def ack(self):
        """Acknowledge the given message.

        Acknowledging a message in Pub/Sub means that you are done
        with it, and it will not be delivered to this subscription again.
        You should avoid acknowledging messages until you have
        *finished* processing them, so that in the event of a failure,
        you receive the message again.

        .. warning::
            Acks in Pub/Sub are best effort. You should always
            ensure that your processing code is idempotent, as you may
            receive any given message more than once.
        """
        time_to_ack = math.ceil(time.time() - self._received_timestamp)
        self._request_queue.put(
            requests.AckRequest(
                ack_id=self._ack_id, byte_size=self.size, time_to_ack=time_to_ack
            )
        )