Fast dataloader and conversion utility for webdataset tar shards. Rust core with Python bindings.

Built for streaming large video and image datasets, but handles any byte data.

Install

pip install webshart

What is this?

Webshart is a fast reader for webdataset tar files with separate JSON index files. This format enables random access to any file in the dataset without downloading the entire archive.

The indexed format provides massive performance benefits:

• Random access: Jump to any file instantly
• Selective downloads: Only fetch the files you need
• True parallelism: Read from multiple shards simultaneously
• Cloud-optimized: Works efficiently with HTTP range requests
• Aspect bucketing: Optionally include image geometry hints width, height and aspect for the ability to bucket images by shape
• Logical sample APIs: Treat image.ext + image.json pairs as one sample while still allowing raw file access
• Caption metadata: Store captions in shard metadata under the plural captions key as either a string or a list of strings
• Custom DataLoader: Includes state dict methods on the DataLoader so that you can resume training deterministically
• Rate-limit friendly: Local caching allows high-frequency random seeking without encountering storage provider rate limits
• Instant start-up with pre-sorted aspect buckets

Growing ecosystem: While not all datasets use this format yet, you can easily create indices for any tar-based dataset (see below).

Quick Start

import webshart

# Find your dataset
dataset = webshart.discover_dataset(
    source="laion/conceptual-captions-12m-webdataset",
    # we're able to upload metadata separately so that we reduce load on huggingface infra.
    metadata="webshart/conceptual-captions-12m-webdataset-metadata",
)
print(f"Found {dataset.num_shards} shards")

loader = webshart.TarDataLoader(dataset)

# File-oriented access is still available.
files = dataset.list_files_in_shard(0)

# Sample-oriented access skips paired JSON sidecars.
samples = dataset.list_samples_in_shard(0)
entry = loader.load_sample(0, 0)
print(entry.path, entry.captions, entry.json_metadata)

Paired datasets

Two datasets can remain independently loadable while also exposing an opt-in join by logical sample key. This works especially well for preference, reference, and slider-training data stored in two subfolders of one repository:

paired = webshart.discover_paired_dataset(
    "webshart/suno-various-94k",
    left_subfolder="original",
    right_subfolder="covers",
)

print(paired.num_pairs)
print(paired.get_pair(0))

loader = webshart.PairedTarDataLoader(paired)
sample = loader.load_pair(0)
print(sample.key, sample.left, sample.right)

The normal contract is unchanged: calling discover_dataset(..., subfolder="original") or subfolder="covers" returns a standalone dataset. Pair indexing is lazy, preserves left-dataset order, and validates identical key sets by default. Pass strict=False to use only the intersection and inspect unmatched_left / unmatched_right.

max_file_size is a visibility limit for loader APIs. Files larger than the configured limit are omitted from iteration, batches, direct sample loading, and aspect buckets instead of being returned with empty data. Direct load_sample() calls return None for an oversized sample. The loader's list_samples_in_shard() returns dictionaries containing sample_idx and filename, so filtered listings retain the stable index required by load_sample().

Common Patterns

For real-world, working examples:

• Use as a DataLoader
• Retrieve data subset/range
• Get dataset statistics without downloading
• List aspect buckets
• Write captions into metadata

Creating Indices for / Converting Existing Datasets

Any tar-based webdataset can benefit from indexing! Webshart includes tools to generate indices:

A command-line tool that auto-discovers tars to process:

% webshart extract-metadata \
    --source laion/conceptual-captions-12m-webdataset \
    --destination laion_output/ \
    --checkpoint-dir ./laion_output/checkpoints \
    --max-workers 2 \
    --include-image-geometry

Or, if you prefer/require direct-integration to an existing Python application, use the API

Uploading Indices to HuggingFace

Once you've generated indices, share them with the community:

# Upload all JSON files to your dataset
huggingface-cli upload --repo-type=dataset \
    username/dataset-name \
    ./indices/ \
    --include "*.json" \
    --path-in-repo "indices/"

