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1. Set message.format.version=0 on brokers. (Broker will write MessageAndOffset V0 to disk)
2. Create internal ApiVersion 0.9.0-1** which uses ProducerRequest V2 and FetchRequest V2.
3. Configure the broker to use ApiVersion 0.9.0 (ProduceRequest V1 and FetchRequest V1).
4. Do a rolling upgrade of the brokers to let the broker pick up the new code supporting ApiVersion 0.9.0-1.
5. Bump up ApiVersion of broker to 0.9.0-1
6. Do a rolling bounce of the brokers to let the broker use FetchRequest V2 for replication.
7. Bump up ProducerRequest and FetchRequest version to V2, which supports both MessageAndOffset V0 and V1.
8. Upgraded brokers support both ProducerRequest V2 and FetchRequest V2 which uses magic byte 1 for MessageAndOffset.
   
   1. When broker sees a producer request V1 (MessageAndOffset = V0), it will decompress the message, assign offsets using definitive absolute offsets and NOT re-compress the message.
      
   2. When broker sees a producer request V2 (MessageAndOffset = V1), it will decompress the message, assign offsets using definitive absolute offsets and do re-compression.  i.e. downconvert the message format to MessageAndOffset V0.
   3. When broker sees a fetch request V1 (Supporting MessageAndOffset = V0), because the data format on disk is MessageAndOffset V0, it will use the zero-copy transfer to reply with fetch response V1 with MessageAndOffset V0.
      
   4. When broker sees a fetch request V2 (Supporting MessageAndOffset = V0, V1), because the data format on disk is MessageAndOffset V0, it will use zero-copy transfer to reply with fetch response V2 with MessageAndOffset V0.
      
9. Upgrade consumer to send FetchRequest V2.
10. Upgrade producer to send ProducerRequest V2.

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1. After most of the consumers are upgraded, Bump up message.format.version=1 and rolling bounce the brokers.
   
2. Upgraded brokers do the followings:
   
   1. When broker sees a producer request V1 (MessageAndOffset = V0), it will decompress the message, assign offsets using relative offsets and re-compress the message. i.e. upconvert the message format to MessageAndOffset V1.
      
   2. When broker sees a producer request V2 (MessageAndOffset = V1), it will decompress the message, assign offsets using relative offsets and NOT do re-compression.
   3. When broker sees a fetch request V1 (Supporting MessageAndOffset = V0), because the data format on disk is MessageAndOffset V1, it will NOT use the zero-copy transfer. Instead the broker will read the message from disk, down convert them to V0 and reply with using fetch response V1 with MessageAndOffset V0.
      
   4. When broker sees a fetch request V2 (Supporting MessageAndOffset = V0, V1), because the data format on disk is MessageAndOffset V1, it will use zero-copy transfer to reply with fetch response V2 with MessageAndOffset V1.

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