h2. 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+. h3. Introduction to Rack bq. 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":http://rack.rubyforge.org/doc/ 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":#resources section below. h3. Rails on Rack h4. 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. h4. +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+ Rails::Server.new.tap { |server| require APP_PATH Dir.chdir(Rails.application.root) server.start } The +Rails::Server+ inherits from +Rack::Server+ and calls the +Rack::Server#start+ method this way: class Server < ::Rack::Server def start ... super end end Here's how it loads the middlewares: def middleware middlewares = [] middlewares << [Rails::Rack::Debugger] if options[:debugger] middlewares << [::Rack::ContentLength] Hash.new(middlewares) end +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| h4. +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 Rack::Debugger use Rack::ContentLength run ApplicationName::Application And start the server: $ rackup config.ru To find out more about different +rackup+ options: $ rackup --help h3. 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. h4. 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 ApplicationName::Application.routes Purpose of each of this middlewares is explained in the "Internal Middlewares":#internal-middleware-stack section. h4. 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. h5. 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 h5. 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 h5. 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 # use Rack::Runtime ... run Blog::Application.routes h4. 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. h4. 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 ApplicationName::Application h3. Resources h4. Learning Rack * "Official Rack Website":http://rack.github.com * "Introducing Rack":http://chneukirchen.org/blog/archive/2007/02/introducing-rack.html * "Ruby on Rack #1 - Hello Rack!":http://m.onkey.org/ruby-on-rack-1-hello-rack * "Ruby on Rack #2 - The Builder":http://m.onkey.org/ruby-on-rack-2-the-builder h4. Understanding Middlewares * "Railscast on Rack Middlewares":http://railscasts.com/episodes/151-rack-middleware