Design Real-Time Comments with Optimistic Updates

Design a real-time comment system with optimistic posting. When a user submits a comment, append it to the visible list immediately so the user can continue interacting while the backend request completes. ### Constraints & Assumptions The source emphasizes optimistic updates. Define ordering and delivery behavior, and include durable history and failure recovery. Do not assume that rendering a pending comment means the server accepted it. ### Clarifying Questions Can comments be edited or deleted? How are comments ordered? What happens after a timeout with an unknown commit outcome? Must another device see the same conversation? ### What a Strong Answer Covers Temporary and server identities, idempotent creation, acknowledgment reconciliation, live events, pending/error states, and retry behavior. ### Follow-up Questions What if a live event arrives before the POST response? How do you avoid duplicate comments on retry? What if the server rejects content or the user navigates away?