Or if you want to contribute to an existing dataset you don't own:

1. Create a community dataset with indices: username/original-dataset-indices
2. Upload the JSON files there
3. Open a discussion on the original dataset suggesting they add the indices

Creating New Indexed Datasets

If you're creating a new dataset, generate indices during creation:

{
  "files": {
    "image_0001.webp": {"offset": 512, "length": 102400},
    "image_0002.webp": {"offset": 102912, "length": 98304},
    ...
  }
}

The JSON index should have the same name as the tar file (e.g., shard_0000.tar → shard_0000.json).

Caption layouts and sidecars

Webshart recognizes both JSON metadata sidecars and plain-text caption sidecars:

sample_0001.webp
sample_0001.json
sample_0002.webp
sample_0002.txt

Paired .json and .txt members are excluded from logical sample indexes. You can inspect the layout from shard metadata without downloading tar members:

layout = dataset.probe_caption_layout(max_shards=16)
print(layout["layout"])  # embedded, json_sidecar, txt_sidecar, mixed, or none

When metadata is extracted or loaded, sidecars are attached to their paired sample entries:

{
  "files": {
    "sample_0001.webp": {
      "offset": 512,
      "length": 102400,
      "width": 1024,
      "height": 1024,
      "aspect": 1.0,
      "json_path": "sample_0001.json",
      "json_offset": 103424,
      "json_length": 128,
      "captions": "a product photo on a white background",
      "json_metadata": {
        "caption": "a product photo on a white background"
      }
    },
    "sample_0001.json": {
      "offset": 103424,
      "length": 128
    }
  }
}

Use file-oriented APIs when you want every archive member, including sidecars:

dataset.list_files_in_shard(0)

reader = dataset.open_shard(0)
raw_file_bytes = reader.read_file(0)

Use sample-oriented APIs when you want training samples:

dataset.list_samples_in_shard(0)
dataset.get_shard_sample_count(0)

reader = dataset.open_shard(0)
image_bytes = reader.read_sample(0)
json_bytes = reader.read_sample_json(0)

entry = loader.load_sample(0, 0)
print(entry.path)
print(entry.captions)
print(entry.json_data)

# Direct caption lookup also handles paired .txt sidecars.
caption = loader.load_caption(0, 0)

Captions are canonicalized to the plural captions metadata key. The value may be a single string, a list of strings, or absent.

webshart.write_captions_to_metadata(
    "shard_0000.json",
    {
        "sample_0001.webp": "a short caption",
        "sample_0002": ["caption one", "caption two"],
    },
)

The writer updates existing webshart metadata JSON in place, removes old singular caption keys from updated samples, and leaves paired .json sidecar entries untouched.

To avoid repeated .txt range reads, fold all sidecar captions into standard webshart metadata files. If metadata caching is enabled, omitting the destination persists the enriched indexes in webshart's cache:

dataset.enable_metadata_cache("cache/metadata", init_shard_count=0)
loader = webshart.TarDataLoader(dataset, load_file_data=False)
loader.coalesce_caption_metadata()

# Or create a portable export tree for copying or upload.
loader.coalesce_caption_metadata("caption-metadata")
webshart.upload_caption_metadata(
    "caption-metadata",
    "organization/dataset-metadata",
    hf_token="hf_...",
)

The CLI provides the same operation. --shard-cache-dir lets coalescing reuse full cached shards instead of issuing one range read per sidecar:

webshart optimize-captions \
  --source organization/dataset \
  --metadata organization/dataset-metadata \
  --destination caption-metadata \
  --shard-cache-dir cache/shards \
  --push-to-hub organization/dataset-metadata

optimize-captions expects existing .tar shards and webshart indexes. To fully repackage a repository of loose media plus .txt/.json sidecars, or a legacy SimpleTuner layout containing unindexed .tar archives whose member filenames are captions, use the rolling optimize-dataset command instead:

webshart optimize-dataset \
  --source stablellama/Qwen-Image-2512_samples \
  --push-to-hub stablellama/Qwen-Image-2512_samples \
  --output-prefix webshart \
  --max-shard-size-gb 1

