Currently the follower broker panics when its LEO is larger than its leader's LEO, and assuming that this is an impossible state to reach, halts the process to prevent any further damage.
Firstly this assumption is invalid and there are legitimate cases (examples below) that this case could actually occur. Secondly halt results into the broker losing its un-flushed data, and if multiple brokers halt simultaneously there is a chance that both leader and followers of a partition are among the halted brokers, which would result into permanent data loss.
Given that this is a legit case, we suggest to replace it with a graceful shutdown to avoid propagating data loss to the entire cluster.
One legit case that this could actually occur is when a troubled broker shrinks its partitions right before crashing (
KAFKA-3410 and KAFKA-3861). In this case the broker has lost some data but the controller cannot still elects the others as the leader. If the crashed broker comes back up, the controller elects it as the leader, and as a result all other brokers who are now following it halt since they have LEOs larger than that of shrunk topics in the restarted broker. We actually had a case that bringing up a crashed broker simultaneously took down the entire cluster and as explained above this could result into data loss.
The other legit case is when multiple brokers ungracefully shutdown at the same time. In this case both of the leader and the followers lose their un-flushed data but one of them has HW larger than the other. Controller elects the one who comes back up sooner as the leader and if its LEO is less than its future follower, the follower will halt (and probably lose more data). Simultaneous ungrateful shutdown could happen due to hardware issue (e.g., rack power failure), operator errors, or software issue (e.g., the case above that is further explained in
KAFKA-3410 and KAFKA-3861 and causes simultaneous halts in multiple brokers)