3 releases (breaking)
0.3.0 | Apr 12, 2024 |
---|---|
0.2.0 | Sep 1, 2023 |
0.1.0 | Jun 9, 2023 |
#121 in Database implementations
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Used in 58 crates
(3 directly)
255KB
6K
SLoC
SSTable
The tantivy-sstable
crate is yet another sstable crate.
It has been designed to be used in quickwit
:
- as an alternative to the default tantivy fst dictionary.
- as a way to store the column index for dynamic fast fields.
The benefit compared to the fst crate is locality. Searching a key in the fst crate requires downloading the entire dictionary.
Once the sstable index is downloaded, running a get
in the sstable
crate only requires a single fetch.
Right now, the block index and the default block size have been thought for quickwit, and the performance of a get is very bad.
Sorted strings?
SSTable stands for Sorted String Table. Strings have to be insert in sorted order.
That sorted order is used in different ways:
- it makes gets and streaming ranges of keys possible.
- it allows incremental encoding of the keys
- the front compression is leveraged to optimize the intersection with an automaton
On disk format
Overview of the SSTable format. Unless noted otherwise, numbers are little-endian.
SSTable
+-------+-------+-----+--------+
| Block | Block | ... | Footer |
+-------+-------+-----+--------+
|----( # of blocks)---|
- Block(
SSTBlock
): list of independent block, terminated by a single empty block. - Footer(
SSTFooter
)
SSTBlock
+----------+----------+--------+-------+-------+-----+
| BlockLen | Compress | Values | Delta | Delta | ... |
+----------+----------+--------+-------+-------+-----+
| |----( # of deltas)---|
|------(maybe compressed)------|
- BlockLen(u32): length of the block, including the compress byte.
- Compress(u8): indicate whether block is compressed. 0 if not compressed, 1 if compressed.
- Values: an application defined format storing a sequence of value, capable of determining it own length
- Delta
Delta
+---------+--------+
| KeepAdd | Suffix |
+---------+--------+
- KeepAdd
- Suffix: KeepAdd.add bytes of key suffix
KeepAdd
KeepAdd can be represented in two different representation, a very compact 1byte one which is enough for most usage, and a longer variable-len one when required
When keep < 16 and add < 16
+-----+------+
| Add | Keep |
+-----+------+
- Add(u4): number of bytes to push
- Keep(u4): number of bytes to pop
Otherwise:
+------+------+-----+
| 0x01 | Keep | Add |
+------+------+-----+
- Add(VInt): number of bytes to push
- Keep(VInt): number of bytes to pop
Note: as the SSTable does not support redundant keys, there is no ambiguity between both representation. Add is always guaranteed to be non-zero, except for the very first key of an SSTable, where Keep is guaranteed to be zero.
SSTFooter
+-----+----------------+-------------+-------------+---------+---------+
| Fst | BlockAddrStore | StoreOffset | IndexOffset | NumTerm | Version |
+-----+----------------+-------------+-------------+---------+---------+
- Fst(Fst): finite state transducer mapping keys to a block number
- BlockAddrStore(BlockAddrStore): store mapping a block number to its BlockAddr
- StoreOffset(u64): Offset to start of the BlockAddrStore. If zero, see the SingleBlockSStable section
- IndexOffset(u64): Offset to the start of the SSTFooter
- NumTerm(u64): number of terms in the sstable
- Version(u32): Currently equal to 3
Fst
Fst is in the format of tantivy_fst
BlockAddrStore
+---------+-----------+-----------+-----+-----------+-----------+-----+ | MetaLen | BlockMeta | BlockMeta | ... | BlockData | BlockData | ... | +---------+-----------+-----------+-----+-----------+-----------+-----+ |---------(N blocks)----------|---------(N blocks)----------|
- MetaLen(u64): length of the BlockMeta section
- BlockMeta(BlockAddrBlockMetadata): metadata to seek through BlockData
- BlockData(CompactedBlockAddr): bitpacked per block metadata
BlockAddrBlockMetadata
+--------+------------+--------------+------------+--------------+-------------------+-----------------+----------+ | Offset | RangeStart | FirstOrdinal | RangeSlope | OrdinalSlope | FirstOrdinalNBits | RangeStartNBits | BlockLen | +--------+------------+--------------+------------+--------------+-------------------+-----------------+----------+
- Offset(u64): offset of the corresponding BlockData in the datastream
- RangeStart(u64): the start position of the first block
- FirstOrdinal(u64): the first ordinal of the first block
- RangeSlope(u32): slope predicted for start range evolution (see computation in BlockData)
- OrdinalSlope(u64): slope predicted for first ordinal evolution (see computation in BlockData)
- FirstOrdinalNBits(u8): number of bits per ordinal in datastream (see computation in BlockData)
- RangeStartNBits(u8): number of bits per range start in datastream (see computation in BlockData)
BlockData
+-----------------+-------------------+---------------+ | RangeStartDelta | FirstOrdinalDelta | FinalRangeEnd | +-----------------+-------------------+---------------+ |------(BlockLen repetitions)---------|
- RangeStartDelta(var): RangeStartNBits bits of little endian number. See below for decoding
- FirstOrdinalDelta(var): FirstOrdinalNBits bits of little endian number. See below for decoding
- FinalRangeEnd(var): RangeStartNBits bits of integer. See below for decoding
converting a BlockData of index Index and a BlockAddrBlockMetadata to an actual block address is done as follow: range_prediction := RangeStart + Index * RangeSlop; range_derivation := RangeStartDelta - (1 << (RangeStartNBits-1)); range_start := range_prediction + range_derivation
The same computation can be done for ordinal.
Note that range_derivation
can take negative value. RangeStartDelta
is just its translation to a positive range.
SingleBlockSStable
The format used for the index is meant to be compact, however it has a constant cost of around 70 bytes, which isn't negligible for a table containing very few keys. To limit the impact of that constant cost, single block sstable omit the Fst and BlockAddrStore from their index. Instead a block with first ordinal of 0, range start of 0 and range end of IndexOffset is implicitly used for every operations.
Dependencies
~6.5MB
~100K SLoC