[jira] [Created] (KAFKA-12378) If a broker is down for more then `delete.retention.ms` deleted records in a compacted topic can come back.

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

Shane created KAFKA-12378:
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             Summary: If a broker is down for more then `delete.retention.ms` deleted records in a compacted topic can come back.
                 Key: KAFKA-12378
                 URL: https://issues.apache.org/jira/browse/KAFKA-12378
             Project: Kafka
          Issue Type: Bug
            Reporter: Shane


If the leader of a compacted topic goes offline, or has replication lag longer than the `delete.retention.ms` of a topic, records that are tombstoned can come back once the leader catches up then becomes the leader.

 

Example of this happening:
 Topic config:
    name: compacted-topic
    settings: delete.retention.ms=0
    Leader: broker 1
    ISR: broker 1, broker 2, broker 3

 

Producer 1 writes a record `1:foo` 
 Producer 1 writes a record `2:bar` 
 broker 1 goes offline 
 broker 2 takes over leadership
 Producer 1 writes a tombstone `1:NULL`
 broker 2 compacts the topic, which leaves the topic with `1:NULL` and `2:bar` in it.
 broker 2 removes the tombstone leaving just `2:bar` in the topic.
 broker 1 comes back online, catches up with replication, takes back leadership
 broker 1 now has `1:foo` and `2:bar` as the data, since the tombstone is deleted

At this point the topic is in a strange state, as the brokers have conflicting data.

 

 

Suggestion:
 I believe this to be quite a hard problem to solve, so I'm not going to suggest any large changes to the codebase, but I think a warning in the docs about `delete.retention.ms` is warranted.
 I think adding something that calls out that brokers are also consumers here: [https://docs.confluent.io/platform/current/installation/configuration/topic-configs.html#topicconfigs_delete.retention.ms] would be helpful, but even further documentation about what happens when a broker is offline for more than `delete.retention.ms` would be nice to see. If it helps I'm happy to take a first draft at updating the docs as well.



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