Switch to the target database to receive event notification messages. 切换到目标数据库以接收事件通知消息。
Notification involves obtaining a lock to the critical section, iterating through the map, and notifying the subscribers. 事件通知首先会获取一个临界区锁，然后遍历关注者映射，对所有的关注者发送事件通知消息。
