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Dropbox / Google Drive HLD: Sync That Only Moves What Changed

A file sync system design (Dropbox / Drive): content-addressed chunks with dedup, delta sync that uploads only changed chunks, the metadata-vs-block split, and conflict handling.

By fiveyearsdevJune 14, 20268 min read

"Design Dropbox." "Upload files, download them on another device" is a weekend project — until you notice the trick that makes Dropbox Dropbox: edit one paragraph of a 500 MB document and it syncs in a blink, moving only a few kilobytes. It does that by never thinking in terms of whole files. It thinks in chunks, each named by the hash of its own contents, and it only ever transfers or stores a chunk the system hasn't seen before — across your edits, across your files, and across every other user. Two ideas carry the design: content-addressed chunking (a chunk's identity is its content, so ide…

What’s inside

Let's start nowhere near a computer

Where this exact shape shows up

Step 1 — Functional requirements (sentences first)

Step 2 — Non-functional requirements

Step 3 — Chunks, hashes, and where things live

Step 4 — Delta sync: send hashes, not bytes

Step 5 — Cross-device sync and conflicts

Step 6 — Trade-offs (each one keeping an NFR)

The complete implementation

Step 7 — Only now, the boxes

Step 8 — Scaling the design, one bottleneck at a time

Step 9 — When a piece fails: designing for failure

The interview corner

Where to go from here

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