Build a file-sync and storage service like Dropbox or Google Drive. A user drops a file into a synced folder on one device; it must appear, byte-identical, on every other device they own and on every device of everyone they've shared the folder with — quickly, durably, and without clobbering a concurrent edit. The product is two systems wearing one trench coat: a content plane that stores the bytes (immutable, content-addressed, deduplicated 4 MB blocks) and a metadata plane that stores the filesystem (a journaled, ordered, strongly-consistent index of which blocks compose which file). Clients sync by following a monotonic cursor over a long-poll loop, and divergent edits are resolved by keeping both — the "conflicted copy."

This canonical models the main sync flow: upload (chunk → dedup → store → commit), download, receive-a-remote-change, and conflict. It deliberately omits the adjacent sprawl (preview/thumbnail rendering, full-text search, sharing-invite UX, billing) to keep the spine sharp.