foo.ex

defmodule TicketBuyer do
  use GenServer
  @ticket_sites [:ticketscalper, :feemaster, :concertoverhead]

  def start_link do
    GenServer.start_link __MODULE__, []
  end
  
  def init(_args) do
    {:ok, %{}}
  end
  
  def fetch_average_price(server, ticket_id) do
    GenServer.call(server, {:fetch_ticket_prices, ticket_id}
  end
  
  def handle_call({:fetch_average_price, ticket_id}, from, state) do
    for site <- @ticket_sites, do: fetch_price(site, ticket_id)
    new_state = Map.put(state, ticket_id, %{from: from, responses: []})
    {:noreply, new_state}
  end
  
  def handle_info({:ticket_client_response, site, ticket_id, price}, state)  do
    new_ticket_state = %{Map.get(state, ticket_id) | responses: [price | ticket_state.responses}
    if length(new_ticket_state.responses) == length(@ticket_sites) do
      average_price =  Enum.sum(new_ticket_state.responses) / Enum.count(new_ticket_state.responses)
      GenServer.reply(new_ticket_state.from, {:ok, average_price})
      {:noreply, Map.drop(state, [ticket_id])}
    else
      {:noreply, Map.put(state, ticket_id, new_ticket_state)}
    end
  end
  
  defp fetch_price(site, ticket_id) do
    # Fetch URL or otherwise asyncronously request data here
    TicketClient.request_price(site, ticket_id)
  end
end