Design a real-time commenting system for a collaborative canvas or document. Users should see new comments quickly, receive updates on threads, and recover conversation history when they reopen the document. <details> <summary>Hint 1</summary> Start by naming the core entities, constraints, and success criteria. </details> <details> <summary>Hint 2</summary> Make the trade-offs explicit before going deep on implementation details. </details> ### Constraints & Assumptions - Users collaborate on shared documents or canvases. - Comments can be created, updated, resolved, and loaded later. - Real-time delivery is important but durable history is required. - The design should support many users viewing the same popular document. ### Clarifying Questions to Ask - Do comments attach to document positions, objects, or free-form coordinates? - Are edits and deletes required? - What latency target defines real-time? - Should offline users receive notifications? - How are permissions inherited from the document? ### What a Strong Answer Covers ```premium-lock What a Strong Answer Covers ``` ### Follow-up Questions - How would you handle a user reconnecting after missing updates? - How would you scale one document with thousands of viewers? - How would you support comment anchors that move? - What consistency guarantee do users need after posting?