Dropbox (File Sync Service)

Phase 1 — Clarify & Scope

Interviewer: Design Dropbox — a file sync service. What do you want to clarify?

Candidate: Key questions:

1. What's the core feature set — sync across devices, share files/folders, collaboration?
2. What's the scale — users, files, total storage?
3. File size distribution — mostly small files (docs) or large files (videos)?
4. What does "sync" mean — immediate (like Dropbox desktop) or eventual?
5. Conflict resolution — what happens when two devices edit the same file offline?

Interviewer: File sync across multiple devices, file sharing with links. 500 million users, 1 billion files. Storage: average file size 1 MB → 1 PB total. Sync latency: changes should appear on other devices within 30 seconds. Conflict: create a "conflicted copy" (Dropbox's approach — don't silently overwrite).

Candidate: Let me size this:

$\text{Upload rate} = 500\text{M users} \times 2 \text{ file changes/day} / 86{,}400 \approx 11{,}574 \text{ uploads/sec}$

$\text{Total storage} = 10^9 \text{ files} \times 1\text{ MB avg} = 1 \text{ PB}$

At 30-second sync SLA: changes must propagate within 30 seconds of upload completion.

$\text{Delta sync bandwidth saving} \approx 80\% \text{ (most edits are small changes to large files)}$

Interviewer: What are the core requirements?

Candidate:

• Upload files, sync to all user's devices within 30 seconds
• Delta sync — only upload/download changed blocks, not the whole file
• File deduplication — same file uploaded by different users stored once
• Conflict detection — detect when two devices modified the same file offline
• File sharing — generate shareable links with optional expiry…