code: def ack(self, uid):
          
          self._redis.pipeline()\
                     .lrem(self.ACKBUF_KEY, 0, uid)\
                     .lrem(self.BACKUP, 0, uid)\
                     .hdel(self.ITEMS_KEY, uid)\
                     .execute()
docs: Acknowledge item as successfully consumed.
  
          Removes uid from ackbuffer and deletes the corresponding item.