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 rackup
To use rackup
instead of Rails' bin/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
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 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
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.
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 theActionDispatch::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.
4 Resources
4.1 Learning Rack
4.2 Understanding Middlewares
Feedback
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