Live-comments during events
Design the live-comments / live-chat system for a streaming platform during a major event: a single stream can have 5M concurrent viewers, comment write bursts of 100k+ messages/sec, and viewers should see a lively but not overwhelming stream of comments within ~1–2s. Requirements: spam/abuse must be filterable in near-real-time, late joiners need recent context, and one mega-stream must not degrade other streams. Walk through ingestion, fan-out, and what you intentionally drop.
What a strong answer looks like
Clarify scale and constraints first. Propose a clean component breakdown, then go deep on the hard parts (data model, bottlenecks, consistency, failure modes) and name the trade-offs you are making.
Clarify5:00 left
Estimate5:00 planned
Design15:00 planned
Deep dive12:00 planned
Failure8:00 planned
Which questions mattered is sealed until you submit. Telling you now would just be handing over the edge cases.
Run or narrate your approach, then ask the coach.