Simulate follower lag exceeding replica.lag.time.max.ms and observe ISR shrinkage.
A follower that falls too far behind the leader (e.g., due to disk I/O, GC pauses, network congestion) is removed from the ISR. This degrades fault tolerance. If ISR drops below min.insync.replicas, acks=all writes are rejected.
The steps below reference the interactive simulator, which is being migrated into /simulate next — until it lands, treat this as the written runbook for the exercise.
Steps
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Verification
ISR should recover to 3/3 after reducing lag. acks=all writes should succeed again.
The interview angle
ISR shrink is caused by replica.lag.time.max.ms breach. Production fix: check follower disk I/O, GC pauses, network. Alert on URP > 0.