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

defmodule Todo.Cache do
  use GenServer

  @impl GenServer
  def init(_init_args) do
    IO.puts("Starting #{__MODULE__}")

    Todo.Database.start()

    {:ok, %{}}
  end

  @impl GenServer
  def handle_call({:server_process, name}, _from, state) do
    case Map.fetch(state, name) do
      {:ok, process} ->
        {:reply, process, state}

      :error ->
        {:ok, new_process} = Todo.Server.start(name)

        new_state = Map.put(state, name, new_process)

        {:reply, new_process, new_state}
    end
  end

  def start_link(_) do
    GenServer.start_link(__MODULE__, nil, name: __MODULE__)
  end

  def server_process(name) do
    GenServer.call(__MODULE__, {:server_process, name})
  end
end