Reddit / Quora / Stack Overflow — Notes

Functional

• Post / question with title, body, tags, media.
• Threaded comments / answers (deeply nested).
• Voting (up/down or accept).
• Listings: hot, new, top, controversial.
• Per-user feed, notifications, mentions.
• Moderation (community + global).

Non-functional

• Read-heavy 50:1; comment voting bursts.
• p99 listing < 200 ms.
• Eventual consistency on vote counts OK; user must see own vote immediately.

Capacity (Reddit-scale)

• 50M+ DAU, 1B+ posts/comments total.
• ~2B page views/day → ~25k/s avg.
• Storage: text ~2 KB avg × 1B posts = 2 TB + comments many TB.

Schema highlights

• posts(id PK, subreddit_id, author, ts, title, body, score, num_comments)
• comments(id, post_id, parent_id, path, author, body, score) materialized path
• votes(user_id, target_id, value, ts) Cassandra; counters in Redis
• subreddits(id, name, type, rules)

Trade-offs

• Materialized-path comments simplify thread queries; cost: rewrite path on move.
• Eventually consistent vote counters allow huge scale; show user-personal vote from local cache.
• Personalized vs algorithm-only: SO uses simple formulas (signals trust); Quora goes full ML.
• Sharding by subreddit localizes hot subs but can hotspot one shard.

Refs

• Reddit "Hot" ranking blog post (Amir Salihefendic).
• Quora engineering posts (Webnode, Quora Search).
• Stack Overflow architecture (Nick Craver's blog series).
• ByteByteGo "Design Reddit".