A Quick Introduction to Rack

What's Rack?

In the words of the author of Rack – Christian Neukirchen: Rack aims to provide a minimal API for connecting web servers supporting Ruby (like WEBrick, Mongrel etc.) and Ruby web frameworks (like Rails, Sinatra etc.).

Web frameworks such as Sinatra are built on top of Rack or have a Rack interface for allowing web application servers to connect to them.

The premise of Rack is simple – it just allows you to easily deal with HTTP requests.

HTTP is a simple protocol: it basically describes the activity of a client sending a HTTP request to a server and the server returning a HTTP response. Both HTTP request and HTTP response in turn have very similar structures. A HTTP request is a triplet consisting of a method and resource pair, a set of headers and an optional body while a HTTP response is in triplet consisting of a response code, a set of headers and an optional body.

Rack maps closely to this. A Rack application is a Ruby object that has a call method, which has a single argument, the environment, (corresponding to a HTTP request) and returns an array of 3 elements, status, headers and body (corresponding to a HTTP response).

Rack includes handlers that connect Rack to all these web application servers (WEBrick, Mongrel etc.).

Rack includes adapters that connect Rack to various web frameworks (Sinatra, Rails etc.).

Between the server and the framework, Rack can be customized to your applications needs using middleware. The fundamental idea behind Rack middleware is – come between the calling client and the server, process the HTTP request before sending it to the server, and processing the HTTP response before returning it to the client.

A Rack App

Installing Rack

Note that I have Ruby 1.9.2 installed on a Windows box and all the programs in this article have been tested using that.

Let's check if we already have rack with us. Open a command window and type:

irb --simple-prompt
>> require 'rack'
=> true
>>

Yes, rack's already there on your machine. If rack's not there you will get an error like:

LoadError: no such file to load -- rack

You can install rack by opening a new command window and typing:

gem install rack

A quick visit to Ruby's proc object

Remember the proc object from Ruby? Blocks are not objects, but they can be converted into objects of class Proc. This can be done by calling the lambda method of the class Object. A block created with lambda acts like a Ruby method. The class Proc has a method call that invokes the block.

In irb type:

>> my_rack_proc = lambda {puts 'Rack Intro'}
=> #<Proc:0x1fc9038@(irb):2(lambda)>
>> # method call invokes the block
?> my_rack_proc.call
Rack Intro
=> nil
>>

A simple Rack app – my_rack_proc

As mentioned earlier, our simple Rack application is a Ruby object (not a class) that responds to call and takes exactly one argument, the environment. The environment must be a true instance of Hash. The app should return an Array of exactly three values: the status code (it must be greater than or equal to 100), the headers (must be a hash), and the body (the body commonly is an Array of Strings, the application instance itself, or a File-like object. The body must respond to method each and must only yield String values.) Let us create our new proc object. Type:

>> my_rack_proc = lambda { |env| [200, {}, ["Hello. The time is #{Time.now}"]] }
=> #<Proc:0x1f4c358@(irb):5(lambda)>
>>

Now we can call the proc object my_rack_proc with the call method. Type:

>> my_rack_proc.call({})
=> [200, {}, ["Hello. The time is 2011-10-24 09:18:56 +0530"]]
>>

my_rack_proc is our single line Rack application.

In the above example, we have used an empty hash for headers. Instead, let's have something in the header as follows:

>> my_rack_proc = lambda { |env| [200, {"Content-Type" => "text/plain"}, ["Hello. The time is #{Time.now}"]] }
=> #<Proc:0x1f4c358@(irb):5(lambda)>
>>

Now we can call the proc object my_rack_proc with the call method. Type:

>> my_rack_proc.call({})
=> [200, {"Content-Type" => "text/plain"}, ["Hello. The time is 2011-10-24 09:18:56 +0530"]]
>>

We can run our Rack application (my_rack_proc) with any of the Rack handlers.

To look at the Rack handlers available, in irb type:

>> Rack::Handler.constants
=> [:CGI, :FastCGI, :Mongrel, :EventedMongrel, :SwiftipliedMongrel, :WEBrick, :LSWS, :SCGI, :Thin]
>>

To get a handler for say WEBrick (the default WEBrick, web application server, that comes along with Ruby), type:

>> Rack::Handler::WEBrick
=> Rack::Handler::WEBrick
>>

All of these handlers have a common method called run to run all the Rack based applications.

