sackli 0.2.2

A format for storing a sequence of byte-array records

Säckli

This is a friendly fork of bagz.

Additions so far:

• Merge some PRs such as S3 support PR by @KefanXIAO and compile fixes.
• Add access_pattern and cache_policy reader hints:
  • On POSIX filesystem, this can add mmap hints or use pread to optimize for random access and larger-than-RAM data.
• Make it compatible to Python versions past 3.13.
• Make it compatible with free-threading (nogil) Python.
• Add CI, stress-tests and automatic wheel releases to PyPI.

Versioning of this fork is detached from the original bagz library at the point it was forked (v0.2.0).

Overview

Säckli is a format for storing a sequence of byte-array records. It supports per-record compression and fast index-based lookup. All indexing is zero based.

Installation

The recommended installation on Linux is via the pre-built wheels on PyPI:

uv pip install sackli

If you want to build locally to work on this, just uv pip install .. However, building can be slow because of GCS and S3 support; to skip both of these dependencies for much faster builds, you can do:

CMAKE_ARGS="-DSACKLI_ENABLE_GCS=OFF -DSACKLI_ENABLE_S3=OFF" uv pip install .

Python API

Python Reader

Reader for reading a single or sharded Säckli file-set.

from collections.abc import Sequence, Iterable

import sackli
import numpy as np

# Säckli Readers support random access. The order of elements within a Säckli
# file is the order in which they are written. Records are returned as `bytes`
# objects.
data = sackli.Reader('/path/to/data.bagz')

# Säckli Readers can be configured like this - here we require that the file was
# written with separate limits.
data_separate_limits = sackli.Reader('/path/to/data.bagz', sackli.Reader.Options(
    limits_placement=sackli.LimitsPlacement.SEPARATE,
))

# Säckli Readers are Sequences and support slicing, iterating, etc.
assert isinstance(data, Sequence)

# Säckli Readers have a length.
assert len(data) > 10

# Can access record by row-index.
fifth_value: bytes = data[5]

# Can slice.
data_from_5: sackli.Reader = data[5:]

# Slices are still Readers.
assert isinstance(data_from_5, sackli.Reader)

assert data_from_5[0] == fifth_value

# Can access records by multiple row-indices.
fourth, second, tenth = data.read_indices([4, 2, 10])
assert fourth == data[4]
assert second == data[2]
assert tenth == data[10]

# Can iterate records.
for record in data:
  do_something_else(record)

# Can read all records. This eager version can be faster than iteration.
all_records = data.read()

# Can iterate sub-range of records.
for record in data[4:9]:
  do_something_else(record)

# Can read a sub-range of records. This eager form can be faster than
# iteration.
sub_range = data[4:9].read()

# Can use an infinite iterator as source of indices. (Reads ahead in parallel.)
def my_generator(size: int) -> Iterable[int]:
  rng = np.random.default_rng(42)
  while True:
    yield rng.integers(size).item()

data_iter: Iterable[bytes] = data.read_indices_iter(my_generator(len(data)))
for i in range(10):
  random_item: bytes = next(data_iter)

Python Reader - Index and MultiIndex

You can use Index to find the first index of a record and MultiIndex to find all instances of an item.

keys = sackli.Reader('/path/to/keys.bag')
# Get the index of the first occurrence of key.
index = sackli.Index(keys)
key_index: int = index[b'example_key']

# Get all occurrences of key.
multi_index = sackli.MultiIndex(keys)
all_indices: list[int] = multi_index[b'example_key']

Python Writer

For writing a single Säckli file.

Example:

import sackli

# Compression is selected based on the file extension:
# `.bagz` will use Zstandard compression with default settings.
# `.bag` will use no compression.
with sackli.Writer('/path/to/data.bagz') as writer:
  for d in generate_records():
    writer.write(d)

# Adjust compression level explicitly.
# Note this will no longer use the extension to detemine whether to compress.
with sackli.Writer(
    '/path/to/data.bagz',
    sackli.Writer.Options(
        compression=sackli.CompressionZstd(level=3)
    ),
) as writer:
  for d in generate_records():
    writer.write(d)

Options

Reader Options

sackli.Reader.Options has these optional arguments.

