rate_limiter.ex

defmodule RateLimiter do
  use GenServer
  require Logger

  @rate_per_minute 30
  @sweep_after :timer.seconds(60)
  @bucket :rate_limiter_requests

  ## Client

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

  def log(id) do
    case :ets.update_counter(@bucket, id, {2, 1}, {id, 0}) do
      count when count > @rate_per_minute -> {:error, :rate_limited}
      _count -> :ok
    end
  end

  ## Server

  def init(_) do
    :ets.new(@bucket, [:set,  :named_table, :public, read_concurrency: true, write_concurrency: true])

    schedule_sweep()
    {:ok, %{}}
  end

  def handle_info(:sweep, state) do
    Logger.debug("Sweeping requests")
    :ets.delete_all_objects(@bucket)
    schedule_sweep()
    {:noreply, state}
  end

  defp schedule_sweep do
    Process.send_after(self(), :sweep, @sweep_after)
  end
end