>> Rack::Handler::WEBrick.run my_rack_proc
[2011-10-24 10:00:45] INFO WEBrick 1.3.1
[2011-10-24 10:00:45] INFO ruby 1.9.2 (2011-07-09) [i386-mingw32]
[2011-10-24 10:00:45] INFO WEBrick::HTTPServer#start: pid=1788 port=80

Open a browser window and type the url: http://localhost/

In your browser window, you should see a string, something like this:

Hello. The time is 2011-10-24 10:02:20 +0530

Note: If you already have something running at port 80, you can run this app at a different port, say 9876. Type:

>> Rack::Handler::WEBrick.run my_rack_proc, :Port => 9876
[2011-10-24 11:32:21] INFO  WEBrick 1.3.1
[2011-10-24 11:32:21] INFO  ruby 1.9.2 (2011-07-09) [i386-mingw32]
[2011-10-24 11:32:21] INFO  WEBrick::HTTPServer#start: pid=480 port=9876

Another Rack app – my_method

A Rack app need not be a lambda; it could be a method. Type:

>> def my_method env
>> [200, {}, ["method called"]]
>> end
=> nil

We declare a method my_method that takes an argument env. The method returns three values.

Next type:

>> Rack::Handler::WEBrick.run method(:my_method)
[2011-10-24 14:32:05] INFO  WEBrick 1.3.1
[2011-10-24 14:32:05] INFO  ruby 1.9.2 (2011-07-09) [i386-mingw32]
[2011-10-24 14:32:05] INFO  WEBrick::HTTPServer#start: pid=1644 port=80

Open a browser window and type the url: http://localhost/

In your browser window, you should see something like this:

method called

Method objects are created by Object#method. They are associated with a particular object (not just with a class). They may be used to invoke the method within the object. The Method.call method invokes the method with the specified arguments, returning the method's return value.

Press Ctrl-C in irb to stop the WEBrick server.

Using rackup

The rack gem comes with a bunch of useful stuff to make life easier for a rack application developer. rackup is one of them.

rackup is a useful tool for running Rack applications. rackup automatically figures out the environment it is run in, and runs your application as FastCGI, CGI, or standalone with Mongrel or WEBrick – all from the same configuration.

To use rackup, you'll need to supply it with a rackup config file. By convention, you should use .ru extension for a rackup config file. Supply it a run RackObject and you're ready to go:

$ rackup config.ru

By default, rackup will start a server on port 9292.

To view rackup help, open a command window and type:

$ rackup --help

Let us create a config.ru file that contains the following:

run lambda { |env| [200, {"Content-Type" => "text/plain"}, ["Hello. The time is #{Time.now}"]] }

This file contains run, which can be called on anything that responds to a .call.

To run our rack app, in the same folder that contains config.ru, type:

$ rackup config.ru
[2011-10-24 15:18:03] INFO WEBrick 1.3.1
[2011-10-24 15:18:03] INFO ruby 1.9.2 (2011-07-09) [i386-mingw32]
[2011-10-24 15:18:03] INFO WEBrick::HTTPServer#start: pid=3304 port=9292

Open a browser window and type the url: http://localhost:9292/

In your browser window, you should see something like this:

Hello. The time is 2011-10-24 15:18:10 +0530

Press Ctrl-C to stop the WEBrick server.

Now let's move our application from the config.ru file to my_app.rb file as follows:

# my_app.rb
class MyApp
  def call env
    [200, {"Content-Type" => "text/html"}, ["Hello Rack Participants"]]
  end
end

Also, our config.ru will change to:

require './my_app'
run MyApp.new

To run our rack app, in the same folder that contains config.ru, type:

rackup config.ru
[2011-10-25 06:18:16] INFO  WEBrick 1.3.1
[2011-10-25 06:18:16] INFO  ruby 1.9.2 (2011-07-09) [i386-mingw32]
[2011-10-25 06:18:16] INFO  WEBrick::HTTPServer#start: pid=2224 port=9292

Open a browser window and type the url: http://localhost:9292/

In your browser window, you should see something like this:

Hello Rack Participants

Using Rack middleware

We mentioned earlier that between the server and the framework, Rack can be customized to your applications needs using middleware. The fundamental idea behind Rack middleware is – come between the calling client and the server, process the HTTP request before sending it to the server, and processing the HTTP response before returning it to the client.

