Proxy for separate backend + frontend apps that work together

config.ru

require 'rack/reverse_proxy'

use Rack::ReverseProxy do
  reverse_proxy_options preserve_host: true, matching: :first
  reverse_proxy '/api', ENV['API_URL'] || 'http://localhost:3000/api'
  reverse_proxy '/', ENV['STATIC_URL'] || 'http://localhost:3333'
end

app = proc do |env|
  [ 200, {'Content-Type' => 'text/plain'}, 'b' ]
end

run app

Gemfile

source 'https://rubygems.org'

gem 'rack-reverse-proxy', require: 'rack/reverse_proxy'

group :development do
  gem 'foreman'
end

Procfile

web: bundle exec rackup config.ru -p $PORT

Ready for deployment on Heroku.