• compression: Can be one of:
  • sackli.CompressionAutoDetect(): Default - Uses extension whether to compress. (.bagz - Compressed (ZStandard), .bag - Uncompressed)
  • sackli.CompressionNone(): Records are not decompressed.
  • sackli.CompressionZstd(): Records are decompressed using Zstandard.
• limits_placement: Can be one of:
  • sackli.LimitsPlacement.TAIL: Default- Reads limits from a tail of file.
  • sackli.LimitsPlacement.SEPARATE: Reads limits from a separate file.
• limits_storage: Can be one of:
  • sackli.LimitsStorage.ON_DISK: Default - Reads limits from disk for each read.
  • sackli.LimitsStorage.IN_MEMORY: Reads all limits from disk in one go.
• access_pattern: Can be one of:
  • sackli.AccessPattern.SYSTEM: Default - no specific hint to the OS.
  • sackli.AccessPattern.RANDOM: Hints that you read entries in random order.
  • sackli.AccessPattern.SEQUENTIAL: Hints that you read entries roughly sequentially.
• cache_policy: Can be one of:
  • sackli.CachePolicy.SYSTEM: Default - no specific hint to the OS.
  • sackli.CachePolicy.DROP_AFTER_READ: Reads data in such a way that the OS is unlikely to hold any of it in cache. For POSIX filesystems, this means using pread with specific flags. This is more efficient when you read more data than your RAM before doing any repeats (ie epoch > RAM).
• max_parallelism: Default number of threads when reading many records.
• sharding_layout: Can be one of:
  • sackli.ShardingLayout.CONCATENATED: Default - See Sharding
  • sackli.ShardingLayout.INTERLEAVED: See Sharding

access_pattern and cache_policy are currently interpreted only for local POSIX files and influence OS-level behaviour on page cache and cache lines.

For tail-formatted files, cache_policy=DROP_AFTER_READ opens a second POSIX read handle to the same file so limits metadata reads keep the default cache policy.

Writer Options

sackli.Writer.Options has these optional arguments.

• compression: Can be one of:
  • sackli.CompressionAutoDetect(): Default - Uses extension whether to compress. (.bagz - Compressed (Zstandard), .bag - Uncompressed)
  • sackli.CompressionNone(): Records are not compressed.
  • sackli.CompressionZstd(level = 3): Records are compressed using Zstandard the level of the compression can be specified.
• limits_placement: Can be one of:
  • sackli.LimitsPlacement.TAIL: Default - Writes limits to a tail of file.
  • sackli.LimitsPlacement.SEPARATE: Writes limits to a separate file.

Apache Beam Support

Säckli also provides Apache Beam connectors for reading and writing Säckli files in Beam pipelines.

Ensure you have Apache Beam installed.

uv pip install apache_beam

Säckli Source

import apache_beam as beam
from sackli.beam import sacklio
import tensorflow as tf

with beam.Pipeline() as pipeline:
  examples = (
      pipeline
      | 'ReadData' >> sacklio.ReadFromSackli('/path/to/your/data@*.bagz')
      | 'Decode' >> beam.Map(tf.train.Example.FromString)
  )
  # Continue your pipeline.

Säckli Sink

from sackli.beam import sacklio
import tensorflow as tf

def create_tf_example(data):
  # Replace with your actual feature creation logic.
  feature = {
      'data': tf.train.Feature(bytes_list=tf.train.BytesList(value=[data])),
  }
  return tf.train.Example(features=tf.train.Features(feature=feature))

with beam.Pipeline() as pipeline:
  data = [b'record1', b'record2', b'record3']

  examples = (
      pipeline
      | 'CreateData' >> beam.Create(data)
      | 'Encode' >> beam.Map(lambda x: create_tf_example(x).SerializeToString())
      | 'WriteData' >> sacklio.WriteToSackli('/path/to/output/data@*.bagz')
  )

GCS Support

Säckli supports Posix file-systems and Google Cloud Storage (GCS). If you have files on GCS you can access them with using the prefix /gs: to the path. These examples assume you have gcloud CLI installed.

