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rack-app GPT assistant

rack-app is a minimalist web framework that focuses on simplicity and maintainability. The framework is meant to be used by seasoned web developers.

rack-app focus on keeping the dependencies as little as possible, while allowing writing functional and minimalist rack-based applications, that will do nothing more than what you defined.

The routing uses a prefix tree, thus adding a large number of API endpoints won't affect the routing lookup time.

It was inspirited by Sinatra, grape, and rack. It's used in production, powering back-end APIs running on the public cloud.

Development Status

The framework is considered stable. I don't have the plan to feature creep the framework without real-life use-cases, since most of the custom edge cases can be resolved with composition.

The next time it will receive further updates, when rack provides a finalized support for http2.

If you have an issue, I weekly check the issues tab, answer and reply, or implement a fix for it.

Since the framework's only dependency is the rack gem, I don't have to update the codebase too often.

Cheers and Happy Coding!


Add this line to your application's Gemfile:

gem 'rack-app'

And then execute:

$ bundle

Or install it yourself as:

$ gem install rack-app

Is it Production ready?

Yes, it's already powering Heroku hosted micro-services.


  • Keep It Simple
  • No Code bloat
  • No on run time processing, or keep at the bare minimum
  • Fully BDD (Behaviour Driven Design)
    • built-in test module to ease the development with easy to use tests
  • Easy to Learn
    • rack-app use well known and easy to understand conventions, such as sinatra like DSL
  • Principle Of Least Surprise
  • Modular design
  • the Only dependency is rack, nothing more
  • Open development
  • Try to create Examples for every feature


  • Easy to understand syntax
    • module method level endpoint definition inspirited heavily by the Sinatra DSL
    • unified error handling
    • syntax sugar for default header definitions
    • namespaces for endpoint request path declarations so it can be dry and unified
  • no Class method bloat, so you can enjoy pure ruby without any surprises
  • App mounting so you can create separated controllers for different task
  • Streaming
  • O(log(n)) lookup routing
    • allows as many endpoint registrations to you as you want, without impact on route lookup speed
  • only basic sets for instance method level for the must need tools, such as params, payload
  • Simple to use class level response serializer
    • so you can choose what type of serialization you want without any enforced convention
  • static file serving so you can mount even filesystem-based endpoints too
  • built-in testing module so your app can be easily written with BDD approach
  • made with minimalism in mind so your app can't rely on the framework when you implement business logic
    • if you need something, you should implement it without any dependency on a web framework, rack-app only mean to be to provide you with easy to use interface to the web layer, nothing less and nothing more
  • per endpoint middleware definitions
    • you can define middleware stack before endpoints and it will only apply to them, similar like protected method workflow
  • File Upload and file download efficiently and elegantly with minimal memory consuming
    • note that this is not only memory friendly way pure rack solution, but also 2x faster than the usual solution which includes buffering in memory
  • params validation with ease

Under the hood

rack-app's router relies on a tree structure which makes heavy use of common prefixes, it is basically a compact prefix tree (or just Radix tree). Nodes with a common prefix also share a common parent.




require 'rack/app'

class App < Rack::App

  desc 'some hello endpoint'
  get '/hello' do
    'Hello World!'



require 'rack/app'

class App < Rack::App

  mount SomeAppClass

  headers 'Access-Control-Allow-Origin' => '*',
          'Access-Control-Expose-Headers' => 'X-My-Custom-Header, X-Another-Custom-Header'

  serializer do |object|

  desc 'some hello endpoint'
  validate_params do
    required 'words', :class => Array, :of => String, :desc => 'some word', :example => ['pug']
    optional 'word', :class => String, :desc => 'one word', :example => 'pug'
    optional 'boolean', :class => :boolean, :desc => 'boolean value', :example => true
  get '/hello' do

    'Hello World!'

  namespace '/users' do

    desc 'some restful endpoint'
    get '/:user_id' do
      response.status = 201
      params['user_id'] #=> restful parameter :user_id
      say #=> "hello world!"


  desc 'some endpoint that has error and will be rescued'
  get '/make_error' do
    raise(StandardError,'error block rescued')

  def say
    "hello #{params['user_id']}!"

  error StandardError, NoMethodError do |ex|

  root '/hello'

  get '/stream' do
    stream do |out|
      out << 'data row'


you can access Rack::Request with the request method and Rack::Response as a response method.

By default, if you don't write anything to the response 'body' the endpoint block logic return will be used

Frontend Example

if you don't mind extending your dependency list then you can use the front_end extension for creating template-based web applications.

require 'rack/app'
require 'rack/app/front_end' # You need to add `gem 'rack-app-front_end'` to your Gemfile

class App < Rack::App

  apply_extensions :front_end

  helpers do

    def method_that_can_be_used_in_template
      'hello world!'


  # use ./app/layout.html.erb as layout, this is optionable
  layout 'layout.html.erb'

  # at '/' the endpoint will serve (render)
  # the ./app/index.html content as response body and wrap around with layout if the layout is given
  get '/' do
    render 'index.html'


this example expects an "app" folder next to the "app.rb" file that included templates being used such as layout.html.erb and index.html.


for testing use rack/test or the bundled testing module for writing unit test for your rack application

require 'spec_helper'
require 'rack/app/test'

describe App do

  include Rack::App::Test

  rack_app described_class

  describe '/hello' do
    # example for params and headers and payload use
    subject { get(url: '/hello', params: {'dog' => 'meat'}, headers: {'X-Cat' => 'fur'}, payload: 'some string') }

    it { expect(subject.status).to eq 200 }

    it { expect(subject.body).to eq "Hello World!" }

  describe '/users/:user_id' do
    # restful endpoint example
    subject { get(url: '/users/1234') }

    it { expect(subject.body).to eq 'hello 1234!'}

    it { expect(subject.status).to eq 201 }


  describe '/make_error' do
    # error handled example
    subject { get(url: '/make_error') }

    it { expect(subject.body).to eq '{:error=>"error block rescued"}' }


Example Apps To start with


After checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to


Bug reports and pull requests are welcome on GitHub at This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the Contributor Covenant code of conduct.

License and Copyright

Rack::App is free software released under the Apache License V2 License. The logo was designed by Zsófia Gebauer. It is Copyright © 2015 Adam Luzsi. All Rights Reserved.