System design interview question
"Design Reddit. Users post to communities, other users vote and reply, replies nest arbitrarily deep, and each community's front page is ranked by a hotness score that decays with time. 500M monthly users."
hierarchical data + vote aggregation + time-decayed ranking. Three specific traps live here: (1) storing and reading a deep comment tree efficiently, (2) counting votes under contention without a hot-row bottleneck, and (3) a ranking score that changes as time passes even when no writes occur.
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