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• When the producer compresses a message, write the relative offset value in the raw message's offset field. Leave the wrapped message's offset blank.
• When broker receives a compressed message, it only needs to
  
  1. Decompress the message to verify the CRC and relative offset.
     NOTE: If the relative offset is not contiguous (e.g., if this is a mirrored compacted topic), the broker will reassign the relative offsets. There are two ways to handle this - (i) reject the ProducerRequest or (ii) just assign the relative offsets. We chose to reassign offsets rather than reject the request because there is a useful use case where mirror maker can do a direct copy from source cluster to destination cluster without even decompressing the message. In this case, the compressed message can have noncontinuous relative offsets (for compacted topics).
  2. Set outer message's base offset. The outer message's base offset will be the offset of the last inner message.  (Since the broker only needs to update the message-set header, there is no need to re-compress message sets.)
• When the log cleaner compacts log segments, it needs to update the inner message's relative offset values. (This will leave "holes" inside the new wrapped message).
• When the consumer receives a message, it converts the relative offset back to actual offset.

Add a message.format.version config to broker configuration

This change is to facilitate protocol migration. Because it is unlikely all the consumer can be migrated to use the new message format at the same time. It is important for the broker to support both new and old message format.

One approach is to let the broker adapt the message format to consumer requirement. It requires the following change:

1. Add a message.format.version configuration on broker.
2. Bump up ProduceRequest/FetchRequest version up from V1 to V2 to indicate a new client.

It works in the following way:

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Compatibility, Deprecation, and Migration Plan

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1. Bump up ProducerRequest and FetchRequest version to V2, which uses MessageAndOffset V1.
2. Create ApiVersion 0.9.0-V1 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-V1.
5. Bump up ApiVersion of broker to 0.9.0-V1
6. Do a rolling bounce of the brokers to let the broker use FetchRequest V2 for replication.
7. Upgraded brokers support both ProducerRequest V2 .
    Upgrade broker to support both ProducerRequest 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 relative offsets and re-compress the message. i.e. upconvert the message format to magMessageAndOffset V1.
      
   2. When broker sees a producer request V2 (MessageAndOffset = V1), it will decompress the message for verification, assign the offset to outer message and NOT do recompression. 
   3. When broker sees a fetch request V1 (MessageAndOffset = V0), because the data format on disk is MessageAndOffset V1, it will no not use the zero-copy transfer, but read the message to memory, do down-conversion, then send fetch response V1.
   4. When broker sees a fetch request V2 (MessageAndOffset = V1), it will use zero-copy transfer to reply with fetch response V2.
      
8. Upgrade consumer to support both V0 and V1send FetchRequest V2.
9. Upgrade producer to send MessageAndOffset V1ProducerRequest V2.

For producer, there will be no impact.

For consumers using MessageAndOffset V0, there will be some performance penalty because there is no zero-copy transfer.

During step 2 7 and step 38, the majority of the consumers may be still using consumers using MessageAndOffset V0, broker could consume more memory.

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