Redis notification is not reliable

[zhuohan123]

According to https://redis.io/topics/notifications

Because Redis Pub/Sub is fire and forget currently there is no way to use this feature if your application demands reliable notification of events, that is, if your Pub/Sub client disconnects, and reconnects later, all the events delivered during the time the client was disconnected are lost.

We need to use something else to implement the notification mechanism

[zhuohan123]

Solved in #12