From the shell:

gcloud config set project your-project-name
gcloud auth application-default login

Then use the 'gs:' file-system prefix.

import pathlib
import sackli

# (This may freeze if you have not configured the project.)
reader = sackli.Reader('gs://your-bucket-name/your-file.bagz')

# Path supports a leading slash to work well with pathlib.
bucket = pathlib.Path('/gs://your-bucket-name')
reader = sackli.Reader(bucket / 'your-file.bagz')

Sharding

An ordered collection of Säckli-formatted files ("shards") may be opened together and indexed via a single global-index. The global-index is mapped to a shard and an index within that shard (shard-index) in one of two ways:

1. Concatenated (default). Indexing is equivalent to the records in each Säckli-formatted shard being concatenated into a single sequence of records.

   Example:

   When opening four Säckli-formatted files with sizes [8, 4, 0, 5], the global-index with range [0, 17) (shown as the table entries) maps to shard and shard-index like this:

                  | shard-index
   shard          |  0  1  2  3  4  5  6  7
   -------------- | -----------------------
   00000-of-00004 |  0  1  2  3  4  5  6  7
   00001-of-00004 |  8  9 10 11
   00002-of-00004 |
   00003-of-00004 | 12 13 14 15 16
   

   Mappings

   global-index	shard	shard-index
   0	00000-of-00004	0
   1	00000-of-00004	1
   2	00000-of-00004	2
   ...	...	...
   8	00001-of-00004	0
   9	00001-of-00004	1
   ...	...	...
   15	00003-of-00004	3
   16	00003-of-00004	4

2. Interleaved where the global-index is interleaved in a round-robin manner across all the shards.

   Example:

   When opening three Säckli-formatted files with sizes [6, 6, 5], the global-index with range [0, 17) (shown as the table entries) maps to shard and shard-index like this:

                  |  shard-index
   shard          |  0  1  2  3  4  5
   -------------- | -----------------
   00000-of-00003 |  0  3  6  9 12 15
   00001-of-00003 |  1  4  7 10 13 16
   00002-of-00003 |  2  5  8 11 14
   

   Mappings

   global-index	shard	shard-index
   0	00000-of-00003	0
   1	00001-of-00003	0
   2	00002-of-00003	0
   ...	...	...
   6	00000-of-00003	2
   7	00001-of-00003	2
   8	00002-of-00003	2
   ...	...	...
   15	00000-of-00003	5
   16	00001-of-00003	5

Säckli file format

The Säckli file format has two parts: the records section and the limits section.

• The records section consists of the concatenation of all (possibly compressed) records. (There are no additional bytes inside or between records, and records are not aligned in any way.)
• The limits section is a dense array of the end-offsets of each record in order, encoded in little-endian 64-bit unsigned integers.

These can be stored with tail-limits in one file, where the limits sections is appended to the record section or separate-limits where they are stored in separate files.

Tail-limits example

Given Säckli file formatted file with the following 3 uncompressed records:

Records
abcdef
123
catcat

The raw bytes of the Säckli file formated file corresponding to the records above:

0x61 a 0x62 b 0x63 c 0x64 d 0x65 e 0x66 f
0x31 1 0x32 2 0x33 3
0x63 c 0x61 a 0x74 t 0x63 c 0x61 a 0x74 t
0x06   0x00   0x00   0x00   0x00   0x00   0x00   0x00  # 6 byte offset
0x09   0x00   0x00   0x00   0x00   0x00   0x00   0x00  # 9 byte offset
0x0f   0x00   0x00   0x00   0x00   0x00   0x00   0x00  # 15 byte offset

The last 8 bytes represent the end-offset of the last record. This is also the start of the limits section. Therefore reading the last 8 bytes will directly tell you the offset of the records/limits boundary.