stubbing_external_service.ex

defmodule AuthController do
  # ...
  def index(conn, _params) do
    #...
    twitter_client(conn).do_something
    #...
  end
  
    # Allow replacing the module used to call twitter. This is used in tests to
  # stub out the external service
  defp twitter_client(conn) do
    case conn.private[:twitter_client] do
      nil -> Twitter
      client -> client
    end
  end
end

# tests
    conn = conn
    |> put_private(:twitter_client, StubTwitter)