How Do Push Notification Recommendations at Reddit Work?
summary
Reddit sends about 150 million recommendation push notifications per day using a semi‑online pipeline: an hourly eligibility job -> notification platform -> Kafka -> ranking platform (consumes events, shares infra with feeds) -> delivery via AWS SNS. Top‑of‑funnel uses subreddit subscriptions, visit proxies, subreddit similarity then two‑tower retrievals; filters use Bloom filters and ML safety/NSFW tags. Heavy ranker is a multitask neural net retrained multiple times a week; a reranking value model can boost fresh posts by ~30% for active users. Daily budgeting predicts activity and disable probability across budgets 1..n.
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chapters (8)
- 0:00Introduction and Reddit overview
- 3:31Mission and system overview
- 9:50Machine learning pipeline overview
- 11:17Candidate generators deep dive
- 17:59Ranking and reranking explained
- 25:00Daily budgeting challenges
- 27:31Real-world vs textbook systems
- 30:20Closing remarks and resources
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