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 custom internal Middleware stack +
+
+
+Understand the best practices for developing a middleware aimed at Rails applications +
+

+ + + +

+ +	This guide assumes a working knowledge of Rack protocol and Rack concepts such as 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 following links:

2. Rails on Rack

2.1. ActionController::Dispatcher.new

ActionController::Dispatcher.new is the primary Rack application object of a Rails application. It responds to call method with a single env argument and returns a Rack response. Any Rack compliant web server should be using ActionController::Dispatcher.new object to serve a Rails application.

2.2. script/server

script/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 script/server creates an instance of Rack::Builder

app = Rack::Builder.new {
+  use Rails::Rack::LogTailer unless options[:detach]
+  use Rails::Rack::Static
+  use Rails::Rack::Debugger if options[:debugger]
+  run ActionController::Dispatcher.new
+}.to_app

Middlewares used in the code above are most useful in development envrionment. The following table explains their usage:

+ +++ + + + + + + + + + + + + + + + + + + + +

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

2.3. rackup

To use rackup instead of Rails' script/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 Rails::Rack::Static
+run ActionController::Dispatcher.new

And start the server:

[lifo@null application]$ rackup

To find out more about different rackup options:

[lifo@null 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.

+ + + +

+ +	+ What is ActionController::MiddlewareStack ? `ActionController::MiddlewareStack` is Rails equivalent of `Rack::Builder`, but built for better flexibility and more features to meet Rails' requirements.

3.1. Adding Middlewares

Rails provides a very simple configuration interface for adding generic Rack middlewares to a Rails applications.

Here’s how you can add middlewares via environment.rb

# environment.rb
+
+config.middleware.use Rack::BounceFavicon

3.2. Internal Middleware Stack

use "ActionController::Lock", :if => lambda {
+  !ActionController::Base.allow_concurrency
+}
+
+use "ActionController::Failsafe"
+
+use "ActiveRecord::QueryCache", :if => lambda { defined?(ActiveRecord) }
+
+["ActionController::Session::CookieStore",
+ "ActionController::Session::MemCacheStore",
+ "ActiveRecord::SessionStore"].each do |store|
+   use(store, ActionController::Base.session_options,
+      :if => lambda {
+        if session_store = ActionController::Base.session_store
+          session_store.name == store
+        end
+      }
+    )
+end
+
+use ActionController::VerbPiggybacking

+ +++ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

Middleware	Purpose
ActionController::Lock	Mutex
ActionController::Failsafe	Never fail
ActiveRecord::QueryCache	Query caching
ActionController::Session::CookieStore	Query caching
ActionController::Session::MemCacheStore	Query caching
ActiveRecord::SessionStore	Query caching
ActionController::VerbPiggybacking	_method hax

3.3. Custom Internal Middleware Stack

VERIFY THIS WORKS. Just a code dump at the moment.

Put the following in an initializer.

ActionController::Dispatcher.middleware = ActionController::MiddlewareStack.new do |m|
+  m.use ActionController::Lock
+  m.use ActionController::Failsafe
+  m.use ActiveRecord::QueryCache
+  m.use ActionController::Session::CookieStore
+  m.use ActionController::VerbPiggybacking
+end

4. Rails Metal Applications

Rails Metal applications are minimal Rack applications specially designed for integrating with a typical Rails applications. As Rails Metal Applications skip all of the Action Controller stack, serving a request has no overhead from the Rails framework itself. This is especially useful for infrequent cases where the performance of the full stack Rails framework is an issue.

4.1. Generating a Metal Application

Rails provides a generator called performance_test for creating new performance tests:

script/generate metal poller

This generates poller.rb in the app/metal directory:

# Allow the metal piece to run in isolation
+require(File.dirname(__FILE__) + "/../../config/environment") unless defined?(Rails)
+
+class Poller
+  def self.call(env)
+    if env["PATH_INFO"] =~ /^\/poller/
+      [200, {"Content-Type" => "text/html"}, ["Hello, World!"]]
+    else
+      [404, {"Content-Type" => "text/html"}, ["Not Found"]]
+    end
+  end
+end

Metal applications are an optimization. You should make sure to understand the related performance implications before using it.

5. Changelog

Lighthouse ticket

+
+January 11, 2009: First version by Pratik +
+

+ +

Ruby on Rails

Sustainable productivity for web-application development

Chapters