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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 a custom Middleware stack


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

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.

- Rack API Documentation

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:

Rails on Rack

Rails Application’s Rack Object is the primary Rack application object of a Rails application. Any Rack compliant web server should be using object to serve a Rails application.


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 = {
use Rails::Rack::LogTailer unless options[:detach]
use Rails::Rack::Debugger if options[:debugger]

map “/” do use Rails::Rack::Static run end


Middlewares used in the code above are primarily useful only in the 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


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

    require “config/environment”

use Rails::Rack::LogTailer
use Rails::Rack::Static

And start the server:

[lifo@null application]$ rackup

To find out more about different rackup options:

[lifo@null application]$ rackup —help

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

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 might produce something like:

use Rack::Lock
use ActionController::Failsafe
use ActionController::Session::CookieStore, , {:secret=>"“, :session_key=>”session"}
use Rails::Rack::Metal
use ActionController::RewindableInput
use ActionController::ParamsParser
use Rack::MethodOverride
use Rack::Head
use ActiveRecord::QueryCache

Purpose of each of this middlewares is explained in Internal Middlewares section.

Configuring Middleware Stack

Rails provides a simple configuration interface config.middleware for adding, removing and modifying the middlewares in the middleware stack via environment.rb or the environment specific configuration file environments/<environment>.rb.

Adding a Middleware

You can add a new middleware to the middleware stack using any of the following methods:

  • config.middleware.add(new_middleware, args) – Adds the new middleware at the bottom of the middleware stack.
  • config.middleware.insert(index, new_middleware, args) – Adds the new middleware at the position specified by index in the middleware stack.
  • config.middleware.insert_before(existing_middleware, new_middleware, args) – Adds the new middleware before the specified existing middleware in the middleware stack.
  • config.middleware.insert_after(existing_middleware, new_middleware, args) – Adds the new middleware after the specified existing middleware in the middleware stack.


  1. environment.rb
  1. Push Rack::BounceFavicon at the bottom
    config.middleware.use Rack::BounceFavicon
  1. Add Lifo::Cache after ActiveRecord::QueryCache.
  2. Pass { :page_cache => false } argument to Lifo::Cache.
    config.middleware.insert_after ActiveRecord::QueryCache, Lifo::Cache, :page_cache => false
Swapping a Middleware

You can swap an existing middleware in the middleware stack using config.middleware.swap.


  1. environment.rb
  1. Replace ActionController::Failsafe with Lifo::Failsafe
    config.middleware.swap ActionController::Failsafe, Lifo::Failsafe

Internal Middleware Stack

Much of Action Controller’s functionality is implemented as Middlewares. The following table explains the purpose of each of them:

Middleware Purpose
Rack::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.
Rack::MethodOverride Sets HTTP method based on _method parameter or env[“HTTP_X_HTTP_METHOD_OVERRIDE”].
Rack::Head Discards the response body if the client sends a HEAD request.

TIP: It’s possible to use any of the above middlewares in your custom Rack stack.

Customizing Internal Middleware Stack

It’s possible to replace the entire middleware stack with a custom stack using ActionController::Dispatcher.middleware=.


Put the following in an initializer:

  1. config/initializers/stack.rb
    ActionController::Dispatcher.middleware = do |m|
    m.use ActionController::Failsafe
    m.use ActiveRecord::QueryCache
    m.use Rack::Head

And now inspecting the middleware stack:

$ rake middleware
(in /Users/lifo/Rails/blog)
use ActionController::Failsafe
use ActiveRecord::QueryCache
use Rack::Head

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.

Generating a Metal Application

Rails provides a generator called metal for creating a new Metal application:

$ script/generate metal poller

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

  1. Allow the metal piece to run in isolation
    require(File.dirname(FILE) + “/../../config/environment”) unless defined?(Rails)

class Poller
if env[“PATH_INFO”] =~ /^\/poller/
[200, {"Content-Type" => “text/html”}, [“Hello, World!”]]
[404, {"Content-Type" => “text/html”}, [“Not Found”]]

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

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 =
return result unless result0.to_i == 404

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.

WARNING: 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.


Lighthouse ticket

  • February 7, 2009: Second version by Pratik
  • January 11, 2009: First version by Pratik
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