the basic idea is, that the zookeeper server will tell the client that he is lagging behind. In your scenario s3 would be a follower of s5 (the leader). So any change is requested to the leader (s5), which will only report successs to the client when all followers have acknowledged the change. But of course in this phase the network between s3 and s5 can fail, and after a timeout, s5 will drop s3 as the follower and still send a success to the client. But at this point s3 also notices that he is no longer a follower, because: 1. the network failed in both directions and s3 has also missed the heartbeat from s5, and therefore now knows that the leader is gone 2. the network connection failed just in the direction s5->s3, then s3 has still processed the update, but s5 will drop s3 as a follower as s5 doesn't receive an acknowledge. It then stops sending heartbeats and sync messages, so for s3 the same situation as in 1. occurs. Besides this situation the most interesting part is the new leader election when the communication between the current leader and a follower is broken. It is important to ensure that not 2 leaders are elected.
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