plug - api - version_router.ex

defmodule Event.Plug.API.VersionRouter do
  import Plug.Conn

  @default_version 1

  def init(options) do
    for {version, router} <- options, into: %{} do
      {version, {router, router.init([])}}
    end
  end

  def call(conn, options) do
    version = get_version_from_conn(conn)
    {router, router_options} = Map.fetch!(options, version)
    router.call(conn, router_options)
  end

  defp get_version_from_conn(conn) do
    version = case Plug.Conn.get_req_header(conn, "accept") do
      [value | _] ->
        Regex.run ~r/version=(\d)/, value, capture: :all_but_first
      _ ->
        @default_version
    end
    :"v#{version}"
  end
end

web - api - v1 - router.ex

defmodule Event.API.V1.Router do
  use Event.Web, :router

  pipeline :api do
    plug :accepts, ["json"]
  end

  scope "/", alias: Event.API, host: "api." do
    pipe_through :api

    scope "/", alias: V1 do
      resources "groups", GroupController

      get "/", GroupController, :index
    end
  end
end

web - api - v2 - router.ex

defmodule Event.API.V2.Router do
  use Event.Web, :router

  pipeline :api do
    plug :accepts, ["json"]
  end

  scope "/", alias: Event.API, host: "api." do
    pipe_through :api

    scope "/", alias: V2 do
      resources "groups", GroupController

      get "/", GroupController, :index
    end
  end
end

web - router.ex

defmodule Event.Router do
  use Event.Web, :router

  pipeline :browser do
    plug :accepts, ["html"]
    plug :fetch_session
    plug :fetch_flash
    plug :protect_from_forgery
    plug :put_secure_browser_headers
  end

  pipeline :api do
    plug Event.Plug.API.VersionRouter, v1: Event.API.V1.Router, v2: Event.API.V2.Router
    plug :accepts, ["json"]
  end

  scope "/", Event, host: "api." do
    pipe_through :api
    get "/", PagesController, :index # redundant, but app wont work without it
  end
end