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https://issues.apache.org/jira/browse/KAFKA-18871?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17931133#comment-17931133
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Daniel Urban commented on KAFKA-18871:
--------------------------------------
[~ppatierno] Thanks for checking the logs. As you said, cluster operator not
rolling due to the ISR is completely normal and works as expected.
The issue is that for the cluster in namespace
csm-op-test-kraft-rollback-f19bca6a broker 1 is never added to the ISR.
I've mentioned that there were multiple kafka clusters managed by this
operator, all of them being rolled back from the KRaft migration, and some
passed successfully (that's probably the one you found when you said "At some
point AFAICS, the broker 0 is rolled because broker 1 seems to catch up."), but
for the specific cluster in ns csm-op-test-kraft-rollback-f19bca6a, the last
roll does not happen, it gets stuck after broker 1 was rolled, and then the
roll is blocked due to the at-min-isr state of 1 partition.
> KRaft migration rollback causes downtime
> ----------------------------------------
>
> Key: KAFKA-18871
> URL: https://issues.apache.org/jira/browse/KAFKA-18871
> Project: Kafka
> Issue Type: Bug
> Components: kraft, migration
> Affects Versions: 3.9.0
> Reporter: Daniel Urban
> Priority: Critical
> Attachments: kraft-rollback-bug.zip
>
>
> When testing the KRaft migration rollback feature, found the following
> scenario:
> # Execute KRaft migration on a 3 broker 3 ZK node cluster to the last step,
> but do not finalize the migration.
> ## In the test, we put a slow but continuous produce+consume load on the
> cluster, with a topic (partitions=3, RF=3, min ISR=2)
> # Start the rollback procedure
> # First we roll back the brokers from KRaft mode to migration mode (both
> controller and ZK configs are set, process roles are removed,
> {{zookeeper.metadata.migration.enable}} is true)
> # Then we delete the KRaft controllers, delete the /controller znode
> # Then we immediately start rolling the brokers 1 by 1 to ZK mode by
> removing the {{zookeeper.metadata.migration.enable}} flag and the
> controller.* configurations.
> # At this point, when we restart the 1st broker (let's call it broker 0)
> into ZK mode, we find an issue which occurs ~1 out of 20 times:
> If broker 0 is not in the ISR for one of the partitions at this point, it can
> simply never become part of the ISR. Since we are aiming for zero downtime,
> we check the ISR states of partitions between broker restarts, and our
> process gets blocked at this point. We have tried multiple workarounds at
> this point, but it seems that there is no workaround which still ensures zero
> downtime.
> Some more details about the process:
> * We are using Strimzi to drive this process, but have verified that Strimzi
> follows the documented steps precisely.
> * When we reach the error state, it doesn't matter which broker became the
> controller through the ZK node, the brokers still in migration mode get
> stuck, and they flood the logs with the following error:
> {code:java}
> 2025-02-26 10:55:21,985 WARN [RaftManager id=0] Error connecting to node
> kraft-rollback-kafka-controller-pool-5.kraft-rollback-kafka-kafka-brokers.csm-op-test-kraft-rollback-e7798bef.svc.cluster.local:9090
> (id: 5 rack: null) (org.apache.kafka.clients.NetworkClient)
> [kafka-raft-outbound-request-thread]
> java.net.UnknownHostException:
> kraft-rollback-kafka-controller-pool-5.kraft-rollback-kafka-kafka-brokers.csm-op-test-kraft-rollback-e7798bef.svc.cluster.local
> at
> java.base/java.net.InetAddress$CachedAddresses.get(InetAddress.java:801)
> at java.base/java.net.InetAddress.getAllByName0(InetAddress.java:1533)
> at java.base/java.net.InetAddress.getAllByName(InetAddress.java:1385)
> at java.base/java.net.InetAddress.getAllByName(InetAddress.java:1306)
> at
> org.apache.kafka.clients.DefaultHostResolver.resolve(DefaultHostResolver.java:27)
> at org.apache.kafka.clients.ClientUtils.resolve(ClientUtils.java:125)
> at
> org.apache.kafka.clients.ClusterConnectionStates$NodeConnectionState.resolveAddresses(ClusterConnectionStates.java:536)
> at
> org.apache.kafka.clients.ClusterConnectionStates$NodeConnectionState.currentAddress(ClusterConnectionStates.java:511)
> at
> org.apache.kafka.clients.ClusterConnectionStates$NodeConnectionState.access$200(ClusterConnectionStates.java:466)
> at
> org.apache.kafka.clients.ClusterConnectionStates.currentAddress(ClusterConnectionStates.java:173)
> at
> org.apache.kafka.clients.NetworkClient.initiateConnect(NetworkClient.java:1075)
> at
> org.apache.kafka.clients.NetworkClient.ready(NetworkClient.java:321)
> at
> org.apache.kafka.server.util.InterBrokerSendThread.sendRequests(InterBrokerSendThread.java:146)
> at
> org.apache.kafka.server.util.InterBrokerSendThread.pollOnce(InterBrokerSendThread.java:109)
> at
> org.apache.kafka.server.util.InterBrokerSendThread.doWork(InterBrokerSendThread.java:137)
> at
> org.apache.kafka.server.util.ShutdownableThread.run(ShutdownableThread.java:136)
> {code}
> * Manually verified the last offsets of the replicas, and broker 0 is caught
> up in the partition.
> * Even after stopping the produce load, the issue persists.
> * Even after removing the /controller node manually (to retrigger election),
> regardless of which broker becomes the controller, the issue persists.
> Based on the above, it seems that during the rollback, brokers in migration
> mode cannot handle the KRaft controllers being removed from the system. Since
> broker 0 is caught up in the partition, we suspect that the other brokers
> (still in migration mode) do not respect the controller state in ZK, and do
> not report changes in the ISR of the partitions they are leading.
> This means that if a replica becomes out of sync in the last restart (e.g.
> due to a slow broker restart), we cannot restart the brokers while ensuring
> zero downtime.
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