[jira] [Created] (KAFKA-13636) Committed offsets could be deleted during a rebalance if a group did not commit for a while

["Damien Gasparina (Jira)" <ji...@apache.org>]

Damien Gasparina created KAFKA-13636:
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             Summary: Committed offsets could be deleted during a rebalance if a group did not commit for a while
                 Key: KAFKA-13636
                 URL: https://issues.apache.org/jira/browse/KAFKA-13636
             Project: Kafka
          Issue Type: Bug
          Components: core, offset manager
    Affects Versions: 3.0.0, 2.8.1, 2.7.2, 2.6.2, 2.5.1, 2.4.0
            Reporter: Damien Gasparina


The group coordinator might delete invalid offsets during a group rebalance. During a rebalance, the coordinator is relying on the last commit timestamp ({_}offsetAndMetadata.commitTimestamp{_}) instead of the last state modification {_}timestampt (currentStateTimestamp{_}) to detect expired offsets.

 

This is relatively easy to reproduce by playing with group.initial.rebalance.delay.ms, offset.retention.minutes and offset.check.retention.interval, I uploaded an example on: [https://github.com/Dabz/kafka-example/tree/master/docker/offsets-retention] .

This script does:
 * Start a broker with: offset.retention.minute=2, o[ffset.check.retention.interval.ms=|http://offset.check.retention.interval.ms/]1000,  group.initial.rebalance.delay=20000
 * Produced 10 messages
 * Create a consumer group to consume 10 messages, and disable auto.commit to only commit a few times
 * Wait 3 minutes, then the Consumer get a {{kill -9}}
 * Restart the consumer after a few seconds
 * The consumer restart from {{auto.offset.reset}} , the offset got removed

 

The cause is due to the GroupMetadata.scala:
 * When the group get emptied, the {{subscribedTopics}} is set to {{Set.empty}} ([https://github.com/apache/kafka/blob/trunk/core/src/main/scala/kafka/coordinator/group/GroupMetadata.scala#L520-L521])
 * When the new member joins, we add the new member right away in the group ; BUT the {{subscribedTopics}} is only updated once the migration is over (in the initNewGeneration) (which could take a while due to the {{{}group.initial.rebalance.delay{}}})
 * When the log cleaner got executed,  {{subscribedTopics.isDefined}} returns true as {{Set.empty != None}} (the underlying condition)
 * Thus we enter [https://github.com/apache/kafka/blob/trunk/core/src/main/scala/kafka/coordinator/group/GroupMetadata.scala#L782-L785] with an empty {{subscribedTopics}} list and we are relying on the {{commitTimestamp}} regardless of the {{currentStateTimestamp}}

 

This seem to be a regression generated by KIP-496 https://cwiki.apache.org/confluence/display/KAFKA/KIP-496%3A+Administrative+API+to+delete+consumer+offsets#KIP496:AdministrativeAPItodeleteconsumeroffsets-ProposedChanges



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