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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 Resources section below.

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.

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

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


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

  1. Rails.root/
    require “config/environment”

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

And start the server:

$ rackup

To find out more about different rackup options:

$ 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 ActionDispatch::Static
use Rack::Lock
use ActiveSupport::Cache::Strategy::LocalCache
use Rack::Runtime
use Rails::Rack::Logger
use ActionDispatch::ShowExceptions
use ActionDispatch::RemoteIp
use Rack::Sendfile
use ActionDispatch::Callbacks
use ActiveRecord::ConnectionAdapters::ConnectionManagement
use ActiveRecord::QueryCache
use ActionDispatch::Cookies
use ActionDispatch::Session::CookieStore
use ActionDispatch::Flash
use ActionDispatch::ParamsParser
use Rack::MethodOverride
use ActionDispatch::Head
use ActionDispatch::BestStandardsSupport
run Blog::Application.routes

Purpose of each of this middlewares is explained in the 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 application.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.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.
  1. config/application.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. config/application.rb
  1. Replace ActionController::Failsafe with Lifo::Failsafe
    config.middleware.swap ActionController::Failsafe, Lifo::Failsafe
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:


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 Enables 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

Using Rack Builder

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

Clear the existing Rails middleware stack

  1. config/application.rb

Add a file to Rails.root

    use MyOwnStackFromScratch


Learning Rack

Understanding Middlewares

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