Reddit's core: submit, vote, comment. Ranking algorithms + community moderation. Scale: 500M+ MAU. Cassandra + PostgreSQL + Redis foundation.

UserWeb / MobileRanking Enginehot / new / topVote ServiceComment TreeAutoMod / SpamNotificationsCassandraposts + commentsPostgreSQLusers + accountsRedisvote cache + hot listsPush Notif Store
Reddit: ranking + votes + comments backed by Cassandra + PostgreSQL + Redis
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Hot algorithm

score = log(votes) + age_penalty. Time-decay + log-scaled votes. Prevents old high-vote posts from dominating.

Hot algorithm

score = log(votes) + age_penalty. Time-decay + log-scaled votes. Prevents old high-vote posts from dominating.

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Comment tree

Threaded comments. Ranked by best/top/controversial. Materialized as tree in Cassandra. Wide row per post.

Vote fuzzing

Displayed vote counts fuzzed to defeat brigading bots. Real counts stored + ranked accurately internally.

AutoMod + moderation

Per-subreddit rules + shared spam filters. AutoMod runs Python-like rules. Human moderators review edge cases.

Redis for hot

Hot lists per subreddit cached in Redis. Recomputed periodically. Reads served without hitting Cassandra.

Ranking + votes + comments + moderation + caching. Reddit's stack is boring by design — reliable at community scale.