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 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.
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 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 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 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 ::File.expand_path('../config/environment', __FILE__) run Rails.application
And start the server:
$ rackup config.ru
To find out more about different rackup
options:
$ rackup --help
2.4 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 built for better flexibility and more features to meet Rails' requirements.
3.1 Inspecting Middleware Stack
Rails has a handy task for inspecting the middleware stack in use:
$ bin/rails middleware
For a freshly generated Rails application, this might produce something like:
use Rack::Sendfile use ActionDispatch::Static use ActionDispatch::Executor use #<ActiveSupport::Cache::Strategy::LocalCache::Middleware:0x000000029a0838> 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::Migration::CheckPending use ActiveRecord::ConnectionAdapters::ConnectionManagement use ActiveRecord::QueryCache use ActionDispatch::Cookies use ActionDispatch::Session::CookieStore use ActionDispatch::Flash use Rack::Head use Rack::ConditionalGet use Rack::ETag run Rails.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 ActiveRecord::QueryCache. # Pass { page_cache: false } argument to Lifo::Cache. config.middleware.insert_after ActiveRecord::QueryCache, 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 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> use Rack::Runtime ... 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
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 files from the public directory. Disabled if
config.public_file_server.enabled
isfalse
.
Rack::Lock
- Sets
env["rack.multithread"]
flag tofalse
and wraps the application within a Mutex.
ActionDispatch::Executor
- Used for thread safe code reloading during development.
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 theActionDispatch::Request#request_id
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
- 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.
ActiveRecord::ConnectionAdapters::ConnectionManagement
- Cleans active connections after each request, unless the
rack.test
key in the request environment is set totrue
.
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.
Rack::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.
It's possible to use any of the above middlewares in your custom Rack stack.
4 Resources
4.1 Learning Rack
4.2 Understanding Middlewares
Feedback
You're encouraged to help improve the quality of this guide.
Please contribute if you see any typos or factual errors. To get started, you can read our documentation contributions section.
You may also find incomplete content, or stuff that is not up to date. Please do add any missing documentation for master. Make sure to check Edge Guides first to verify if the issues are already fixed or not on the master branch. Check the Ruby on Rails Guides Guidelines for style and conventions.
If for whatever reason you spot something to fix but cannot patch it yourself, please open an issue.
And last but not least, any kind of discussion regarding Ruby on Rails documentation is very welcome in the rubyonrails-docs mailing list.