System designMediumsd-g271

Subject Live commentsLevel Mid–Senior~45 minCommon in Networking & APIs · Distributed systems interviewsIndustries Technology

Question

Design the live-comment / live-chat system for a streamed event (sports final, product launch) with up to 5 million concurrent viewers in one room. Viewers post short comments; everyone sees a fast-moving feed. At peak there are 100k comments/min inbound but no human can read 5M/min, so the displayed stream must be sampled. Comments need light moderation (spam/abuse) before display, and the experience must feel live (<2 s). How do you ingest, moderate, and fan out at this scale?

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.

Learn the concepts

Narrate your design

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Run or narrate your approach, then ask the coach.