Rack is a Ruby web server interface that provides a minimal specification for building web servers and web applications. It allows web servers and frameworks to be developed independently while remaining compatible. Rack defines a simple interface between web servers and Ruby frameworks or applications - a Rack application must respond to the 'call' method and return an array containing the response status, headers, and body. Middleware can be used to modify requests and responses and are configured in a rackup file to build a modular stack.

1. Introduction to Rack
Kerry Buckley, 26 October 2010

2. Introduction to Rack
Why was it needed?
How does it work?
Building a stack with middleware
Writing a simple rack application

3. Web frameworks
and many more…

4. Web servers
Mongrel
CGI
Webrick

5. Before Rack

6. With Rack
Rack-compliant interface

7. Introduction to Rack
Why was it needed?
How does it work?
Building a stack with middleware
Writing a simple rack application

8. Rack is just an
interface specification

9. A Rack app is…
• An object (not a class)…
• …which responds to call(env)…
• …and returns an array containing:
• response code
• a hash of headers
• the body*
*this is a slight simplification

10. The environment hash
• REQUEST_METHOD
• SERVER_NAME, SERVER_PORT
• SCRIPT_NAME, PATH_INFO, QUERY_STRING
• HTTP_ variables from request headers
• Some rack-specific variables

11. A simple Rack app
require 'rubygems'
require 'rack'
class HelloWorld
def call(env)
[200, {"Content-Type" => "text/html"}, "Hello World!"]
end
end
Rack::Handler::Mongrel.run HelloWorld.new, :Port => 9292

12. Even simpler Rack app
require 'rubygems'
require 'rack'
Rack::Handler::Mongrel.run proc { |env|
[200, {"Content-Type" => "text/html"}, "Hello World!"]
}, :Port => 9292

13. Revisiting the body
• An object (not a class)…
• …which responds to call(env)…
• …and returns an array containing:
• response code
• a hash of headers
• the body*
*this is a slight simplification

14. Revisiting the body
• Must respond to each, yielding strings
• Strings work in 1.8, but not 1.9
• close will be called if present
• to_path can provide a file location

15. Body as array
require 'rubygems'
require 'rack'
class HelloWorld
def call(env)
[200, {"Content-Type" => "text/html"}, ["Hello ", "World!"]]
end
end
Rack::Handler::Mongrel.run HelloWorld.new, :Port => 9292

16. Body as IO object
require 'rubygems'
require 'rack'
class HelloWorld
def call(env)
[200, {"Content-Type" => "text/html"},
StringIO.new("Hello World!")]
end
end
Rack::Handler::Mongrel.run HelloWorld.new, :Port => 9292

17. Body as self
require 'rubygems'
require 'rack'
class HelloWorld
def call(env)
[200, {"Content-Type" => "text/html"}, self]
end
def each
yield "Hello "
yield "World!"
end
end
Rack::Handler::Mongrel.run HelloWorld.new, :Port => 9292

18. The rackup file
• Configuration DSL for a Rack app
• Server-independent
• Allows stacking of middleware
• Provides simple route mapping

19. The rackup file
Your config.ru file:
class HelloWorld
def call(env)
[200, {"Content-Type" => "text/html"}, "Hello World!"]
end
end
run HelloWorld.new
Rack loads it like this:
config_file = File.read(config)
rack_application = eval("Rack::Builder.new { #{config_file} }")
server.run rack_application, options

20. The rack gem
Provides a bunch of helper classes
• Request/response wrappers
• Logging
• Authentication (basic and digest)
• Cookies and sessions
• Mock requests and responses

21. Introduction to Rack
Why was it needed?
How does it work?
Building a stack with middleware
Writing a simple rack application

22. Middleware
Request Response
Middleware A
Request Response
Middleware B
Request Response
Application

23. Middleware
• A middleware is just a rack application
• Constructor takes next app down
• Can modify request or response
• Can call layer below, or just return
• Configured with ‘use’ in rackup file

24. Middleware in Rails
• Used internally for cookies, parameter
parsing etc
• Add your own in environment.rb:
Rails::Initializer.run do |config|
config.middleware.use "MyMiddlewareClass"
end

25. Introduction to Rack
Why was it needed?
How does it work?
Building a stack with middleware
Writing a simple rack application

26. Demo!