Design a Synchronized Live Comment Overlay

Design a chat-like comment service whose messages appear as a live overlay synchronized to a video or event timeline. Viewers can publish short comments at their current playback position and receive relevant comments while watching live or replaying later.

Cover publish and read APIs, message ordering, real-time delivery, storage, timeline queries, fanout, moderation, rate limits, and client synchronization. State what ordering and latency guarantees are realistic when viewers and servers have different clocks.

Constraints & Assumptions

• A popular event may have a very large concurrent audience and bursty comment traffic.
• Clients may pause, seek, reconnect, or watch at different playback positions.
• The overlay should remain usable under heavy load rather than rendering every accepted comment.
• Comments require moderation and abuse controls.

Clarifying Questions to Ask

• Is the canonical timeline live wall-clock time, media position, or both?
• Must every viewer see the same comments in the same order?
• What latency, retention, replay, and regional requirements apply?
• Are likes, replies, filtering preferences, or creator moderation in scope?