docs: def add(self, event, pk, ts=None, ttl=None):
          """Add an event to event store.
  
          All events were stored in a sorted set in redis with timestamp as
          rank  score.
  
          :param event: the event to be added, format should be ``table_action``
          :param pk: the primary key of event
          :param ts: timestamp of the event, default to redis_server's
           current timestamp
          :param ttl: the expiration time of event since the last update
          :return: bool
          """
          key = self._keygen(event, ts)
          try:
              self._zadd(key, pk, ts, ttl)
              return True
          except redis.ConnectionError as e:
              # connection error typically happens when redis server can't be
              # reached or timed out, the error will be silent with an error
              # log and return None.
              self.logger.error(
                  "redis event store failed with connection error %r" % e)
              return False