state_from_disk.ex

CrawlerExample.Queue do
  use GenServer

  #client
  def start_link(opts \\ []) do
    GenServer.start_link(__MODULE__, nil, opts)
  end

  def load_from_file(file_path) do
    GenServer.cast(__MODULE__, {:load_state_from_disk, file_path})
  end

  #server
  @impl true
  def init(_init_arg) do
    schedule_write_to_disk(5000)
    {:ok, :queue.new}
  end

  @impl true
  def handle_info(:schedule_write_to_disk, state) do
    current_state = :queue.to_list(state)

    state_json = Poison.encode!(current_state)

    File.write("/tmp/crawler_state.json", state_json)

    schedule_write_to_disk(interval)
  end

  @impl true
  def handle_cast({:load_state_from_disk, file_path}, _) do
    contents = File.read!(file_path)
    from_json = Poison.decode!(contents)

    {:noreply, :queue.from_list(from_json)}
  end

  defp schedule_write_to_disk(interval), do: :erlang.send_after(interval, self(), :schedule_write_to_disk)
end