Memorex - a cache for expensive code paths

example.ex

require Memorex

Memorex.start_link([])

# Cache this expensive function call, e.g. external service API
Memorex.get("unique_cache_key", ttl: 30) do
  do_some_expensive_shit
end

memor.ex

defmodule Memorex do
  use GenServer

  @default_options [ttl: 60]

  def init(_args) do
    {:ok, :ets.new(__MODULE__, [:private])}
  end

  def start_link() do
    GenServer.start_link(__MODULE__, [], name: __MODULE__)
  end

  defmacro get(key, opts \\ [], proc) do
    quote do
      opts   = unquote(@default_options) |> Keyword.merge(unquote(opts))
      key    = :crypto.hash(:sha256, unquote(key))
      result = GenServer.call(unquote(__MODULE__), {:get, key})

      case result do
        nil ->
          GenServer.call(unquote(__MODULE__), {:put, key, unquote(proc)[:do], opts[:ttl]})
        value ->
          value
      end
    end
  end

  def handle_call({:get, key}, _from, tid) do
    result = case :ets.lookup(tid, key) do
      [{^key, value}] -> value
      _ -> nil
    end

    {:reply, result, tid}
  end

  def handle_call({:put, key, value, ttl}, _from, tid) do
    :ets.insert(tid, {key, value})
    :erlang.send_after(ttl * 1000, self, {:purge, key})

    {:reply, value, tid}
  end

  def handle_info({:purge, key}, tid) do
    :ets.delete(tid, key)

    {:noreply, tid}
  end
end