# Design Comments for a Collaborative Whiteboard Design comments for a real-time collaborative whiteboard. The product details below are practice assumptions used to make the prompt self-contained. ### Constraints & Assumptions - A comment thread can be anchored to an object or a board coordinate. - Authorized collaborators can create, reply, edit, delete, resolve, mention, and observe comments. - A board can have thousands of active collaborators and far more occasional viewers. - Comment durability matters more than sub-second presence; live updates should normally arrive within one second. - Object edits, moves, and deletion can occur concurrently with comment actions. ### Clarifying Questions to Ask - What permissions differ among board owners, editors, commenters, and viewers? - Should anchors move with objects, and what happens when an object is deleted? - Are edit history, offline clients, moderation, search, and notifications required? - What ordering guarantee do clients expect within a thread? ### Part 1: Data and API Contract Define comment-thread, reply, anchor, mention, and permission data. Specify write APIs, idempotency, pagination, and conflict behavior. #### Hints - Treat a comment body and its anchor as independently evolving data. #### What This Part Should Cover - Stable identifiers and versions - Permission checks and auditability - Anchor lifecycle ### Part 2: Real-Time Delivery Describe how clients subscribe, catch up after disconnection, handle reordered or duplicate events, and avoid coupling durable writes to slow viewers. #### Hints - A snapshot plus a position in an event stream can simplify recovery. #### What This Part Should Cover - Durable-to-live event path - Ordering and deduplication - Fan-out and backpressure ### Part 3: Reliability and Product Edges Cover concurrent edits, object deletion, mentions, notification retries, abuse, hot boards, and observability. #### Hints - Decide which operations may be eventually consistent. #### What This Part Should Cover - Explicit conflict semantics - Decoupled side effects - Hot-key and failure handling ### What a Strong Answer Covers - A durable, permission-aware comment model - A coherent anchor and deletion policy - Reliable live synchronization with reconnect semantics - Scalable fan-out, asynchronous notifications, and measurable correctness ### Follow-up Questions - How would offline comment creation synchronize? - How would you support text ranges inside editable objects? - What changes for a board viewed by a million people during a launch? - How would legal retention interact with user-visible deletion?