@callback is_new?(String.t(), any()) :: {:error, term()} | boolean()

@callback load(String.t()) :: {:ok, term()} | {:not_found, %{}} | {:error, term()}

@callback save(String.t(), term()) ::
  {:ok, term(), String.t()}
  | {:error, term(), term(), term()}
  | {:error, term(), term()}

@callback save_async(String.t(), term(), integer()) ::
  {:ok, term(), String.t()}
  | {:error, term(), term(), term()}
  | {:error, term(), term()}