Combination of default values of log.retention.hours (168 hours = 7 days) and offsets.retention.minutes (1440 minutes = 1 day) may be dangerous in special cases. Offset retention should be always greater than log retention.
We have observed the following scenario and issue:
- Producing of data to a topic was disabled two days ago by producer update, topic wasn't deleted.
- Consumer consumed all data and properly committed offsets to Kafka.
- Consumer made no more offset commits for that topic because there was no more incoming data and there was nothing to confirm. (We have auto-commit disabled, I'm not sure how behaves enabled auto-commit.)
- After one day: Kafka cleared too old offsets according to offsets.retention.minutes.
- After two days: Long-term running consumer was restarted after update, it didn't find any committed offsets for that topic since they were deleted by offsets.retention.minutes so it started consuming from the beginning.
- The messages were still in Kafka due to larger log.retention.hours, about 5 days of messages were read again.
Known workaround to solve this issue:
- Explicitly configure log.retention.hours and offsets.retention.minutes, don't use defaults.
- Prolong default value of offsets.retention.minutes to be at least twice larger than log.retention.hours.
- Check these values during Kafka startup and log a warning if offsets.retention.minutes is smaller than log.retention.hours.
- Add a note to migration guide about differences between storing of offsets in ZooKeeper and Kafka (http://kafka.apache.org/documentation.html#upgrade).