System designMediumsd-g567

Subject System designLevel Mid–Senior~35 minCommon in Storage & CDN · Networking & APIs · Concurrency · Algorithms & data structures interviewsIndustries Software development, Technology

Question

Design a comment + threading system for a news/discussion site with ~10M daily readers. Threads can nest arbitrarily deep, popular articles get 50K+ comments, and the read:write ratio is roughly 100:1. Requirements: load a comment thread fast (top-level + a few replies each), 'load more replies' pagination, sort by top/newest, soft-delete that keeps replies visible, and live updates as new comments arrive. Most reads hit a small set of hot articles.

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.