master_proxy.ex

defmodule MasterProxy do
  use Application

  # See http://elixir-lang.org/docs/stable/elixir/Application.html
  # for more information on OTP Applications
  def start(_type, _args) do
    import Supervisor.Spec, warn: false

    cowboy = Plug.Adapters.Cowboy.child_spec(:http, MasterProxy.Plug, [])

    children = [
      cowboy
      # Define workers and child supervisors to be supervised
      # worker(MasterProxy.Worker, [arg1, arg2, arg3])
    ]

    # See http://elixir-lang.org/docs/stable/elixir/Supervisor.html
    # for other strategies and supported options
    opts = [strategy: :one_for_one, name: MasterProxy.Supervisor]
    Supervisor.start_link(children, opts)
  end
end

mix.exs

defmodule MasterProxy.Mixfile do
  use Mix.Project

  def project do
    [app: :master_proxy,
     version: "0.0.1",
     deps_path: "../../deps",
     lockfile: "../../mix.lock",
     elixir: "~> 1.0",
     build_embedded: Mix.env == :prod,
     start_permanent: Mix.env == :prod,
     deps: deps]
  end

  # Configuration for the OTP application
  #
  # Type `mix help compile.app` for more information
  def application do
    [applications: [:logger, :cowboy, :plug, :my_app, :my_app_admin],
     mod: {MasterProxy, []}]
  end

  # Dependencies can be Hex packages:
  #
  #   {:mydep, "~> 0.3.0"}
  #
  # Or git/path repositories:
  #
  #   {:mydep, git: "https://github.com/elixir-lang/mydep.git", tag: "0.1.0"}
  #
  # To depend on another app inside the umbrella:
  #
  #   {:myapp, in_umbrella: true}
  #
  # Type `mix help deps` for more examples and options
  defp deps do
    [{:my_app_admin, in_umbrella: true},
     {:plug, "~> 1.0.0"},
     {:cowboy, "~> 1.0.0"},
     {:my_app, in_umbrella: true}]
  end
end

plug.ex

defmodule MasterProxy.Plug do
  import Plug.Conn

  def init(options) do
    # initialize options

    options
  end

  def call(conn, _opts) do
    case some_condition do
      foo -> MyApp.Endpoint.call(conn, [])
      _   -> MyAppAdmin.Endpoint.call(conn, [])
    end
  end
end