code: def callback(self, timestamp, event_type, payload):
          
          try:
              data = (event_type, payload)
              LOG.debug('RX NOTIFICATION ==>\nevent_type: %(event)s, '
                        'payload: %(payload)s\n', (
                            {'event': event_type, 'payload': payload}))
              if 'create' in event_type:
                  pri = self._create_pri
              elif 'delete' in event_type:
                  pri = self._delete_pri
              elif 'update' in event_type:
                  pri = self._update_pri
              else:
                  pri = self._delete_pri
              self._pq.put((pri, timestamp, data))
  
          except Exception as exc:
              LOG.exception('Error: %(err)s for event %(event)s',
                            {'err': str(exc), 'event': event_type})
docs: Callback method for processing events in notification queue.
  
          :param timestamp: time the message is received.
          :param event_type: event type in the notification queue such as
                             identity.project.created, identity.project.deleted.
          :param payload: Contains information of an event