack protocol and Rack concepts such as middlewares, url maps and Rack::Builder.

1. Introduction to Rack

Rack provides a minimal, modular and adaptable interface for developing web applications in Ruby. By wrapping HTTP requests and responses in the simplest way possible, it unifies and distills the API for web servers, web frameworks, and software in between (the so-called middleware) into a single method call.

Explaining Rack is not really in the scope of this guide. In case you are not familiar with Rack’s basics, you should check out the following links:

2. Rails on Rack

2.1. ActionController::Dispatcher.new

ActionController::Dispatcher.new is the primary Rack application object of a Rails application. It responds to call method with a single env argument and returns a Rack response. Any Rack compliant web server should be using ActionController::Dispatcher.new object to serve a Rails application.

2.2. script/server

script/server does the basic job of creating a Rack::Builder object and starting the webserver. This is Rails equivalent of Rack’s rackup script.

Here’s how script/server creates an instance of Rack::Builder

app = Rack::Builder.new {
  use Rails::Rack::LogTailer unless options[:detach]
  use Rails::Rack::Static
  use Rails::Rack::Debugger if options[:debugger]
  run ActionController::Dispatcher.new
}.to_app

Middlewares used in the code above are most useful in development envrionment. The following table explains their usage:

Middleware Purpose

Rails::Rack::LogTailer

Appends log file output to console

Rails::Rack::Static

Serves static files inside RAILS_ROOT/public directory

Rails::Rack::Debugger

Starts Debugger

2.3. rackup

To use rackup instead of Rails' script/server, you can put the following inside config.ru of your Rails application’s root directory:

# RAILS_ROOT/config.ru
require "config/environment"

use Rails::Rack::LogTailer
use Rails::Rack::Static
run ActionController::Dispatcher.new

And start the server:

[lifo@null application]$ rackup

To find out more about different rackup options:

[lifo@null application]$ rackup --help

3. Action Controller Middleware Stack

Many of Action Controller’s internal components are implemented as Rack middlewares. ActionController::Dispatcher uses ActionController::MiddlewareStack to combine various internal and external middlewares to form a complete Rails Rack application.

Note
What is ActionController::MiddlewareStack ?
ActionController::MiddlewareStack is Rails equivalent of Rack::Builder, but built for better flexibility and more features to meet Rails' requirements.

3.1. Inspecting Middleware Stack

Rails has a handy rake task for inspecting the middleware stack in use:

$ rake middleware

For a freshly generated Rails application, this will produce:

use ActionController::Lock
use ActionController::Failsafe
use ActiveRecord::QueryCache
use ActionController::Session::CookieStore, {:secret=>"<secret>", :session_key=>"_<app>_session"}
use Rails::Rack::Metal
use ActionController::VerbPiggybacking
run ActionController::Dispatcher.new

3.2. Adding Middlewares

Rails provides a very simple configuration interface for adding generic Rack middlewares to a Rails applications.

Here’s how you can add middlewares via environment.rb

# environment.rb

config.middleware.use Rack::BounceFavicon

3.3. Internal Middleware Stack

use "ActionController::Lock", :if => lambda {
  !ActionController::Base.allow_concurrency
}

use "ActionController::Failsafe"

use "ActiveRecord::QueryCache", :if => lambda { defined?(ActiveRecord) }

["ActionController::Session::CookieStore",
 "ActionController::Session::MemCacheStore",
 "ActiveRecord::SessionStore"].each do |store|
   use(store, ActionController::Base.session_options,
      :if => lambda {
        if session_store = ActionController::Base.session_store
          session_store.name == store
        end
      }
    )
end

use ActionController::VerbPiggybacking
Middleware Purpose

ActionController::Lock

Sets env["rack.multithread"] flag to true and wraps the application within a Mutex.

ActionController::Failsafe

Returns HTTP Status 500 to the client if an exception gets raised while dispatching.

ActiveRecord::QueryCache

Enable the Active Record query cache.

ActionController::Session::CookieStore

Uses the cookie based session store.

ActionController::Session::MemCacheStore

Uses the memcached based session store.

ActiveRecord::SessionStore

Uses the database based session store.

ActionController::VerbPiggybacking

Sets HTTP method based on _method parameter or env["HTTP_X_HTTP_METHOD_OVERRIDE"].

3.4. Customizing Internal Middleware Stack

VERIFY THIS WORKS. Just a code dump at the moment.

Put the following in an initializer.

ActionController::Dispatcher.middleware = ActionController::MiddlewareStack.new do |m|
  m.use ActionController::Lock
  m.use ActionController::Failsafe
  m.use ActiveRecord::QueryCache
  m.use ActionController::Session::CookieStore
  m.use ActionController::VerbPiggybacking
end

Josh says :

3.3: I wouldn’t recommend this: custom internal stack i’d recommend using config.middleware.use api we still need a better api for swapping out existing middleware, etc config.middleware.swap AC::Sessions, My::Sessoins or something like that

4. Rails Metal Applications

Rails Metal applications are minimal Rack applications specially designed for integrating with a typical Rails application. As Rails Metal Applications skip all of the Action Controller stack, serving a request has no overhead from the Rails framework itself. This is especially useful for infrequent cases where the performance of the full stack Rails framework is an issue.

4.1. Generating a Metal Application

Rails provides a generator called performance_test for creating new performance tests:

script/generate metal poller

This generates poller.rb in the app/metal directory:

# Allow the metal piece to run in isolation
require(File.dirname(__FILE__) + "/../../config/environment") unless defined?(Rails)

class Poller
  def self.call(env)
    if env["PATH_INFO"] =~ /^\/poller/
      [200, {"Content-Type" => "text/html"}, ["Hello, World!"]]
    else
      [404, {"Content-Type" => "text/html"}, ["Not Found"]]
    end
  end
end

Metal applications are an optimization. You should make sure to understand the related performance implications before using it.

4.2. Execution Order

All Metal Applications are executed by Rails::Rack::Metal middleware, which is a part of the ActionController::MiddlewareStack chain.

Here’s the primary method responsible for running the Metal applications:

def call(env)
  @metals.keys.each do |app|
    result = app.call(env)
    return result unless result[0].to_i == 404
  end
  @app.call(env)
end

In the code above, @metals is an ordered ( alphabetical ) hash of metal applications. Due to the alphabetical ordering, aaa.rb will come before bbb.rb in the metal chain.

Important Metal applications cannot return the HTTP Status 404 to a client, as it is used for continuing the Metal chain execution. Please use normal Rails controllers or a custom middleware if returning 404 is a requirement.

5. Middlewares and Rails

6. Changelog

Lighthouse ticket

  • January 11, 2009: First version by Pratik