Design a live video comments system

System Design: Live Video Comments (FB Live-style)

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

Core user experience

• Viewers watching the same live stream should see new comments appear in near real-time.
• Users can post comments; everyone watching should receive them quickly.
• Comments are ordered in a sensible way (define your ordering semantics).

Functional requirements

• Post a comment to a specific live stream.
• Fetch recent comments when a viewer joins (initial load + pagination/scrollback).
• Real-time delivery of new comments to connected viewers.
• Basic moderation support (e.g., remove a comment; block a user; optionally profanity/spam filtering).

Non-functional requirements

• High fan-out: a single stream can have very large concurrent audiences.
• Low latency for real-time delivery (define a target, e.g., p95 < 1s).
• Reliability: comments should not be silently lost; define durability/consistency tradeoffs.
• Handle abuse: rate limiting, spam, and reconnect storms.

What to cover

• APIs (HTTP/WebSocket) and data model.
• High-level architecture and key components.
• Scaling strategy (partitioning/sharding, fan-out model).
• Consistency/order guarantees and how you implement them.
• Storage, caching, and retention.
• Moderation and deletion propagation.
• Key metrics and failure modes.