Design File Storage and Sync

Requirements

A file storage and sync service keeps your files in the cloud and mirrors them across your devices. Edit a file on your laptop and it appears on your phone. Dropbox, Google Drive and OneDrive work this way.

Functional:

• Upload, download and delete files and folders from web, desktop and mobile.
• Sync: a change on one device reaches the user's other devices automatically.
• Share files and folders with other users.
• Version history: view and restore earlier versions.

Non-functional:

• Reliable: never lose a file. This is the most important requirement.
• Efficient: editing a small part of a large file must not re-upload the whole file.
• Fast sync: other devices learn about a change within seconds.
• Consistent metadata: a user must not see a half-uploaded file or two different folder trees.