Status

Current state: Under Discussion

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

Motivation

Today the broker needs to decompress compressed messages, assign offsets to each message and recompress the messages again. This causes additional CPU cost. This KIP is trying to avoid server side recompression.

This KIP is a distilled/improved version of an earlier discussion that we started.

Public Interfaces

We propose the following change to the message format

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

Proposed Changes

Wire protocol change

Change the usage of offset field

• 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. (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.

Compatibility, Deprecation, and Migration Plan

NOTE: This part is drafted based on the assumption that KIP-31 and KIP-32 will be implemented in one patch.

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For upgraded consumers, they can handle both V0 and V1.

Rejected Alternatives

None