Example MasterProxy for Elixir Umbrella applications supporting websockets

application.ex

defmodule MasterProxy.Application do
  @moduledoc """
  Custom Cowboy specification to support hosting multiple Phoenix apps within the same umbrella, 
  all served via the same port. This is only really necessary if you're deploying your umbrella 
  to a restricted hosting environment like Heroku where they only expose 1 port per dyno.
  
  Don't for get to configure your endpoints, something like this should work:
  
  config :myapp_web, MyAppWeb.Endpoint,
    url: [scheme: "https", host: "myapp.herokuapp.com", port: 443],
    force_ssl: [rewrite_on: [:x_forwarded_proto]]
   
  """
  
  use Application

  def start(_type, _args) do
    import Supervisor.Spec, warn: false

    port = (System.get_env("PORT") || "4000") |> String.to_integer

    cowboy = Plug.Adapters.Cowboy.child_spec(:http, MasterProxy.Plug, [], [
      port: port,
      dispatch: [{:_, [phoenix_live_reload(), websocket(), master_proxy()]}]
    ])

    children = [
      cowboy
    ]

    opts = [strategy: :one_for_one, name: MasterProxy.Supervisor]
    Supervisor.start_link(children, opts)
  end

  def phoenix_live_reload do
    {
      "/phoenix/live_reload/socket/websocket",
      Phoenix.Endpoint.CowboyWebSocket,
      {
        Phoenix.Transports.WebSocket,
        {MyAppWeb.Endpoint, Phoenix.LiveReloader.Socket, :websocket}
      }
    }
  end

  def websocket do
    {"/socket/websocket",
      Phoenix.Endpoint.CowboyWebSocket,
      {Phoenix.Transports.WebSocket,
        {MyAppWeb.Endpoint, MyAppWeb.UserSocket, :websocket}}
    }
  end

  def master_proxy do
    {:_, Plug.Adapters.Cowboy.Handler, {MasterProxy.Plug, []}}
  end
end