Rails on Rack

This guide covers Rails integration with Rack and interfacing with other Rack components.

After reading this guide, you will know:

How to use Rack Middlewares in your Rails applications.
Action Pack's internal Middleware stack.
How to define a custom Middleware stack.

This guide assumes a working knowledge of Rack protocol and Rack concepts such as middlewares, URL maps, and Rack::Builder.

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.

Explaining how Rack works 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

Rails.application is the primary Rack application object of a Rails application. Any Rack compliant web server should be using Rails.application object to serve a Rails application.

2.2. `bin/rails server`

bin/rails server does the basic job of creating a Rack::Server object and starting the web server.

Here's how bin/rails server creates an instance of Rack::Server

Rails::Server.new.tap do |server|
  require APP_PATH
  Dir.chdir(Rails.application.root)
  server.start
end

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

2.3. Development and Auto-reloading

Middlewares are loaded once and are not monitored for changes. You will have to restart the server for changes to be reflected in the running application.

3. Action Dispatcher Middleware Stack

Many of Action Dispatcher'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.

ActionDispatch::MiddlewareStack is Rails' equivalent of Rack::Builder, but is built for better flexibility and more features to meet Rails' requirements.

3.1. Inspecting Middleware Stack

Rails has a handy command for inspecting the middleware stack in use:

$ bin/rails middleware

For a freshly generated Rails application, this might produce something like:

use ActionDispatch::HostAuthorization
use Rack::Sendfile
use ActionDispatch::Static
use ActionDispatch::Executor
use ActionDispatch::ServerTiming
use ActiveSupport::Cache::Strategy::LocalCache::Middleware
use Rack::Runtime
use Rack::MethodOverride
use ActionDispatch::RequestId
use ActionDispatch::RemoteIp
use Sprockets::Rails::QuietAssets
use Rails::Rack::Logger
use ActionDispatch::ShowExceptions
use WebConsole::Middleware
use ActionDispatch::DebugExceptions
use ActionDispatch::ActionableExceptions
use ActionDispatch::Reloader
use ActionDispatch::Callbacks
use ActiveRecord::Migration::CheckPending
use ActionDispatch::Cookies
use ActionDispatch::Session::CookieStore
use ActionDispatch::Flash
use ActionDispatch::ContentSecurityPolicy::Middleware
use Rack::Head
use Rack::ConditionalGet
use Rack::ETag
use Rack::TempfileReaper
run MyApp::Application.routes

The default middlewares shown here (and some others) are each summarized in the Internal Middlewares section, below.

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 ActionDispatch::Executor.
# Pass { page_cache: false } argument to Lifo::Cache.
config.middleware.insert_after ActionDispatch::Executor, 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 ActionDispatch::ShowExceptions with Lifo::ShowExceptions
config.middleware.swap ActionDispatch::ShowExceptions, Lifo::ShowExceptions

3.2.3. Moving a Middleware

You can move an existing middleware in the middleware stack using config.middleware.move_before and config.middleware.move_after.

# config/application.rb

# Move ActionDispatch::ShowExceptions to before Lifo::ShowExceptions
config.middleware.move_before Lifo::ShowExceptions, ActionDispatch::ShowExceptions

# config/application.rb

# Move ActionDispatch::ShowExceptions to after Lifo::ShowExceptions
config.middleware.move_after Lifo::ShowExceptions, ActionDispatch::ShowExceptions

3.2.4. Deleting a Middleware

Add the following lines to your application configuration:

# config/application.rb
config.middleware.delete Rack::Runtime

And now if you inspect the middleware stack, you'll find that Rack::Runtime is not a part of it.

$ bin/rails middleware
(in /Users/lifo/Rails/blog)
use ActionDispatch::Static
use #<ActiveSupport::Cache::Strategy::LocalCache::Middleware:0x00000001c304c8>
...
run Rails.application.routes

If you want to remove session related middleware, do the following:

# config/application.rb
config.middleware.delete ActionDispatch::Cookies
config.middleware.delete ActionDispatch::Session::CookieStore
config.middleware.delete ActionDispatch::Flash

And to remove browser related middleware,

# config/application.rb
config.middleware.delete Rack::MethodOverride

If you want an error to be raised when you try to delete a non-existent item, use delete! instead.

# config/application.rb
config.middleware.delete! ActionDispatch::Executor

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:

ActionDispatch::HostAuthorization

Guards from DNS rebinding attacks by explicitly permitting the hosts a request can be sent to. See the configuration guide for configuration instructions.

Rack::Sendfile

Sets server specific X-Sendfile header. Configure this via config.action_dispatch.x_sendfile_header option.

ActionDispatch::Static

Used to serve static files from the public directory. Disabled if config.public_file_server.enabled is false.

Rack::Lock

Sets env["rack.multithread"] flag to false and wraps the application within a Mutex.

ActionDispatch::Executor

Used for thread safe code reloading during development.

ActionDispatch::ServerTiming

Sets a Server-Timing header containing performance metrics for the request.

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#request_id method.

ActionDispatch::RemoteIp

Checks for IP spoofing attacks.

Sprockets::Rails::QuietAssets

Suppresses logger output for asset requests.

Rails::Rack::Logger

Notifies the logs that the request has begun. After the 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::ActionableExceptions

Provides a way to dispatch actions from Rails' error pages.

ActionDispatch::Reloader

Provides prepare and cleanup callbacks, intended to assist with code reloading during development.

ActionDispatch::Callbacks

Provides callbacks to be executed before and after dispatching the request.

ActiveRecord::Migration::CheckPending

Checks pending migrations and raises ActiveRecord::PendingMigrationError if any migrations are pending.

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.session_store is set to a value.

ActionDispatch::ContentSecurityPolicy::Middleware

Provides a DSL to configure a Content-Security-Policy header.

Rack::Head

Returns an empty body for all HEAD requests. It leaves all other requests unchanged.

Rack::ConditionalGet

Adds support for "Conditional GET" so that server responds with nothing if the page wasn't changed.

Rack::ETag

Adds ETag header on all String bodies. ETags are used to validate cache.

Rack::TempfileReaper

Cleans up tempfiles used to buffer multipart requests.

It's possible to use any of the above middlewares in your custom Rack stack.

Railscast on Rack Middlewares