This guide covers Rails integration with Rack and interfacing with other Rack components.
After reading this guide, you will know:
This guide assumes a working knowledge of Rack protocol and Rack concepts such as middlewares, URL maps, and Rack::Builder
.
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.
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.
rails server
rails server
does the basic job of creating a Rack::Server
object and starting the web server.
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
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_relative 'config/environment' run Rails.application
And start the server:
$ rackup config.ru
To find out more about different rackup
options, you can run:
$ rackup --help
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.
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.
Rails has a handy command for inspecting the middleware stack in use:
$ 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 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::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.
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
.
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
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
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.
$ 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
Much of Action Controller's functionality is implemented as Middlewares. The following list explains the purpose of each of them:
Rack::Sendfile
config.action_dispatch.x_sendfile_header
option.ActionDispatch::Static
config.public_file_server.enabled
is false
.Rack::Lock
env["rack.multithread"]
flag to false
and wraps the application within a Mutex.ActionDispatch::Executor
ActiveSupport::Cache::Strategy::LocalCache::Middleware
Rack::Runtime
Rack::MethodOverride
params[:_method]
is set. This is the middleware which supports the PUT and DELETE HTTP method types.ActionDispatch::RequestId
X-Request-Id
header available to the response and enables the ActionDispatch::Request#request_id
method.ActionDispatch::RemoteIp
Sprockets::Rails::QuietAssets
Rails::Rack::Logger
ActionDispatch::ShowExceptions
ActionDispatch::DebugExceptions
ActionDispatch::Reloader
ActionDispatch::Callbacks
ActiveRecord::Migration::CheckPending
ActiveRecord::PendingMigrationError
if any migrations are pending.ActionDispatch::Cookies
ActionDispatch::Session::CookieStore
ActionDispatch::Flash
config.action_controller.session_store
is set to a value.ActionDispatch::ContentSecurityPolicy::Middleware
Rack::Head
GET
requests and serves them as so.Rack::ConditionalGet
GET
" so that server responds with nothing if the page wasn't changed.Rack::ETag
Rack::TempfileReaper
It's possible to use any of the above middlewares in your custom Rack stack.
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