storage.exs

defmodule Store do
  def start_link do
    Agent.start_link(fn -> %{} end)
  end

  def set(pid, key, value) do
    Agent.update(pid, fn state ->
      Map.put(state, key, value)
    end)
  end

  def get(pid, key) do
    Agent.get(pid, fn state ->
      Map.get(state, key)
    end)
  end
end

{:ok, pid} = Store.start_link

Store.set(pid, :foo, "foo")
Store.set(pid, :bar, "bar")
Store.set(pid, :baz, "baz")

{Store.get(pid, :foo), Store.get(pid, :bar)}