System designHardsd-g011

Subject Live commentsLevel Senior–Staff~45 minCommon in ML systems · Distributed systems interviewsIndustries Technology, Software development

Question

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.

Learn the concepts

Narrate your design

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Run or narrate your approach, then ask the coach.