[KAFKA-18095] Consider allowing member to join without subscription under new consumer protocol

• Type: Improvement
• Status: Resolved
• Priority: Minor
• Resolution: Fixed
• Fix Version/s: 4.0.0
• Component/s: group-coordinator
• Labels: kip-848
• Assignee: David Jacot
• Reporter: Lianet Magrans
• Created: 26/Nov/24 14:59
• Updated: 03/Dec/24 10:12
• Resolved: 03/Dec/24 10:12

Description

With the new consumer group protocol we currently enforce that members must provide subscribed topics or a regex when joining the group. This prevents members to join in cases where it subscribes to a java pattern for which the client-side resolution finds no matching topics.

If we relax this validation, we would end up with a more consistent behaviour for the regex subscriptions under the new protocol, given that currently, if the regex is resolved on the broker and finds no matching topics, the member is still allowed in the group (with no assignments), but if resolved on the client it doesn't join.

Also, the consumer could continue to guarantee that it joins after subscribe(Pattern), no matter if there are matching topics or not.

If we implement this change on the group coordinator, we also need to update the new consumer to actually attempt to join after a call to subscribe(Pattern) if no matching topics found (related jira)

Issue Links

• is required by: KAFKA-18096 Allow new consumer to join group after subscribe to pattern if no matching topics
• links to: GitHub Pull Request #18003