Colin Leroy created KAFKA-16296:
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             Summary: Broker shrinks ISR when restarting
                 Key: KAFKA-16296
                 URL: https://issues.apache.org/jira/browse/KAFKA-16296
             Project: Kafka
          Issue Type: Bug
          Components: core
    Affects Versions: 3.6.1
            Reporter: Colin Leroy


We have a rolling-restart problem we don't understand on a 3-node cluster.

When stopping a broker, everything goes fine and the partitions are reassigned 
to the other brokers.

When that broker restarts, it shrinks ISR because of "Out of sync replicas":
{code:java}
[2024-02-22 10:18:02,069] INFO [Partition OSS.PREPROD.Monitoring.Metric-5 
broker=3] Shrinking ISR from 2,1,3 to 3. Leader: (highWatermark: 704389542, 
endOffset: 704395843). Out of sync replicas: (brokerId: 2, endOffset: -1, 
lastCaughtUpTimeMs: 1708593437335) (brokerId: 1, endOffset: -1, 
lastCaughtUpTimeMs: 1708593437335). (kafka.cluster.Partition)

[2024-02-22 10:18:02,124] INFO [Partition OSS.PREPROD.Monitoring.Metric-5 
broker=3] ISR updated to 3 (under-min-isr) and version updated to 1075 
(kafka.cluster.Partition) {code}
I do not understand why brokers 1 and 2 would be out of sync, it seems to me 
that given that brokers 1 and 2 were not restarted, they should be in sync.

This, of course, causes problems as producers reconnect to broker 3 only to 
find the min ISR requirement is not fullfilled.

Thanks in advance,

Colin



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