main_api.rb

begin
  require 'bundler/inline'
rescue LoadError => e
  $stderr.puts 'Bundler version 1.10 or later is required. Please update your Bundler'
  raise e
end

gemfile(true) do
  source 'https://rubygems.org'

  gem 'sqlite3'
  # gem 'pg'
  gem 'rails', '~> 6.0'
  gem 'grape', "~> 1.3"
  gem 'puma'
end

require 'rails'
require 'active_model/railtie'
require 'action_controller/railtie'
require 'action_controller/railtie'

require 'rack/handler/puma'
require "grape"

class API < Grape::API
  version :v1, using: :path
  format :json

  content_type :json, "application/json; charset=UTF-8"

  resource :test do
    desc "Test endpoint"
    get do
      { status: "OK" }
    end
  end
end

class TestApp < Rails::Application
  config.load_defaults 6.0
  secrets.secret_token = 'secret_token'
  secrets.secret_key_base = 'secret_key_base'
  config.eager_load = false

  config.logger = Logger.new($stdout)
  Rails.logger = config.logger
end

Rails.application.initialize!

Rails.application.routes.draw do
  mount API => "/"
end

Rack::Handler::Puma.run(TestApp, Port: 3000)

# Run the server:
#   $ ruby main_api.rb

proxy_api.rb

begin
  require 'bundler/inline'
rescue LoadError => e
  $stderr.puts 'Bundler version 1.10 or later is required. Please update your Bundler'
  raise e
end

gemfile(true) do
  source 'https://rubygems.org'

  gem "sinatra", "~> 2.0", ">= 2.0.7"
  gem "sinatra-contrib", "~> 2.0", ">= 2.0.7"
  gem "puma", "~> 4.3"
  gem "curb", "~> 0.9"
end

require 'sinatra'
require 'rack/handler/puma'
require 'curb'

LOGGER = Logger.new($stdout)

class Server < Sinatra::Base
  register Sinatra::MultiRoute

  route :head, :delete, :get, :options, :path, :post, :put, :patch, "/*" do |path|
    LOGGER.info("Started request: /#{path}")

    curl = Curl::Easy.new("http://localhost:3000/v1/test.json")
    curl.perform

    LOGGER.info("Headers:")

    # copy headers
    curl.header_str.to_s.split("\r\n").each do |header|
      name, value = header.split(":", 2)
      LOGGER.info("\t#{name}:#{value}")
      response.headers[name] = value.strip if value
    end

    LOGGER.info("Ready")
    [curl.status.split(/\s+/, 2).first.to_i, curl.body]
  end 
end

Rack::Handler::Puma.run(Server, Port: 3001)

# Run the proxy server:
#   $ ruby proxy_api.rb

TESTME.md

1. Run main_api.rb (ruby main_api.rb)
2. Run proxy_api.rb (ruby proxy_api.rb)
3. Open browser and go to http://localhost:3001/test
4. See infinite load :(