docs/retrieval.md

Retrieving and verifying files

The catalog holds no file content. To use a file, download it from the address in its pointer and check it against the size and hashes the pointer gives. The fields are described in fields.md.

Steps

1. Choose an address: uri, or one from alt_uris, which serve the same bytes. s3:// addresses usually have a public https address in alt_uris. hf://datasets/<repo>@<revision>/<path> stands for https://huggingface.co/datasets/<repo>/resolve/<revision>/<path>.
2. If access.needs_auth says that a login, a guestbook or a form is needed, follow the source's procedure first.
3. If the pointer has range, request only those bytes with the HTTP header Range: bytes=<offset>-<offset + length - 1>. The file is that byte range of uri, for example one record inside a WARC file, and size_bytes equals length.
4. Compare the number of bytes with size_bytes, unless it is null (the source does not publish the size).
5. Compute the hashes named in hashes and compare every value; hash_scope says what they cover (below). Treat a mismatch as a failure; do not substitute another file.
6. If hashes is empty (hash_source is computed_on_fetch), the source publishes no hash: compute your own after download and keep it with the file.

What the hashes cover (hash_scope)

• file (the default when the field is absent): the bytes you downloaded, that is the whole file, or the byte range when the pointer has range.
• http_payload: the range is one gzip-compressed WARC record. Decompress it, remove the WARC and HTTP headers, undo chunked transfer encoding, and hash the HTTP response body. size_bytes stays the compressed length of the record.
• api_body: uri returns JSON from an API. Hash the UTF-8 bytes of the field named in source_meta.api_body_field (a dotted path in which numbers index lists; items.0.body when absent), not the whole response.

Files inside archives

A pointer with parent_pointer_id and member_path is a file inside an archive that has its own pointer: retrieve and verify the archive, then extract member_path.

Source notes and the manifests themselves

Each collection.json adds notes for its source under fetch and notes. Its shards list gives the sha256 of every part file of the batch, so the downloaded manifests can be checked too.