tomFlowee
c63519fa47
The leveldb and univalue 3rd party libraries are not installed and not needed by anyone outside of the Hub. So move them there, making it easier for 3rd party usage.
76 lines
2.7 KiB
Plaintext
76 lines
2.7 KiB
Plaintext
The log file contents are a sequence of 32KB blocks. The only
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exception is that the tail of the file may contain a partial block.
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Each block consists of a sequence of records:
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block := record* trailer?
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record :=
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checksum: uint32 // crc32c of type and data[] ; little-endian
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length: uint16 // little-endian
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type: uint8 // One of FULL, FIRST, MIDDLE, LAST
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data: uint8[length]
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A record never starts within the last six bytes of a block (since it
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won't fit). Any leftover bytes here form the trailer, which must
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consist entirely of zero bytes and must be skipped by readers.
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Aside: if exactly seven bytes are left in the current block, and a new
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non-zero length record is added, the writer must emit a FIRST record
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(which contains zero bytes of user data) to fill up the trailing seven
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bytes of the block and then emit all of the user data in subsequent
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blocks.
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More types may be added in the future. Some Readers may skip record
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types they do not understand, others may report that some data was
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skipped.
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FULL == 1
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FIRST == 2
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MIDDLE == 3
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LAST == 4
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The FULL record contains the contents of an entire user record.
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FIRST, MIDDLE, LAST are types used for user records that have been
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split into multiple fragments (typically because of block boundaries).
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FIRST is the type of the first fragment of a user record, LAST is the
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type of the last fragment of a user record, and MIDDLE is the type of
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all interior fragments of a user record.
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Example: consider a sequence of user records:
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A: length 1000
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B: length 97270
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C: length 8000
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A will be stored as a FULL record in the first block.
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B will be split into three fragments: first fragment occupies the rest
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of the first block, second fragment occupies the entirety of the
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second block, and the third fragment occupies a prefix of the third
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block. This will leave six bytes free in the third block, which will
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be left empty as the trailer.
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C will be stored as a FULL record in the fourth block.
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===================
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Some benefits over the recordio format:
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(1) We do not need any heuristics for resyncing - just go to next
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block boundary and scan. If there is a corruption, skip to the next
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block. As a side-benefit, we do not get confused when part of the
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contents of one log file are embedded as a record inside another log
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file.
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(2) Splitting at approximate boundaries (e.g., for mapreduce) is
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simple: find the next block boundary and skip records until we
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hit a FULL or FIRST record.
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(3) We do not need extra buffering for large records.
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Some downsides compared to recordio format:
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(1) No packing of tiny records. This could be fixed by adding a new
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record type, so it is a shortcoming of the current implementation,
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not necessarily the format.
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(2) No compression. Again, this could be fixed by adding new record types.
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