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!