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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:

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.

- 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:

2 Rails on Rack

2.1 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.

2.2 rails server

rails 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 rails server creates an instance of Rack::Builder

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

Middlewares used in the code above are primarily useful only in the development environment. The following table explains their usage:

Middleware Purpose
Rails::Rack::LogTailer Appends log file output to console
ActionDispatch::Static Serves static files inside Rails.root/public directory
Rails::Rack::Debugger Starts Debugger

2.3 rackup

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

# Rails.root/ require "config/environment" use Rails::Rack::LogTailer use ActionDispatch::Static run

And start the server:

[[email protected] application]$ rackup

To find out more about different rackup options:

[[email protected] 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.

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 might produce something like:

use Rack::Lock use ActionController::Failsafe use ActionController::Session::CookieStore, , {:secret=>"<secret>", :session_key=>"_<app>_session"} use Rails::Rack::Metal use ActionDispatch::RewindableInput use ActionController::ParamsParser use Rack::MethodOverride use Rack::Head use ActiveRecord::QueryCache run

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

3.2 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.

3.2.1 Adding a Middleware

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

  • config.middleware.use(new_middleware, args) – Adds the new middleware at the bottom of 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.


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

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


# config/environment.rb # Replace ActionController::Failsafe with Lifo::Failsafe config.middleware.swap ActionController::Failsafe, Lifo::Failsafe
3.2.3 Middleware Stack is an Array

The middleware stack behaves just like a normal Array. You can use any Array methods to insert, reorder, or remove items from the stack. Methods described in the section above are just convenience methods.

For example, the following removes the middleware matching the supplied class name:


3.3 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.

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

3.4 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:

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

And now inspecting the middleware stack:

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

3.5 Using Rack Builder

The following shows how to replace use Rack::Builder instead of the Rails supplied MiddlewareStack.

Clear the existing Rails middleware stack

# environment.rb config.middleware.clear

Add a file to Rails.root

# use MyOwnStackFromStratch run

4 Resources

4.1 Learning Rack

4.2 Understanding Middlewares

5 Changelog

Lighthouse ticket

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