Rails Middleware Stack

3.3 Internal Middleware Stack

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

Rack::Sendfile

• Sets server specific X-Sendfile header. Configure this via config.action_dispatch.x_sendfile_header option.
  ActionDispatch::Static
• Used to serve static assets. Disabled if config.serve_static_assets is false.
  Rack::Lock
• Sets env["rack.multithread"] flag to false 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.

API application (using ActionController::API) comes with the following controller modules by default:

• ActionController::UrlFor: Makes url_for and friends available
• ActionController::Redirecting: Support for redirect_to
• ActionController::Rendering: Basic support for rendering
• ActionController::Renderers::All: Support for render :json and friends
• ActionController::ConditionalGet: Support for stale?
• ActionController::ForceSSL: Support for force_ssl
• ActionController::RackDelegation: Support for the request and response methods returningActionDispatch::Request and ActionDispatch::Response objects.
• ActionController::DataStreaming: Support for send_file and send_data
• AbstractController::Callbacks: Support for before_filter and friends
• ActionController::Instrumentation: Support for the instrumentation hooks defined by ActionController (seethe source for more).
• ActionController::Rescue: Support for rescue_from.

• AbstractController::Translation: Support for the l and t localization and translation methods. These delegate to I18n.translate and I18n.localize.
• ActionController::HttpAuthentication::Basic::ControllerMethods (or Digest or Token): Support for basic, digest or token HTTP authentication.
• AbstractController::Layouts: Support for layouts when rendering.
• ActionController::MimeResponds (and ActionController::ImplicitRender for Rails 4): Support for content negotiation (respond_to, respond_with).
• ActionController::Cookies: Support for cookies, which includes support for signed and encrypted cookies. This requires the cookie middleware.