All About Rack

As it turns out, I made it through just about 10 months of professional web development without really digging into Rack. That's probably the case for a lot of web developers who work with abstractions and can tidily forget all about the middleware that's packaging up a request object for use in our applications.

But since I'm revisiting a lot of the curriculum we covered in my bootcamp, I thought it'd be a great time to write up a post and do a deeper dive on Rack.

What Is Rack?

The need for Rack becomes apparent when we think about the request-response cycle. The browser, or client, sends a request to a server, which is just some lines of information including the request method, the request target, and an optional body. The server parses that information and sends back information in the form of a response.

Why Do We Need It?

In the olden days, web servers would parse the request information differently, so applications would need to factor that into their responses.

Enter Rack, which is middleware that sits between the client and server and that ensures that web servers parse requests in a particular way, turning that request into a hash with specific keys.

For an application to be Rack compliant, you need the following:

  • for the application to call run on some object
  • for that object to respond to a method call
  • for that call method to accept the environment hash as a parameter and return an array with three items: the status code, the headers, and a body that responds to the .each

Usually that body is nothing more than a string of HTML in an array.

How Do We Use It?

Using Rack for a simple app is fairly straightforward.

gem install rack or add it to your Gemfile and bundle install. Running rackup from your application's root directory will execute the code in That's where you'd call the method run on some object.

Your call method is what would take in the environment and return different responses based on the HTTP method and target specified in the request.

Something even as simple as this in your would work:

require 'rack'

class App
  def call(env)
    [200, {}, ['hello world']]


And that's it. Now if you inspect the env, you'll see that it's no longer some ugly string, but an beautiful hash you can work with. That's Rack working its magic.