Build a real-time collaborative editor along the lines of Google Docs / Office Online. Many users open the same doc simultaneously, type in shared paragraphs, see each other's cursors live, leave comments, and pick up exactly where they left off after closing the laptop for a week. The hard problems are not "how do I store text" — they are causal ordering of concurrent ops, real conflict-freedom under partial failure, sub-200ms keystroke→broadcast at planet scale, and *durability of edits the user already saw on their other devices*. The load-bearing claim of this design: **server-authoritative single-writer doc-actor, with WAL-before-ack to a per-doc Kafka partition fenced by a Raft-issued lease epoch.** Everything else is variation on that spine.