Possible subscription agent?

defmodule Consumer.Subscriptions do
  def start_link do
    Agent.start_link(fn -> %{} end, name: __MODULE__)
  end

  def get_subscribers(topic) do
    Agent.get(__MODULE__, fn map -> Map.get(map, topic, []) end)
  end

  def add_subscription(sub, topic) do
    Agent.update(__MODULE__, fn map -> Map.put(map, topic, [sub|Map.get(map, topic, [])]) end)
  end

  def remove_subscriber(sub, topic) do
    Agent.update(__MODULE__, fn map ->
      Map.put(map, topic, List.delete(Map.get(map, topic, []), sub))
    end)
  end
end