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1. If a consumer receives a higher generation id for it's group, it stops fetching and commits existing offsets before sending a JoinGroupRequest to the co-ordinator.
   
2. The co-ordinator checks the generation id in the OffsetCommitRequest and rejects it if the generation id in the request is higher than the generation id on the co-ordinator. This indicates a bug in the consumer code.
   
3. The co-ordinator does not allow offset commit requests with generation ids older than the current group generation id in zookeeper either. This constraint is not a problem during a rebalance since until all consumers have sent a JoinGroupRequest, the co-ordinator does not increment the group's generation id. And from that point until the point the co-ordinator sends a JoinGroupResponse, it does not expect to receive any OffsetCommitRequests from any of the consumers in the group in the current generation. So the generation id on every OffsetCommitRequest sent by the consumer should always match the current generation id on the co-ordinator. Another case worth discussing is when a consumer goes through a soft failure e.g. a long GC pause during a rebalance. If a consumer pauses for more than session.timeout.ms, it is marked dead by the co-ordinator. It The co-ordinator will expire the JoinGroupRequests received so far, fail the current rebalance attempt and trigger another rebalance operation.
   

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