Design a live-stream comments system

System Design: Live-stream comments (YouTube Live-style)

Design a system that supports real-time comments for live video streams.

Core features

• Users can post comments to a specific live stream.
• Viewers of that stream see new comments appear in near real time.
• The system must handle “hot” streams with very high QPS (orders of magnitude larger than normal streams).

Requirements to clarify

• Latency target (e.g., p95 end-to-end publish → visible under 500 ms).
• Ordering expectations (strict total order vs best-effort vs per-user ordering).
• Fanout scale (viewers per stream), and whether all viewers must see all comments.
• Moderation features (spam detection, toxic content filtering, blocklists, slow mode).
• Retention/replay (store comments for VOD playback and auditing).

Deliverables

• High-level architecture and major components.
• Data model and APIs.
• How you scale ingestion and fanout, especially for hot streams.
• Justify the choice of streaming/messaging tech (e.g., Kafka vs other MQ/pubsub options).
• Backpressure, rate limiting, and failure handling.