def _callback_handler(self, message, callback):
        """Call callback with deserialized message.

        Messages that are processed without exception are ack'ed.

        If the message processing generates an exception, it will be
        ack'ed if ack_on_error=True. Otherwise it will be .reject()'ed.
        Rejection is better than waiting for the message to timeout.
        Rejected messages are immediately requeued.
        """

        ack_msg = False
        try:
            msg = rpc_common.deserialize_msg(message.payload)
            callback(msg)
            ack_msg = True
        except Exception:
            if self.ack_on_error:
                ack_msg = True
                LOG.exception(_("Failed to process message"
                                " ... skipping it."))
            else:
                LOG.exception(_("Failed to process message"
                                " ... will requeue."))
        finally:
            if ack_msg:
                message.ack()
            else:
                message.reject()