Status

Current state: Under Discussion Accepted

Discussion thread: here

JIRA: KAFKA-2511

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MessageAndOffset => MessageSize Offset Message
  MessageSize => int32Message
  Offset => int64 <------ CHANGE: Base offset for wrapper message of compressed message, relative offset for inner compressed message.
  MessageSize => int32
  
  Message => Crc MagicByte Attributes Timestamp KeyLength Key ValueLength Value
    Crc => int32
    MagicByte => int8
    Attributes => int8
    KeyLength => int32
    Key => bytes
    ValueLength => int32
    Value => bytes

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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, if multiple message sets are compacted into one message set, the broker 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 configuration to the broker

• The message.format.version controls the message format written to disk. The value equals to the magic byte of the message format. Introducing this configuration is to avoid doing version up/down conversion for the majority of users.
• If a consumer supports message.format.version, the broker will just use zero-copy transfer to send back the FetchResponse.
• If a consumer does not support message.format.version, the broker will have to do down conversion and send FetchResponse without using zero-copy.

Compatibility, Deprecation, and Migration Plan

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The proposed protocol is not backward compatible. The migration plan are as below::

Phase 1 (MessageAndOffset V0 on disk):

1. Set message.format.version=0 on brokers. (Broker will write MessageAndOffset V0 to disk)Bump up ProducerRequest and FetchRequest version to V2, which uses MessageAndOffset V1.
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-V11.
5. Bump up ApiVersion of broker to 0.9.0-1Do a rolling bounce of the brokers . to let the broker use FetchRequest V2 for replication.
6. 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 relative absolute offsets and re-compress the message.
      
   2. When broker sees a producer request V2 (MessageAndOffset = V1), it will decompress the message, assign offsets using absolute offsets and do re-compression.  i.e. upconvert down-convert the message format to MessageAndOffset V0.(This is no worse than what the brokers are currently doing.)
   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.
      
7. Upgrade consumer to send FetchRequest V2.
8. Upgrade producer to send ProducerRequest V2.

Phase 2 (MessageAndOffset V1 on disk):

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 producer request V2 (MessageAndOffset = V1), it will decompress the message for verification, assign the offset to outer message and NOT do recompression.offsets using relative offsets and re-compress the message. i.e. up-convert 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 NOT use the zero-copy transfer, but . Instead the broker will read the message to memoryfrom disk, do down-conversion, then send convert them to V0 and reply 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.
3. Upgrade consumer to send FetchRequest V2.
4. Upgrade producer to send ProducerRequest V2.

For producer, there will be no impact.

In phase 1For producer, there will be no impact for consumers.

For consumers using MessageAndOffset V0In phase 2, there will be some performance penalty for consumers that only supports MessageAndOffset V0, because there is no zero-copy transfer.

During step 7 and step 8, the majority of the consumers may be still using consumers using MessageAndOffset V0, broker could consume more memoryAt the beginning of phase 2, there will be some time the log segment contains both MessageAndOffset V0 and V1. The broker will always do down conversion for FetchRequest V1 and zero-copy transfer for FetchRequest V2.

** We introduce internal ApiVersion here to help the user who are running on trunk to upgrade in the future. Otherwise the interim ApiVersion between two official releases will require users to downgrade ApiVersion then upgrade.

To canary a broker

After phase 1, it is possible for user to canary a broker in phase 2 and roll back if something goes wrong. The procedure is:

1. Set message.format.version=1 on one of the brokers (broker B).
2. Broker B will start to act like what described in phase 2.
   
   1. It will sends FetchRequest V2 to other brokers for replication.
   2. It will only see ProduceRequest/FetchRequest V1 from other brokers and clietns.
      
3. If something goes wrong, we can do the following to rollback:
   1. shutdown broker B
      
   2. nuke the data of the topics it was serving as leader before shutdown
      
   3. set message.format.version=0
   4. restart the broker to let the broker replicate from leaders. At this point the data on disk will be in MessageAndOffset V0.
      

In step 2, it is recommended to put only small amount of leaders on the broker, because at that point the broker needs to do down conversion for all the fetch requests.

Rejected Alternatives

None

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