API Version router using `send_resp` vs a custom exceptin.

version_router_exception.ex

defmodule Event.Plug.API.VersionRouter do
  @default_api_version "1"

  def init(options) do
    case Keyword.fetch!(options, :versions) do
      versions ->
        for {version, router} <- versions, into: %{} do
          {version, {router, router.init([])}}
        end
    end
  end

  def call(conn, options) do
    case Map.fetch(options, get_api_version_from_conn(conn)) do
      {:ok, {router, router_options}} ->
        router.call(conn, router_options)
      _ ->
        raise InvalidAPIVersionError, conn: conn, valid_api_versions: Map.keys(options)
    end
  end

  defp get_api_version_from_conn(conn) do
    case Plug.Conn.get_req_header(conn, "api-version") do
      [version] ->
        version
      _ ->
        @default_api_version
    end
  end

  defmodule InvalidAPIVersionError do
    @moduledoc """
    Exception raised when an invalid API version is passed in the `Api-Version` header.
    """
    defexception plug_status: 422, message: "invalid API version", conn: nil

    def exception(options) do
      conn = Keyword.fetch!(options, :conn)
      valid_api_versions = Keyword.fetch!(options, :valid_api_versions)
      requested_version = Plug.Conn.get_req_header(conn, "api-version")

      %InvalidAPIVersionError{
        conn: conn,
        message: "#{requested_version} is not a valid API version. Valid versions are: #{Enum.join(valid_api_versions, ", ")}"
      }
    end
  end
end

version_router_send_resp.ex

defmodule Event.Plug.API.VersionRouter do
  use Event.Web, :router

  @default_version "1"

  def init(options) do
    case Keyword.fetch!(options, :versions) do
      versions ->
        for {version, router} <- versions, into: %{} do
          {version, {router, router.init([])}}
        end
    end
  end

  def call(conn, options) do
    version = get_api_version_from_conn(conn)
    case Map.fetch(options, version) do
      {:ok, {router, router_options}} ->
        router.call(conn, router_options)
      _ ->
        send_resp conn, :unprocessable_entity,
          "#{version} is not a valid API version. Valid versions are #{Map.keys(options) |> Enum.join(", ")}"
      end
  end

  defp get_api_version_from_conn(conn) do
    case Plug.Conn.get_req_header(conn, "api-version") do
      [version] ->
        version
      _ ->
        @default_version
    end
  end
end