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Status

Current state: "Under Discussiondiscarded"

Discussion thread: here

JIRA: here

Note: discarding this KIP since the benefits of the space savings outweigh the cost incurred for message conversion. And there are also planned changes for version v.3, like:

3. v3 of the record format should make it cheaper to make changes in the future (perhaps it could support tagged fields or similar)

4. We'd want to fix other known issues at the same time (eg log append time should always be available, there may be others)

5. We should consider whether we would want to introduce a user header that is at the batch level vs record level for efficiency reasons

[Additional proposal] Changing the batch format proposal

Problem: Currently, we perform validation of data schema (such as verifying monotonically increasing offsets etc.). To do that, we have to read the headers of each message in a batch. For a compressed batch, this means that we have to decompress the entire batch including the payload (key/value) even though we don't really require any information out of them.

Solution: If we change the ordering of messages in a batch to prefix with headers of all messages first and followed by the payload of all messages, it will lead to an optimization where we have to "partially" decompress the batch (i.e. the prefix).

Ideas for message format v.3 wiki: ideas for kafka message format v.3

Motivation

Logs in kafka consists of batches, and one batch consists of many messages. So if the size of each message header can be reduced, it'll improve the network traffic and storage size (and money, of course) a lot, even it's a small improvement. Some companies now handle trillions of messages per day using Kafka, so, if we can reduce just 1 byte per message, the save is really considerable.

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length: varint
attributes: int8
    bit 0~7: unused // <-- unused attributes field
timestampDelta: varlong
offsetDelta: varint
keyLength: varint
key: byte[]
valueLen: varint
value: byte[]
Headers => [Header]

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baseOffset: int64
batchLength: int32 // <-- will change to varint
partitionLeaderEpoch: int32
magic: int8 (current magic value is 2) // <-- will changebump to 3
crc: int32
attributes: int16
    bit 0~2:
        0: no compression
        1: gzip
        2: snappy
        3: lz4
        4: zstd
    bit 3: timestampType
    bit 4: isTransactional (0 means not transactional)
    bit 5: isControlBatch (0 means not a control batch)
    bit 6: hasDeleteHorizonMs (0 means baseTimestamp is not set as the delete horizon for compaction)
    // new added attribute below
    bit 7: ignoreMessageAttributes (0 means not to ignore)
    bit 8~15: unused

producerId: int64
producerEpoch: int16
baseSequence: int32
lastOffsetDelta: int32 // <-- will change to varint
baseTimestamp: int64 // <-- will change to varlong
maxTimestamp: int64 // <-- will change semantic to varlong
producerId: int64
producerEpoch: int16
baseSequence: int32
 maxTimestampDelta and change type to varint records: [Record]

Furthermore, the record batch header can also be smaller. I'd also like to improve them by:

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2. maxTimestamp: change the semantic to maxTimestampDelta, and change type from int64 to varlongvarint. In most case, the timestamp for each record inside the batch should be very close. So, changing to varlong varint will save space. The maxTimestamp can be calculated via [baseTimestamp + maxTimestampDelta]

3. lastOffsetDelta: change the type from int32 to varint. Same as above, In most case, the offset delta should be small. So, changing to varint will save space.

4. Length: It means the size of this batch. This change needs more explanation. 

The default producer `batch.size` config is 16384, and `linger.ms` is 0, that means, in most case, this value should be smaller than 16384. Besides, when producer send batches to a node, it’ll also send other (in-progress) batches that also belong to this node together. For example: 

the leader of tp-0, tp-1, tp-2 are node 1, and now, the batch of tp-0 is full, ready to send to node 1, the producer will also send batches in tp-1, tp-2 (even though the batch size is small) to node 1. Because of this characteristic, in the log segment, there should be many “small batches”. So, if changing the length field from int32 to varint should also be good.move above 3 fields to the trail of the batch headers.

Note: 

The biggest value of varint 2 bytes is 16383

The biggest value of varint 3 bytes is 2097151

With the above 4 3 record batch field fields changes, in a normal batch, with close timestamp between each record and offsets, the save can be:

MaxTimestamp from int64 (8 bytes) to 2 ~ 3 bytes. Suppose the max timestamp delta has a long 30 16 seconds, which only need 3 2 bytes for varlongvarint. The offset delta save from int32 (4 bytes) to 1~2 bytes (the biggest value for 1 byte2 bytes of varint can be 16383).About the length, we use int32 (4 bytes) for now. But as analyzed above, we should have many small batches in the log. Even if it’s the default batch.size 16384, we only need 3 bytes to store by using varint. 

In all, we can save around:

[8 - 6 (baseTimestamp)] +  [8 - 3 2 (max timestamp delta)] + [4 - 2 (offset delta)] + [4 - 3 (length)] = 10 Bytes for each batch.

18% space save for batch overhead, compared with original batch overhead 53 bytes.

For the each message header space save, using the example in KIP-98:

For example, assuming a fixed message size of 1K with 100 byte keys and reasonably close timestamps, the overhead increases by only 7 bytes for each additional batched message (2 bytes for the message size, 1 byte for attributes, 2 bytes for timestamp delta, 1 byte for offset delta, and 1 byte for key size)

14% space save for each message overhead, compared with originally 7 bytes, now only 6 bytes needed.

Proposed Changes

When writing the batch using the new version of message format, we'll default set ignoreMessageAttributes field to 1 in record batch header, and create records without attribute field.

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