How Reddit Delivers Notifications to Tens of Millions of Users

Reddit's notification system processes millions of posts daily to deliver personalized push notifications to tens of millions of users. The system uses a four-stage pipeline: budgeting determines daily notification limits per user using causal modeling, retrieval narrows content using rule-based and two-tower model approaches, ranking employs deep neural networks with multi-task learning to predict user engagement, and reranking applies business logic for diversity and personalization. The architecture emphasizes real-time processing, prevents notification fatigue, and balances engagement with user experience through careful volume control and relevance optimization.