Design a Parallel and Resumable Model Downloader

Design a model downloader that downloads a model artifact in parallel pieces, resumes interrupted work, and makes a complete model available to its consumer.

Constraints & Assumptions

• The source is a compilation of third-party interview reports and names sharded model downloading as a system-design task. It supplies no proprietary architecture, measured performance, or detailed original interface.
• Practice scope: a published model version has an immutable manifest listing its files, expected lengths, and integrity digests. Downloads may use file-level parallelism and byte ranges when the server supports them. The consumer must not mistake partial files for a complete model.
• Network requests and the downloader can fail. A retry should reuse verified work when its artifact binding still matches.
• No artifact size, concurrency limit, bandwidth target, or distribution topology is supplied; explain how these inputs determine the design.

Clarifying Questions to Ask

• Is the source content immutable or versioned, and does it support byte-range requests with version validation?
• Are integrity digests available for complete files, individual chunks, or both?
• Must several processes or machines share a local cache, and who owns an in-progress download?
• Does the consumer require atomic publication of the complete model version or only independent files?

Part 1 — Plan and Download Pieces

Describe the manifest, piece plan, concurrency control, and transfer validation. Explain how to avoid combining ranges from different remote object versions.

What This Part Should Cover

• Stable artifact identity and explicit byte ranges.
• Bounded network, disk, and memory use.
• Verification of status, returned ranges, content length, and object version.

Part 2 — Resume and Publish Safely

Explain persistent progress, restart behavior, integrity checks, and the final transition from partial download to usable model.

What This Part Should Cover

• Durable verified-piece records bound to the correct manifest and object version.
• Safe handling of incomplete writes or stale progress metadata.
• Final validation and atomic visibility at the granularity the consumer requires.