def get_cached_object(cache_key)
  expensive_object = Memcached.get cache_key

  if expensive_object == typeof(ExpensiveObject)
    return expensive_object
  elsif expensive_object == null
    # try to "add" the cache_key, setting the value to a
    # timeout during which all other threads will wait and
    # poll; note that add only sets the value if no key exists yet
    we_set_it = Memcached.add cache_key, Time.now + 5.seconds
    if we_set_it
      # go build expensive_object, then set it in the cache
      Memcached.set cache_key, expensive_object
      return expensive_object
    end
  end

  # we're not building the object, someone else is;
  # poll and sleep until it is ready or we timeout
  timeout = expensive_object
  while (Time.now < timeout)
    expensive_object = Memcached.get cache_key
    if expensive_object == typeof(ExpensiveObject)
      return expensive_object
    end
    sleep(300) # ms
  end
  throw TimeoutException
end