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For Kafka Streams that needs to be backward compatible to older broker versions, we add a new config value for config parameter processing.guarantees that parameter PROCESSING_GUARANTEE that we call "exaclty_once_beta". Hence, users with new broker versions can opt-in the new feature (ie, using a single producer per thread for EOS instead of a producer per task).

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1. Broker must be upgraded to 2.5 first.
2. Upgrade the stream application binary and choose to set UPGRADE_FROM_CONFIG config to 2.5 (note that 2.5 refers to the Streams version, not the broker version) or lower and set PROCESSING_GUARATNEE to "exacltyexactly_once_beta". Do the first rolling bounce, and make sure the group is stable with every instance on 2.6 binary.
3. Remove/unset UPGRADE_FROM_CONFIG, in order to point the application to actual Kafka client version. Do a second rolling bounce and now the application officially starts using new thread producer for EOS.

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