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https://issues.apache.org/jira/browse/KAFKA-6774?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16487767#comment-16487767
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Vahid Hashemian commented on KAFKA-6774:
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[~hachikuji] Yes that makes sense. I believe this has been noted in the KIP as
well.
> Improve default groupId behavior in consumer
> --------------------------------------------
>
> Key: KAFKA-6774
> URL: https://issues.apache.org/jira/browse/KAFKA-6774
> Project: Kafka
> Issue Type: Improvement
> Components: consumer
> Reporter: Jason Gustafson
> Assignee: Vahid Hashemian
> Priority: Major
> Labels: needs-kip
>
> At the moment, the default groupId in the consumer is "". If you try to use
> this to subscribe() to a topic, the broker will reject the group as invalid.
> On the other hand, if you use it with assign(), then the user will be able to
> fetch and commit offsets using the empty groupId. Probably 99% of the time,
> this is not what the user expects. Instead you would probably expect that if
> no groupId is provided, then no committed offsets will be fetched at all and
> we'll just use the auto reset behavior if we don't have a current position.
> Here are two potential solutions (both requiring a KIP):
> 1. Change the default to null. We will preserve the current behavior for
> subscribe(). When using assign(), we will not bother fetching committed
> offsets for the null groupId, and any attempt to commit offsets will raise an
> error. The user can still use the empty groupId, but they have to specify it
> explicitly.
> 2. Keep the current default, but change the consumer to treat this value as
> if it were null as described in option 1. The argument for this behavior is
> that using the empty groupId to commit offsets is inherently a dangerous
> practice and should not be permitted. We'd have to convince ourselves that
> we're fine not needing to allow the empty groupId for backwards compatibility
> though.
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