Status

Current state: Under Discussion

Discussion thread: TBD

JIRA: https://issues.apache.org/jira/browse/KAFKA-7610

Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).

Motivation

In the current consumer protocol, the field `member.id` is assigned by broker to track group member status. A new consumer joins the group with `member.id` field set as UNKNOWN_MEMBER_ID (empty string), since it needs to receive the identity assignment from broker first. For request with unknown member id, Broker will blindly accept the new join group request, store the member metadata and return a UUID to consumer. The edge case is that if initial join group request fails, or the consumer keeps restarting, there will be many leftover MemberMetadata within group metadata cache which will eventually burst broker memory. The detection and fencing of invalid join group request is crucial for broker stability.

This KIP is a parallel work with KIP-389 which tries to enforce hard cap on the group metadata size, and an important compliment for KIP-345 which introduces static membership

Public Interfaces

We will introduce a new join group response type call MISSING_MEMBER_ID

Proposed Changes

Describe the new thing you want to do in appropriate detail. This may be fairly extensive and have large subsections of its own. Or it may be a few sentences. Use judgement based on the scope of the change.

Compatibility, Deprecation, and Migration Plan

• What impact (if any) will there be on existing users?
• If we are changing behavior how will we phase out the older behavior?
• If we need special migration tools, describe them here.
• When will we remove the existing behavior?

Rejected Alternatives

If there are alternative ways of accomplishing the same thing, what were they? The purpose of this section is to motivate why the design is the way it is and not some other way.