We experienced an unexpected unclean leader election after network glitch happened on the leader of partition. We have replication factor 2.
Here is the sequence of event gathered from various logs:
1. ZK session timeout happens for leader of partition
2. New ZK session is established for leader
3. Leader removes the follower from ISR (which might be caused by replication delay due to the network problem) and updates the ISR in ZK
4. Controller processes the BrokerChangeListener event happened at step 1 where the leader seems to be offline
5. Because the ISR in ZK is already updated by leader to remove the follower, controller makes an unclean leader election
6. Controller processes the second BrokerChangeListener event happened at step 2 to mark the broker online again
It seems to me that step 4 happens too late. If it happens right after step 1, it will be a clean leader election and hopefully the producer will immediately switch to the new leader, thus avoiding consumer offset reset.