Documentation

A record batch. The following fields are not made explicit since they are only for framing and checksum:

• batchLength
• magic (always the number 2)
• crc

From kafka documentation:

baseOffset: int64
batchLength: int32
partitionLeaderEpoch: int32
magic: int8 (current magic value is 2)
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)
    bit 7~15: unused
lastOffsetDelta: int32
baseTimestamp: int64
maxTimestamp: int64
producerId: int64
producerEpoch: int16
baseSequence: int32
records: [Record]

A few of my own observations:

• The docs add a note that that last field records is not really what it looks like. The array length is always serialized in the usual way, but the payload might be compressed.
• The field batchLength includes the size of everything after it. So, not itself and not baseOffset.