Guides.rubyonrails.org

Chapters

1. Introduction to Rack
2. Rails on Rack
   • Rails Application’s Rack Object
   • rails server
   • rackup
3. Action Controller Middleware Stack
   • Inspecting Middleware Stack
   • Configuring Middleware Stack
   • Internal Middleware Stack
   • Customizing Internal Middleware Stack
   • Using Rack Builder
4. Resources
   • Learning Rack
   • Understanding Middlewares

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 Resources section below.

2 Rails on Rack

2.1 Rails Application’s Rack Object

ActionController::Dispatcher.new is the primary Rack application object of a Rails application. Any Rack compliant web server should be using ActionController::Dispatcher.new 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 = Rack::Builder.new {
  use Rails::Rack::LogTailer unless options[:detach]
  use Rails::Rack::Debugger if options[:debugger]
  use ActionDispatch::Static
  run ActionController::Dispatcher.new
}.to_app

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

2.3 rackup

To use rackup instead of Rails’ rails 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 ActionDispatch::Static
run ActionController::Dispatcher.new

And start the server:

$ rackup config.ru

To find out more about different rackup options:

$ 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.

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 ActionDispatch::Static
use Rack::Lock
use ActiveSupport::Cache::Strategy::LocalCache
use Rack::Runtime
use Rails::Rack::Logger
use ActionDispatch::ShowExceptions
use ActionDispatch::DebugExceptions
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.

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 application.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/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

3.2.2 Swapping a Middleware

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

# config/application.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:

config.middleware.delete(middleware)

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:

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 = ActionController::MiddlewareStack.new 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 ActionController::Dispatcher.new

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

# config/application.rb
config.middleware.clear

Add a config.ru file to Rails.root

# config.ru
use MyOwnStackFromScratch
run ActionController::Dispatcher.new

4 Resources

4.1 Learning Rack

• Official Rack Website
• Introducing Rack
• Ruby on Rack #1 – Hello Rack!
• Ruby on Rack #2 – The Builder

4.2 Understanding Middlewares

• Railscast on Rack Middlewares

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