The target is always a Hugging Face dataset repository. It may be the same repository as the source because generated files live under --output-prefix. The input layout is detected automatically. Loose sidecars are coalesced into metadata. Legacy tar members are repacked into bounded shards and their filename stems become captions, matching SimpleTuner's filename strategy (underscores become spaces). Remote legacy inputs use aligned HTTP ranges from the saved member offset and retain only the current output shard locally.

After each shard is indexed, its sidecar captions are embedded in the JSON index and the tar, index, and .webshart-optimize-state.json are uploaded in a single commit. The state records relative positions and conversion settings, never local absolute paths. For legacy tars this includes the source archive index and tar-block member offset, so a rerun resumes within an archive after the last committed output shard. Use --max-shards N to bound each worker invocation.

For a local-only conversion, replace --push-to-hub with a local destination:

webshart optimize-dataset \
  --source /datasets/loose-pairs \
  --destination /datasets/indexed \
  --max-shards 10

Plain-text sidecars are omitted from the tar after their captions are embedded. JSON sidecars are likewise coalesced: recognized caption fields become the canonical captions value and the complete object is retained as json_metadata in the index.

Hub reads accept hf_token= and also honor HF_TOKEN. This includes gated datasets and separately hosted metadata. Local discovery recursively pairs tar and JSON indexes, preserving their relative subdirectories.

Aspect Bucketing Samples

list_shard_aspect_buckets() is file-oriented and buckets any indexed file that has width and height.

For training pipelines, prefer list_shard_sample_aspect_buckets():

loader = webshart.TarDataLoader(dataset)
buckets = loader.list_shard_sample_aspect_buckets(
    [0],
    key="geometry-tuple",
    target_pixel_area=1024**2,
)[0]["buckets"]

for bucket_key, entries in buckets.items():
    for item in entries:
        virtual_id = f"webshart://0/{item['sample_idx']}/{item['filename']}"
        image = loader.load_sample(0, item["sample_idx"])

This uses logical samples from metadata.sample_range() / get_sample_by_index() and excludes paired JSON sidecars before bucketing. Each bucket entry includes sample_idx, so callers can build stable IDs and load images directly with loader.load_sample(shard_idx, sample_idx).

Why is it fast?

Problem: Standard tar files require sequential reading. To get file #10,000, you must read through files #1-9,999 first.

Solution: The indexed format stores byte offsets and sample metadata in a separate JSON file, enabling:

• HTTP range requests for any file
• True random access over network
• Parallel reads from multiple shards
• Large scale, aspect-bucketed datasets
• No wasted bandwidth

The Rust implementation provides:

• Real parallelism (no Python GIL)
• Zero-copy operations where possible
• Efficient HTTP connection pooling
• Optimized tokio async runtime
• Optional local caching for metadata and shards
• Fast aspect bucketing for image data

Datasets Using This Format

I discovered after creating this library that cheesechaser is the origin of the indexed tar format, which webshart has formalised and extended to include aspect bucketing support.

• NebulaeWis/e621-2024-webp-4Mpixel
• picollect/danbooru2 (subfolder: images)
• webshart/OpenVid-1M-webshart-indices (indices for Dev-Jahn/OpenVid-1M-wds)
• Many picollect image datasets
• Your dataset could be next! See "Creating Indices" above

Requirements

• Python 3.12+
• Linux/macOS/Windows

Roadmap

• image decoding is currently not handled by this library, but it will be added with zero-copy.
• more informative API for caching and other Rust implementation details
• multi-gpu/multi-node friendly dataloader

Projects using webshart

• CaptionFlow uses this library to solve memory use and seek performance issues typical to webdatasets

License

MIT