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

You fix a single typo in a 300-page thesis — one character in a 40 MB file — and the little sync icon spins for half a heartbeat and settles. On the train, on hotel Wi-Fi, on your phone’s last bar of signal, the same thing: the change lands on your other devices in seconds. That’s the trick that makes Dropbox Dropbox, and it isn’t "upload the file fast." It’s that the system never thinks in whole files. It thinks in chunks, each one named by the hash of its own bytes, and it only ever moves a chunk it hasn’t seen before — across your edits, across your files, across every other user on Earth. …