elixir-todo-list/lib/todo/database_worker.ex

defmodule Todo.DatabaseWorker do
  use GenServer

  def start_link({_db_folder, worker_id} = state) do
    GenServer.start_link(__MODULE__, state, name: via_tuple(worker_id))
  end

  def store(worker_id, key, data) do
    worker_id
    |> via_tuple()
    |> GenServer.cast({:store, key, data})
  end

  def get(worker_id, key) do
    worker_id
    |> via_tuple()
    |> GenServer.call({:get, key})
  end

  defp via_tuple(worker_id) do
    Todo.ProcessRegistry.via_tuple({__MODULE__, worker_id})
  end

  @impl GenServer
  def init({db_folder, worker_id} = state) do
    IO.puts("Starting #{__MODULE__} #{worker_id} with db folder #{db_folder}.")

    {:ok, state}
  end

  @impl GenServer
  def handle_cast({:store, key, data}, {db_folder, _worker_id} = state) do
    {db_folder, key}
    |> file_name()
    |> File.write!(:erlang.term_to_binary(data))

    {:noreply, state}
  end

  @impl GenServer
  def handle_call({:get, key}, _, {db_folder, _worker_id} = state) do
    data =
      case File.read(file_name({db_folder, key})) do
        {:ok, contents} ->
          :erlang.binary_to_term(contents)

        {:error, :enoent} ->
          nil
      end

    {:reply, data, state}
  end

  def file_name({db_folder, key}) do
    Path.join(db_folder, to_string(key))
  end
end