James Cheng commented on KAFKA-4682:

Yes, that's a reasonable summary.

Other details to consider:
* [~hachikuji] asked whether we have a single broker-level config for the 
expiration time that applies to all groups, or if we need to allow individual 
consumer groups to specify their own expiration time. In the discussion for 
 [~hachikuji] raised the same question. I'm not sure.
* Offsets are normally saved per (partition, groupId). Do we want to allow 
offsets to be expired for individual partitions separately from the group? As 
an example, say I have a groupId="foo" that commits for (Topic A, partition 0) 
and (Topic B, partition 0). And then groupId stops subscribing to Topic B, and 
only subscribes to (Topic A, partition 0). Should the offset for (Topic B, 
partition 0) stay around as long as the group is active? Or, should it be 
expired, since it is not part of the group anymore?

> Committed offsets should not be deleted if a consumer is still active
> ---------------------------------------------------------------------
>                 Key: KAFKA-4682
>                 URL: https://issues.apache.org/jira/browse/KAFKA-4682
>             Project: Kafka
>          Issue Type: Bug
>            Reporter: James Cheng
> Kafka will delete committed offsets that are older than 
> offsets.retention.minutes
> If there is an active consumer on a low traffic partition, it is possible 
> that Kafka will delete the committed offset for that consumer. Once the 
> offset is deleted, a restart or a rebalance of that consumer will cause the 
> consumer to not find any committed offset and start consuming from 
> earliest/latest (depending on auto.offset.reset). I'm not sure, but a broker 
> failover might also cause you to start reading from auto.offset.reset (due to 
> broker restart, or coordinator failover).
> I think that Kafka should only delete offsets for inactive consumers. The 
> timer should only start after a consumer group goes inactive. For example, if 
> a consumer group goes inactive, then after 1 week, delete the offsets for 
> that consumer group. This is a solution that [~junrao] mentioned in 
> https://issues.apache.org/jira/browse/KAFKA-3806?focusedCommentId=15323521&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-15323521
> The current workarounds are to:
> # Commit an offset on every partition you own on a regular basis, making sure 
> that it is more frequent than offsets.retention.minutes (a broker-side 
> setting that a consumer might not be aware of)
> or
> # Turn the value of offsets.retention.minutes up really really high. You have 
> to make sure it is higher than any valid low-traffic rate that you want to 
> support. For example, if you want to support a topic where someone produces 
> once a month, you would have to set offsetes.retention.mintues to 1 month. 
> or
> # Turn on enable.auto.commit (this is essentially #1, but easier to 
> implement).
> None of these are ideal. 
> #1 can be spammy. It requires your consumers know something about how the 
> brokers are configured. Sometimes it is out of your control. Mirrormaker, for 
> example, only commits offsets on partitions where it receives data. And it is 
> duplication that you need to put into all of your consumers.
> #2 has disk-space impact on the broker (in __consumer_offsets) as well as 
> memory-size on the broker (to answer OffsetFetch).
> #3 I think has the potential for message loss (the consumer might commit on 
> messages that are not yet fully processed)

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