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Current state: "Under Discussion"Accepted

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JIRA: KAFKA-12446

Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).

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{BYTE_ARRAY oldValue}{BYTE newOldFlag=0}
{BYTE_ARRAY newValue}{BYTE newOldFlag=1}
{UINT32INT newDataLength}{BYTE_ARRAY newValue}{BYTE_ARRAY oldValue}{BYTE newOldFlag=2}

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1. In the first rolling bounce, we replace the byte code (i.e. swap the jars), set the config upgrade.from="older version"  (possible values are "0.10.0" - "3.34"), and then bounce each instance to upgrade it.
   
   The upgrade.from="older version"  config will ensure we are still writing out only the old serialization format until all instances are on the new byte code, at which point we can be sure that all instances in the group will be able to successfully deserialize the new format if we were to start writing it. 
2. The second rolling bounce is to simply remove the upgrade.from="older version"  config and bounce each instance for it to begin writing in the new serialization format. 

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