How Reddit Delivers Notifications to Tens of Millions of Users

Reddit's notification system balances engaging users without overwhelming them, using a sophisticated four-stage pipeline that processes millions of posts daily. The system determines personalized notification limits through causal modeling, retrieves potential content through subscriptions and recommendation models, ranks posts using deep learning that predicts multiple engagement behaviors, and finally reranks candidates based on product strategy and user experience goals. This infrastructure delivers timely, relevant notifications to tens of millions of users while carefully preventing notification fatigue.