Nested comment threads
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.
Clarify4:00 left
Estimate4:00 planned
Design11:30 planned
Deep dive9:30 planned
Failure6:00 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.