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Right now we have  `AddOffsetsToTxnRequest` which contains a consumer group id. By leveraging this information, we could build a reverse mapping from topic partition to producer.id. Thus when we upgrade to the new API, group coordinator will be looking at this reverse mapping and send out abort transaction requests to all the affected transaction coordinators, which ensures us a smooth upgrade without worrying about old pending transactions. To remain compatible with current semantic during upgrade, we would proactively reject old version producer if it tries to init transaction towards a topic partition that is owned by some other producer. This may potentially requires a new Request type to let group coordinator inform other transaction coordinator to fence old producer and let them crash immediately.

Rejected Alternatives

• Producer Pooling:
• Producer support multiple transactional ids:
• Tricky rebalance synchronization:

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