Rackup also has a use method that accepts a middleware. Let us use one of Rack's built-in middleware.

You must have observed that every time we make a change to our config.ru or my_app.rb files, we need to restart the WEBrick server. To avoid this, let's use one of Rack's built-in middleware – Rack::Reloader. Edit config.ru to have:

require './my_app'
use Rack::Reloader
run MyApp.new

In the same folder, we have the my_app.rb file. It's contents are:

# my_app.rb
class MyApp
  def call env
    [200, {"Content-Type" => "text/html"}, ["Hello Rack Participants from across the globe"]]
  end
end

To run our rack app, in the same folder that contains config.ru, type:

$ rackup config.ru

Open a browser window and type the url: http://localhost:9292/

In your browser window, you should see something like this:

Hello Rack Participants from across the globe

Modify my_app.rb and instead of "Hello Rack Participants from across the globe" type "Hello Rack Participants from across the world!"

Refresh the browser window and you should see:

Hello Rack Participants from across the world!

There was no need to re-start the WEBrick server.

Writing our own Middleware

For all practical purposes, a middleware is a rack application that wraps up an inner application.

Our middleware is going to do something very simple. We will append some text to the body being sent by our inner application. Let us write our own middleware. We will create a middleware class called MyRackMiddleware. Here's the skeleton program:

class MyRackMiddleware
  def initialize(appl)
    @appl = appl
  end
  def call(env)
  end
end

In the code above, to get the original body, we initialize the class MyRackMiddleware by passing in the inner application (appl).

Next we need to call this appl:

def call(env)
  status, headers, body = @appl.call(env) # we now call the inner application
end

In the above code, we now have access to the original body, which we can now modify. Rack does not guarantee you that the body would be a string. It could be an object too. However, we are guaranteed that if we call .each on the body, everything it returns will be a string. Let's use this in our call method:

def call(env)
  status, headers, body = @appl.call(env)
  append_s = "... greetings from RubyLearning!!"
  [status, headers, body << append_s]
end

Here's our completed middleware class:

class MyRackMiddleware
  def initialize(appl)
    @appl = appl
  end
  def call(env)
    status, headers, body = @appl.call(env)
    append_s = "... greetings from RubyLearning!!"
    [status, headers, body << append_s]
  end
end

Our my_app.rb file remains the same. It's contents are:

# my_app.rb
class MyApp
  def call(env)
    [200, {"Content-Type" => "text/html"}, ["Hello Rack Participants from across the globe"]]
  end
end

Edit config.ru to have:

require './my_app'
require './myrackmiddleware'
use Rack::Reloader
use MyRackMiddleware
run MyApp.new

To run our rack app, in the same folder that contains config.ru, type:

$ rackup config.ru

Open a browser window and type the url: http://localhost:9292/

In your browser window, you should see something like this:

Hello Rack Participants from across the globe... greetings from RubyLearning!!

Rack and Sinatra

Let us see how to use Rack with a Sinatra app. Create a folder racksinatra. Next, let us install Sinatra:

gem install sinatra

Here's a trivial Sinatra program my_sinatra.rb in the folder racksinatra:

require 'sinatra'
get '/' do
  'Welcome to all'
end

Sinatra can use Rack middleware. Our trivial middleware class will intercept the request from the user, calculate and display the response time on the console and pass on the unaltered request to the Sinatra app. In the folder racksinatra write the middleware class namely rackmiddleware.rb.

class RackMiddleware
  def initialize(appl)
    @appl = appl
  end
  def call(env)
    start = Time.now
    status, headers, body = @appl.call(env) # call our Sinatra app
    stop = Time.now
    puts "Response Time: #{stop-start}" # display on console
    [status, headers, body]
  end
end

In order for Heroku to know what to do with our Sinatra app, create a config.ru in the folder racksinatra:

require "./my_sinatra"
run Sinatra::Application

Modify the Sinatra app to use our middleware:

# my_sinatra.rb
require 'sinatra'
require './rackmiddleware'
use RackMiddleware
get '/' do
  'Welcome to all'
end

That's it. I hope you liked this introductory article on Rack. Happy Racking!!