# Console
rails new APP_NAME --api

# Console
rake routes
   Prefix Verb   URI Pattern          Controller#Action
api_users GET    /users(.:format)     api/users#index {:subdomain=>"api"}
          POST   /users(.:format)     api/users#create {:subdomain=>"api"}
 api_user GET    /users/:id(.:format) api/users#show {:subdomain=>"api"}
          PATCH  /users/:id(.:format) api/users#update {:subdomain=>"api"}
          PUT    /users/:id(.:format) api/users#update {:subdomain=>"api"}
          DELETE /users/:id(.:format) api/users#destroy {:subdomain=>"api"}

# routes.rb
Rails.application.routes.draw do
  namespace :api, path: '/', constraints: { subdomain: 'api' } do
    resources :users
  end

  # Same as doing:
  # constraints subdomain: 'api' do
  #   namespace :api, path: '/' do
  #     resources :users
  #   end
  # end
end

# controllers/api/users_controller.rb
module Api
  class UsersController < ApplicationController
   ...
  end
end

# config/application.rb
module ApiDemo
  class Application < Rails::Application
    # Only loads a smaller set of middleware suitable for API only apps.
    # Middleware like session, flash, cookies can be added back manually.
    # Skip views, helpers and assets when generating a new resource.
    config.api_only = true
  end
end