7. Design a File Storage Service

The brief

Design a file storage and sharing service for web and desktop clients. Separate file bytes from metadata and permissions, and explain resumable uploads, downloads and version creation. Define what a user sees after an interrupted upload or conflicting edits from two devices.

• Support 1 million users uploading 100 TB/day in total; files range from 1 KB to 10 GB.
• Networks can disconnect mid-upload and desktop devices may edit offline.
• Users can share a folder with named collaborators and revoke access later.

Constraints

Supported file size≥ 10 GB

Declare the maximum supported single-file size.

Atomic file visibility

Do not publish metadata pointing to an incomplete or missing file version.

Revocation contract

Define authorization for bytes and metadata, including the lifetime of already-issued download links.

What to cover

1. 01

   Metadata and versions

   Define ownership, folders, permissions, version identity and conflict behavior.

2. 02

   Transfer protocol

   Describe multipart upload, retry verification, completion and download access.

3. 03

   Storage and cleanup

   Estimate daily and retained byte volume; handle unreferenced chunks and deleted versions.

4. 04

   Failure walkthrough

   Trace a dropped upload, simultaneous edits and access revocation; compare two conflict policies.

Worked designs

Explore the architecture and decisions, then build on an example with Coach.

Official design

Design a File Sync Service like Dropbox

Sync files up to 50 GB across devices: presigned multipart uploads, blocks named by hash, a journal per namespace with WebSocket hints, and cold tiers: one-hour boards for junior, senior and staff, with the theory behind them.

Explore the design

Review rubric

AI feedback uses these criteria. Scores are practice feedback.