Action Cable Presence Tracking (Lightweight)

                  class Presence
  TTL = 60

  def initialize(redis: Redis.current)
    @redis = redis
  end

  def touch(room_id, member_id)
    key = "presence:room:#{room_id}"
    @redis.sadd(key, member_id)
    @redis.expire(key, TTL)
  end

  def members(room_id)
    @redis.smembers("presence:room:#{room_id}").map(&:to_i)
  end
end