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Rack Walkthrough

Rack is the HTTP interface for Ruby. Rack defines a standard interface for interacting with HTTP and connecting web servers. Rack makes it easy to write HTTP facing applications in Ruby. Rack applications are shockingly simple. There is the code that accepts a request and code serves the response. Rack defines the interface between the two.

This walkthrough covers Rack from the beginning up to applications, middleware, middleware stacks, testing, integrations, and finally webservers. You’ll have a much better understanding of how the Ruby web stack works by the end of this walkthrough.

Rack applications are objects that respond to call. They must return a "triplet". A triplet contains the status code, headers, and body. Here’s an example class that shows "hello world."

This class is not a Rack application. It demonstrates what a triplet looks like. The first element is the HTTP response code. The second is a hash of headers. The third is an enumerable object representing the body. We can use our HelloWorld class to create a simple rack application. We know that we need to create an object that responds to call. call takes one argument: the rack environment. We’ll come back to the envlater.

Now for a runnable server script:

Here’s what happens when you run this script:

NOTE: The output you see may be different. Rack::Server chooses a server based off what’s installed in order of preference. It uses Webrick if you have nothing else installed because that’s part of Ruby’s standard library. More on servers later on.

Simply open http://localhost:8080 and you’ll see "Hello World" in the browser. It’s not fancy but you just wrote your first rack app! We didn’t write our own server and that’s ok. Matter of fact, that’s fantastic. Odds are you will never need to write your own server. There are plenty of servers to choose from: Thin, Unicorn, Rainbows, Goliath, Puma, and Passenger. You don’t want to write those. You want to write applications.

Now visit http://localhost:8080?message=foo and you’ll see "message=foo" on the page. If you’re more curious about env you can do this:

Now curl the URL and to see everything included in env. It’s a standard Hashinstance.

You may have noticed that the env doesn’t do any fancy parsing. The query string wasn’t a hash. It was the string. It is raw data. Rack is simple to understand and use. You could only work with hashes and triplets. However that’s tedious and doesn’t scale. Complex applications need abstractions. Enter Rack::Request and Rack::Response.

Further Reading

Rack::Request is simply a proxy for the env hash. The underlying env hash is modified so keep that in mind.

Rack::Response is an abstraction around response triplets. It simplifies access to headers, cookies, and the body. Here’s an example:

These are basic abstractions. They don’t require much explanation. You can learn more about them by reading the documentation.

Now we build more complex applications now on these abstracations. It’s hard to make an application when all the logic is in one class. Applications are composed of different classes with different responsiblities. These discrete chunks are called “middleware”.

Further Reading

Rack applications are objects that respond to call. We can do whatever we want inside call, for instance we can delegate to another class. Here’s an example:

Here’s “null op” middleware:

Another middleware that modifies the upstream response:

Lastly, a middleware that modifies the request before downstream processing:

The middleware stack exmplifies the builder pattern. Composing Rack applications is so common (and required) that Rack includes a class to make this easy.

Rack::Builder creates a middleware stack. Each object calls the next one and returns its return value. Rack contains a bunch of handy middlewares. They have one for caching and encodings. Let’s increase our test applications performance.

app has a call method that generates this call tree:

We’ll skip the functionality of each middleware because that’s not important. This is an example of how you can build up functionality in applications. Middlewares are powerful. You can add manipulate incoming data before hitting the next one or modify the response from an existing one. Let’s create some for practice.

And another:

Now we can use those middlewares in our app.

We’ve just written our own middeware and learned how to generate a runnable application with a middleware stack. This is how rack apps are written in practice. Now onto the final piece of the puzzle: config.ru

Further Resources

Now navigate into the correct directory and run: rackup and you’ll see:

NOTE: The exact output may vary system to system depending on which gems are installed and their versions.

rackup prefers better servers like Thin over WeBrick. The code inside config.ru is evaluated and built using a Rack::Builder which generates an rack API compliant object. The object is passed to the rack server (Thin). The rack server puts the application online.

TIP: Your webserver (e.g. puma, thin) usually provides their own CLI with server specific options. rackup provides a way to forward options to the underlying application server but this may not work in all cases. The underlying application server may also have a separate configuration file for its specific configuration (e.g. a control port/socket). You’re advised to understand that you can launch your webserver in multiple ways. Investigate and learn which fits your use case.

Rails 3+ is fully Rack compliant. A Rails 3 application is more complex Rack application. It uses a complex middleware stack. The dispatcher is the final middlware. The dispatcher reads the routing table and calls the correct controller and method. Here’s the stock middleware stack used in production:

And rackup file:

You know that Example::Application must have a call method. Here’s the implementation of this method from 3.2 stable:

We’ve covered how rackup and Rack::Server use available server libraries to start the actual HTTP server. The Ruby community offers many options for various use cases. Let’s review them so you can make the best choice.

This server is included in the standard library. It’s meant only for development. Webrick is a toy implementation. It should never be used for anything remotely close to production.

This server uses reactor pattern provided Eventmachine. It used to be popular but has been superseeded by better alternatives. If you cannot use anything better, then go for thin. It’s better than Webrick.

Puma is multi-threaded and multi-process. This the most flexible server out there for pure Ruby application. You can use multi-process (a.k.a. clustered mode) if your Ruby platform has a GIL (read: MRI) and your application is CPU bound. IO bound applications (most web applications) can go ahead with the mutli-threaded mode (default mode) since the GIL blocks true concurrency. Puma also includes a HTTP control interface for programmatical manipulation or getting stats. Check out puma before going for anything else — especially if you’re using JRuby or Rubinius.

Unicorn uses a pre-forking model. This means it will start up multiple processes and use system calls to “load balance” requests across child processes. Users must note that connections to external resources (such as a database) must happen in child proess. Unicorn provides an “after fork” for this. Users locked on MRI should investigate this option.

Passenger is a bit different than the others in the list. Passenger is is an nginx or apache module. It does also have a standalone mode also. It supports multi-threaded and multi-process models. You should consider passenger if you need to deploy the application to an existing apache or nginx server.

This is the end of the walkthrough. We’ve covered:

Now you’re better equipped to leverage these abstractions in your applications and libraries.

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Rack Walkthrough

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