Scaling

Design a file-sync service like Dropbox: a folder of files lives on many devices, and when a user edits a file on one device, the change appears on every other device within seconds.

In scope: syncing changes across devices, uploading and downloading files efficiently, transferring only what changed, and resolving conflicting edits. Real-time character-by-character collaborative editing, full-text search over file contents, and sharing permissions are out of scope.