This is basically one giant question with like seven sub-questions baked in.
Start by clarifying requirements and scale, then propose a hybrid push-pull architecture using WebSockets for real-time delivery and a pub/sub system for fan-out. Walk through the write path (comment ingestion, ordering, deduplication) and read path (client connection, message delivery, back-pressure), emphasizing trade-offs and scalability to millions of viewers.
Pro tip: Demonstrate awareness of the 'thundering herd' problem and propose solutions like sharded pub/sub channels or edge caching to avoid overwhelming the origin. Also, mention the importance of idempotency and client-side deduplication to handle at-least-once delivery.
Ask questions to understand expected read/write ratio, latency requirements, ordering guarantees, and scale (e.g., millions of concurrent viewers, thousands of comments per second).
Choose WebSockets for bidirectional real-time communication, with fallback to long-polling. Discuss connection management, heartbeats, and reconnection strategies.
Use a distributed pub/sub system (e.g., Kafka, Redis Pub/Sub) to fan out comments to all subscribers. Ensure ordering via per-post sequence numbers and deduplication using unique comment IDs.
Write path: ingest comments via API, validate, assign sequence numbers, persist to database, and publish to pub/sub. Read path: clients subscribe to post-specific channels, receive comments via WebSocket, and apply back-pressure if needed.
Scale horizontally with sharded pub/sub channels, use edge servers/CDNs for fan-out, implement back-pressure by dropping or batching messages, and monitor system health.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.