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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

endprologue.

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

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.

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:

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

rackup

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

  1. 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

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 #
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 ActionDispatch::Head
use Rack::ConditionalGet
use Rack::ETag
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:

config.middleware.delete(middleware)

Internal Middleware Stack

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

ActionDispatch::Static
  • Used to serve static assets. Disabled if config.serve_static_assets is true.
Rack::Lock
  • Sets env[“rack.multithread”] flag to true and wraps the application within a Mutex.
ActiveSupport::Cache::Strategy::LocalCache::Middleware
  • Used for memory caching. This cache is not thread safe.
Rack::Runtime
  • Sets an X-Runtime header, containing the time (in seconds) taken to execute the request.
Rack::MethodOverride
  • Allows the method to be overridden if params[:_method] is set. This is the middleware which supports the PUT and DELETE HTTP method types.
ActionDispatch::RequestId
  • Makes a unique X-Request-Id header available to the response and enables the ActionDispatch::Request#uuid method.
Rails::Rack::Logger
  • Notifies the logs that the request has began. After request is complete, flushes all the logs.
ActionDispatch::ShowExceptions
  • Rescues any exception returned by the application and calls an exceptions app that will wrap it in a format for the end user.
ActionDispatch::DebugExceptions
  • Responsible for logging exceptions and showing a debugging page in case the request is local.
ActionDispatch::RemoteIp
  • Checks for IP spoofing attacks.
ActionDispatch::Reloader
  • Provides prepare and cleanup callbacks, intended to assist with code reloading during development.
ActionDispatch::Callbacks
  • Runs the prepare callbacks before serving the request.
ActiveRecord::ConnectionAdapters::ConnectionManagement
  • Cleans active connections after each request, unless the rack.test key in the request environment is set to true.
ActiveRecord::QueryCache
  • Enables the Active Record query cache.
ActionDispatch::Cookies
  • Sets cookies for the request.
ActionDispatch::Session::CookieStore
  • Responsible for storing the session in cookies.
ActionDispatch::Flash
  • Sets up the flash keys. Only available if config.action_controller.session_store is set to a value.
ActionDispatch::ParamsParser
  • Parses out parameters from the request into params.
ActionDispatch::Head
  • Converts HEAD requests to GET requests and serves them as so.
Rack::ConditionalGet
  • Adds support for “Conditional GET” so that server responds with nothing if page wasn’t changed.
Rack::ETag
  • Adds ETag header on all String bodies. ETags are used to validate cache.
ActionDispatch::BestStandardsSupport
  • 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.

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 = 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

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
    config.middleware.clear


Add a config.ru file to Rails.root

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

Resources

Learning Rack

Understanding Middlewares

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