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In this KIP, we therefore propose to relax the condition for offset commit fencing.

Identifying zombies using the epoch a partition was assigned to the member

To  To derive a more relaxed check, we need to identify an epoch which separates zombie commits from commits of the current owner. As mentioned above, zombie commit requests are commit requests that include a partition, member ID and member epoch combination, so that the member owned the partition at that epoch. However, the partition has since been reassigned to a different member.

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In both cases, the zombie commit will therefore be rejected by the relaxed check.

Proposed Changes

Introducing

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an epoch per assigned partition

We extend the model of a consumer group with one integer per assigned partition for each member of a group. This includes both partitions directly assigned to the member, and partitions pending revocation. The assignment epoch is set to the epoch in which the partition was assigned to the member, and we have the invariant AssignmentEpoch <= MemberEpoch <= TargetAssignmentEpoch <= GroupEpoch.

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We add a new field, AssignmentEpochs to TopicPartitions. To keep the representation compact, we store it as an array aligned with Partitions, that is, for every item in partition there is one item in AssignmentEpochs.The field is nullable and tagged. For legacy records that do not include the assignment epochs, it will be null, and assignment epochs for all partitions in assignedPartitions and partitionsPendingRevocation are considered to be equal to memberEpoch.

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