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  • KIP-447: Producer scalability for exactly once semantics

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This is a server-client integrated change, and it's required to upgrade the broker first with `inter.broker.protocol.version` to the latest. Any produce request with higher version will automatically get fenced because of no support.  If this is the case on a Kafka Streams application, you will be recommended to unset `CONSUMER_GROUP_AWARE_TRANSACTION` config as necessary to just upgrade the client without using new thread producer.

To make the upgrade completely compatible with current EOS transaction semantics, we need to be able to distinguish clients who are making progress on the same input source but use using different transactional id. It is possible to have two different types of clients within the same consumer group. Imagine a case as a Kafka Streams applications, where half of the instances are using old task producer API, while the other half of them use new consumer group API. This fencing could be done by leveraging  `AddOffsetsToTxnRequest` which contains a consumer group id and topic partitions. Group coordinator could build a reverse mapping from topic partition to producer.id, and remember which producer has been contributing offset commits to each specific topic partition. In this way, when we upgrade to the new API, group coordinator will be actively checking this map upon receiving `AddOffsetsToTxnRequest`. If the stored `producer.id` doesn't match the one defined in request, coordinator would send out a `ConcurrentProducerCommitException` to stored producer by aborting any ongoing transaction associated with it. This ensures us a smooth upgrade without worrying about old pending transactions.

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