WARNING: This guide assumes a working knowledge of Rack protocol and Rack concepts such as middlewares, url maps and Rack::Builder.
h3. Introduction to Rack
bq. 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.
* "Ruby on Rack #1 - Hello Rack!":http://m.onkey.org/2008/11/17/ruby-on-rack-1
* "Ruby on Rack #2 - The Builder":http://m.onkey.org/2008/11/18/ruby-on-rack-2-rack-builder
h3. Rails on Rack
h4. Rails Application's Rack Object
<tt>ActionController::Dispatcher.new</tt> is the primary Rack application object of a Rails application. Any Rack compliant web server should be using +ActionController::Dispatcher.new+ object to serve a Rails application.</p>
<tt>script/server</tt> does the basic job of creating a +Rack::Builder+ object and starting the webserver. This is Rails' equivalent of Rack's +rackup+ script.
Here's how +script/server+ creates an instance of +Rack::Builder+
<ruby>
app = Rack::Builder.new {
use Rails::Rack::LogTailer unless options[:detach]
use Rails::Rack::Debugger if options[:debugger]
map "/" do
use Rails::Rack::Static
run ActionController::Dispatcher.new
end
}.to_app
</ruby>
Middlewares used in the code above are primarily useful only in the development envrionment. The following table explains their usage:
|_.Middleware|_.Purpose|
|Rails::Rack::LogTailer|Appends log file output to console|
To use +rackup+ instead of Rails' +script/server+, you can put the following inside +config.ru+ of your Rails application's root directory:
<ruby>
# RAILS_ROOT/config.ru
require "config/environment"
use Rails::Rack::LogTailer
use Rails::Rack::Static
run ActionController::Dispatcher.new
</ruby>
And start the server:
<shell>
[lifo@null application]$ rackup
</shell>
To find out more about different +rackup+ options:
<shell>
[lifo@null application]$ rackup --help
</shell>
h3. Action Controller Middleware Stack
Many of Action Controller's internal components are implemented as Rack middlewares. +ActionController::Dispatcher+ uses +ActionController::MiddlewareStack+ to combine various internal and external middlewares to form a complete Rails Rack application.
NOTE: +ActionController::MiddlewareStack+ is Rails' equivalent of +Rack::Builder+, but built for better flexibility and more features to meet Rails' requirements.
h4. Inspecting Middleware Stack
Rails has a handy rake task for inspecting the middleware stack in use:
<shell>
$ rake middleware
</shell>
For a freshly generated Rails application, this might produce something like:
<ruby>
use Rack::Lock
use ActionController::Failsafe
use ActionController::Session::CookieStore, , {:secret=>"<secret>", :session_key=>"_<app>_session"}
use Rails::Rack::Metal
use ActionController::RewindableInput
use ActionController::ParamsParser
use Rack::MethodOverride
use Rack::Head
use ActiveRecord::QueryCache
run ActionController::Dispatcher.new
</ruby>
Purpose of each of this middlewares is explained in "Internal Middlewares":#internal-middleware-stack section.
h4. Configuring Middleware Stack
Rails provides a simple configuration interface +config.middleware+ for adding, removing and modifying the middlewares in the middleware stack via +environment.rb+ or the environment specific configuration file <tt>environments/<environment>.rb</tt>.
h5. Adding a Middleware
You can add a new middleware to the middleware stack using any of the following methods:
* +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.
<strong>Example:</strong>
<ruby>
# environment.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.
The middleware stack behaves just like a normal +Array+. You can use any +Array+ methods to insert, reorder, or remove items from the stack. Methods described in the section above are just convenience methods.
For example, the following removes the middleware matching the supplied class name:
Rails Metal applications are minimal Rack applications specially designed for integrating with a typical Rails application. As Rails Metal Applications skip all of the Action Controller stack, serving a request has no overhead from the Rails framework itself. This is especially useful for infrequent cases where the performance of the full stack Rails framework is an issue.
Ryan Bates' railscast on the "Rails Metal":http://railscasts.com/episodes/150-rails-metal provides a nice walkthrough generating and using Rails Metal.
Metal applications are an optimization. You should make sure to "understand the related performance implications":http://weblog.rubyonrails.org/2008/12/20/performance-of-rails-metal before using it.
h4. Execution Order
All Metal Applications are executed by +Rails::Rack::Metal+ middleware, which is a part of the +ActionController::MiddlewareStack+ chain.
Here's the primary method responsible for running the Metal applications:
In the code above, +@metals+ is an ordered hash of metal applications. Due to the default alphabetical ordering, +aaa.rb+ will come before +bbb.rb+ in the metal chain.
It is, however, possible to override the default ordering in your environment. Simply add a line like the following to +config/environment.rb+
<ruby>
config.metals = ["Bbb", "Aaa"]
</ruby>
Each string in the array should be the name of your metal class. If you do this then be warned that any metal applications not listed will not be loaded.
WARNING: Metal applications cannot return the HTTP Status +404+ to a client, as it is used for continuing the Metal chain execution. Please use normal Rails controllers or a custom middleware if returning +404+ is a requirement.
