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Rails on Rack

This guide covers Rails integration with Rack and interfacing with other Rack components.

After reading this guide, you will know:

  • How to create Rails Metal applications.
  • How to use Rack Middlewares in your Rails applications.
  • Action Pack's internal Middleware stack.
  • How to 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.

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

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

rails server

rails server does the basic job of creating a Rack::Server object and starting the webserver.

Here's how rails server creates an instance of Rack::Server do |server|
  require APP_PATH

The Rails::Server inherits from Rack::Server and calls the Rack::Server#start method this way:

class Server < ::Rack::Server
  def start

Here's how it loads the middlewares:

def middleware
  middlewares = []
  middlewares << [Rails::Rack::Debugger] if options[:debugger]
  middlewares << [::Rack::ContentLength]

Rails::Rack::Debugger is primarily useful only in the development environment. The following table explains the usage of the loaded middlewares:

Middleware Purpose
Rails::Rack::Debugger Starts Debugger
Rack::ContentLength Counts the number of bytes in the response and set the HTTP Content-Length header


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 Rack::Debugger
use Rack::ContentLength
run ApplicationName::Application

And start the server:

$ rackup

To find out more about different rackup options:

$ rackup --help

Action Dispatcher Middleware Stack

Many of Action Dispatchers's internal components are implemented as Rack middlewares. Rails::Application uses ActionDispatch::MiddlewareStack to combine various internal and external middlewares to form a complete Rails Rack application.

NOTE: ActionDispatch::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::Middleware:0x000000029a0838>
use Rack::Runtime
use Rack::MethodOverride
use ActionDispatch::RequestId
use Rails::Rack::Logger
use ActionDispatch::ShowExceptions
use ActionDispatch::DebugExceptions
use ActionDispatch::RemoteIp
use ActionDispatch::Reloader
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::Head
use Rack::ConditionalGet
use Rack::ETag
use ActionDispatch::BestStandardsSupport
run MyApp::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.

# config/application.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

Swapping a Middleware

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

# config/application.rb

# Replace ActionDispatch::ShowExceptions with Lifo::ShowExceptions
config.middleware.swap ActionDispatch::ShowExceptions, Lifo::ShowExceptions

Middleware Stack is an Enumerable

The middleware stack behaves just like a normal Enumerable. You can use any Enumerable methods to manipulate or interrogate the stack. The middleware stack also implements some Array methods including [], unshift and delete. Methods described in the section above are just convenience methods.

Append following lines to your application configuration:

# config/application.rb
config.middleware.delete "Rack::Lock"

And now if you inspect the middleware stack, you'll find that Rack::Lock will not be part of it.

$ rake middleware
(in /Users/lifo/Rails/blog)
use ActionDispatch::Static
use #<ActiveSupport::Cache::Strategy::LocalCache::Middleware:0x00000001c304c8>
use Rack::Runtime
run Blog::Application.routes

If you want to remove session related middleware, do the following:

# config/application.rb
config.middleware.delete "ActionDispatch::Cookies"
config.middleware.delete "ActionDispatch::Session::CookieStore"
config.middleware.delete "ActionDispatch::Flash"

And to remove browser related middleware,

# config/application.rb
config.middleware.delete "ActionDispatch::BestStandardsSupport"
config.middleware.delete "Rack::MethodOverride"

Internal Middleware Stack

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


  • Used to serve static assets. Disabled if config.serve_static_assets is true.


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


  • Used for memory caching. This cache is not thread safe.


  • Sets an X-Runtime header, containing the time (in seconds) taken to execute the request.


  • Allows the method to be overridden if params[:_method] is set. This is the middleware which supports the PUT and DELETE HTTP method types.


  • Makes a unique X-Request-Id header available to the response and enables the ActionDispatch::Request#uuid method.


  • Notifies the logs that the request has began. After request is complete, flushes all the logs.


  • Rescues any exception returned by the application and calls an exceptions app that will wrap it in a format for the end user.


  • Responsible for logging exceptions and showing a debugging page in case the request is local.


  • Checks for IP spoofing attacks.


  • Provides prepare and cleanup callbacks, intended to assist with code reloading during development.


  • Runs the prepare callbacks before serving the request.


  • Cleans active connections after each request, unless the rack.test key in the request environment is set to true.


  • Enables the Active Record query cache.


  • Sets cookies for the request.


  • Responsible for storing the session in cookies.


  • Sets up the flash keys. Only available if config.action_controller.session_store is set to a value.


  • Parses out parameters from the request into params.


  • Converts HEAD requests to GET requests and serves them as so.


  • Adds support for "Conditional GET" so that server responds with nothing if page wasn't changed.


  • Adds ETag header on all String bodies. ETags are used to validate cache.


  • Enables “best standards support” so that IE8 renders some elements correctly.

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

Using Rack Builder

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

Clear the existing Rails middleware stack

# config/application.rb

Add a file to Rails.root

use MyOwnStackFromScratch
run ApplicationName::Application


Learning Rack

Understanding Middlewares

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