genserver behaviour

Keyvaluestore.ex

defmodule KeyValueStore do
  use GenServer
  
  def init(_) do
    {:ok, HashDict.new}
  end
  
  def handle_cast({:put, key, value}, state) do
    {:noreply, HashDict.put(state, key, value)}
  end
  
  def handle_call({:get, key}, _, state) do
    {:reply, HadhDict.get(state, key), state}
  end
  
  def start do
    GenServer.start(KeyValueStore, nil)
  end
  
  def put(pid, key, value) do
    GenServer.cast(pid, {:put, key, value})
  end
  
  def get(pid, key) do
    GenServer.call(pid, {:get, key})
  end
end

timer.ex

defmodule KeyValueStore do
  use GenServer
    
  def init(_) do
    :timer.send_interval(5000, :cleanup)
    {:ok, HashDict.new}
  end
  
  def handle_info(:cleanup, state) do
    IO.puts "performing cleanup..."
    {:noreply, state}
  end
  
  def handle_info(_, state), do: {:noreply, state}
end

Todo_database.ex

defmodule Todo.Database do
  use GenServer
  
  def start(db_folder) do
    GenServer.start(__MODULE__, db_folder,
      name: :database_server)
  end
  
  def store(key, data) do
    GenServer.cast(:database_server, {:store, key, data})
  end
  
  def get(key) do
    GenServer.call(:database_server, {:get, key})
  end
  
  def init(db_folder) do
    File.mkdir_p(db_folder)
    {:ok, db_folder)
  end
  
  def handle_cast({:store, key, data}, db_folder) do
    file_name(db_folder, key)
    |> File.write!(:erlang.term_to_binary(data))
    
    {:noreply, db_folder}
  end
  
  def handle_call({:get, key}, _, db_folder) do
    data = case File.read(file_name(db_folder, key)) do
       {:ok, contents} -> :erlang.binary_to_term(contents)
       _ -> nil
    end
    
    {:reply, data, db_folder}
  end
  
  defp file_name(db_folder, key), do: "#{db_folder}/#{key}"
end