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https://issues.apache.org/jira/browse/KAFKA-14201?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17599744#comment-17599744
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Jason Gustafson commented on KAFKA-14201:
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Worth mentioning that the alternative is to make the server more permissive and
just ignore instance ID if it is set in OffsetCommit while memberId is empty.
> Consumer should not send group instance ID if committing with empty member ID
> -----------------------------------------------------------------------------
>
> Key: KAFKA-14201
> URL: https://issues.apache.org/jira/browse/KAFKA-14201
> Project: Kafka
> Issue Type: Bug
> Reporter: Jason Gustafson
> Priority: Major
>
> The consumer group instance ID is used to support a notion of "static"
> consumer groups. The idea is to be able to identify the same group instance
> across restarts so that a rebalance is not needed. However, if the user sets
> `group.instance.id` in the consumer configuration, but uses "simple"
> assignment with `assign()`, then the instance ID nevertheless is sent in the
> OffsetCommit request to the coordinator. This may result in a surprising
> UNKNOWN_MEMBER_ID error. The consumer should probably be smart enough to only
> send the instance ID when committing as part of a consumer group.
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