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https://issues.apache.org/jira/browse/KAFKA-18856?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17929871#comment-17929871
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José Armando García Sancio commented on KAFKA-18856:
----------------------------------------------------

Yes, a 3.9 node (broker or controller) cannot join Kafka cluster which has 4.0 
as the finalized metadata version. Which node is logging this error? The 3.9 
broker?
{code:java}
 [2025-02-25 01:05:06,530] ERROR Encountered metadata loading fault: Unhandled 
error initializing new publishers 
(org.apache.kafka.server.fault.LoggingFaultHandler)
org.apache.kafka.image.writer.UnwritableMetadataException: Metadata has been 
lost because the following could not be represented in metadata.version 
3.0-IV1: feature flag(s): group.version, transaction.version {code}
I would have assumed that the broker would have failed to register to the Kafka 
cluster because it doesn't support the 4.0 metadata version? Do you see such 
registration errors?

It is possible that the issue is that the 3.9 broker is fetching and applying 
metadata records from the active controller even though it is not able to 
register to the cluster. Having said that, it is a confusing error message: 
"following could not be represented in metadata.version 3.0-IV1: feature 
flag(s): group.version, transaction.version."

What do you think [~cmccabe] ?

> 3.9 broker unable to join dynamic quorum with 4.0 controller
> ------------------------------------------------------------
>
>                 Key: KAFKA-18856
>                 URL: https://issues.apache.org/jira/browse/KAFKA-18856
>             Project: Kafka
>          Issue Type: Bug
>            Reporter: TengYao Chi
>            Assignee: TengYao Chi
>            Priority: Major
>
> Test step:
>  * format 4.0 controller /w controller.properties & standalone
>  ** bin/kafka-storage.sh format --standalone -t "dAtBluZORiuJEKV2uSWB2Q" -c 
> config/controller.properties
>  * start 4.0 controller
>  * init 3.9 broker /w kraft/broker.properties & no-initial-controllers 
>  ** bin/kafka-storage.sh format -t "dAtBluZORiuJEKV2uSWB2Q" --config 
> config/kraft/broker.properties -N
>  * start 3.9 broker
> error message:
> {code:java}
> [2025-02-24 23:13:21,271] ERROR Encountered metadata loading fault: Unhandled 
> error initializing new publishers 
> (org.apache.kafka.server.fault.LoggingFaultHandler)
> org.apache.kafka.image.writer.UnwritableMetadataException: Metadata has been 
> lost because the following could not be represented in metadata.version 
> 3.0-IV1: feature flag(s): group.version, transaction.version
>     at 
> org.apache.kafka.image.writer.ImageWriterOptions.handleLoss(ImageWriterOptions.java:94)
>     at 
> org.apache.kafka.image.FeaturesImage.handleFeatureLevelNotSupported(FeaturesImage.java:108)
>     at org.apache.kafka.image.FeaturesImage.write(FeaturesImage.java:88)
>     at org.apache.kafka.image.MetadataImage.write(MetadataImage.java:153)
>     at 
> org.apache.kafka.image.loader.MetadataLoader.initializeNewPublishers(MetadataLoader.java:296)
>     at 
> org.apache.kafka.image.loader.MetadataLoader.lambda$scheduleInitializeNewPublishers$0(MetadataLoader.java:267)
>     at 
> org.apache.kafka.queue.KafkaEventQueue$EventContext.run(KafkaEventQueue.java:132)
>     at 
> org.apache.kafka.queue.KafkaEventQueue$EventHandler.handleEvents(KafkaEventQueue.java:215)
>     at 
> org.apache.kafka.queue.KafkaEventQueue$EventHandler.run(KafkaEventQueue.java:186)
>     at java.base/java.lang.Thread.run(Thread.java:1583)
> ^C[2025-02-24 23:13:21,658] INFO Terminating process due to signal SIGINT 
> (org.apache.kafka.common.utils.LoggingSignalHandler)
> [2025-02-24 23:13:21,660] INFO App info kafka.server for 2 unregistered 
> (org.apache.kafka.common.utils.AppInfoParser) {code}



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