An example of how to write a simple JSON REST API with a simplistic router directly on top of Rack (i.e. without the use of a framework like Sinatra).

rack-rest-api-example.rb

#################################################################
#
# File: lib/api.rb
#
#################################################################

require 'user' # ActiveRecord model

# Connect to the db
ActiveRecord::Base.establish_connection(ENV['DATABASE_URL'])

class Api
  def self.routes
    [
      {method: "GET", path: %r{^/users/(?<id>\d+)}, api_method: :find},
      {method: "POST", path: %r{^/users}, api_method: :create},
      {method: "PUT", path: %r{^/users/(?<id>\d+)}, api_method: :update},
      {method: "GET", path: %r{^/authenticate}, api_method: :authenticate}
    ]    
  end

  def self.find(params)
    User.find(params[:id])
  end
  
  def self.create(params)
    User.create!(params[:user])
  end
  
  def self.update(params)
    User.update_attributes!(params[:user])
  end
  
  def self.authenticate(params)
    User.authenticate(params[:email], params[:password])
  end
end


#################################################################
#
# File: config.ru
#
#################################################################

require "rubygems"
require "bundler/setup"

require "json"

$: << File.join(File.dirname(__FILE__), "lib")
require "api"

# Simplistic router based on path and request method
class Router
  def self.find_match(request_method, path, routes)
    routes.each do |route|
      if match = match_route(request_method, path, route)
        return match
      end
    end
    return nil
  end

  def self.match_route(request_method, path, route)
    if route[:method] == request_method && match_data = route[:path].match(path)
      route_match = route.slice(:api_method)
      if match_data.names.present?
        route_match[:params] = match_data.names.inject({}) { |params, name| params[name] = match_data[name]; params }
      else
        route_match[:params] = {}
      end
      route_match
    else
      nil
    end
  end
end

run(->(env) {
  request = Rack::Request.new(env)  
  if route_match = Router.find_match(request.request_method, request.path, Api.routes)
    begin
      api_params = HashWithIndifferentAccess.new(request.params.merge(route_match[:params]))
      response_data = Api.send(route_match[:api_method], api_params)
      [200, {'Content-Type' => 'application/json'}, StringIO.new(response_data.to_json)]
    rescue ActiveRecord::RecordNotFound => e
      [404, {'Content-Type' => 'text/plain'}, StringIO.new(e.message)]
    rescue ActiveRecord::RecordInvalid => e
      [422, {'Content-Type' => 'text/plain'}, StringIO.new(e.record.full_error_messages)]
    end
  else
    [404, {'Content-Type' => 'text/plain'}, StringIO.new('file not found')]
  end
})