It uses Redis distributed pub/sub server in order to support App Server load-balancing so it needs to be able to relay messages received from 1 App Server to clients that’s connected to any of the other App Servers, so messages are published to Redis Pub/Sub which notifies all App Servers, which are all subscribed to the same pub/sub topic. The App Server that maintains the matching local client connection is the one that notifies the target client/s.
The implementation of RedisServerEvents is pretty simple and straight-forward to follow.