Your question is Real-Time Notification System for Millions. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
Design a real-time notification system capable of engaging millions of users without degrading service reliability.
Clarify the product and reliability requirements before proposing an end-to-end ML system. Cover event ingestion, notification candidate generation, ranking and re-ranking, delivery orchestration, and user controls. Explain how you would separate online decisions from batch computation, evaluate engagement without encouraging notification spam, and protect core services during traffic spikes or model failures.
Address scale assumptions, latency and availability targets, data freshness, model training and serving, observability, experimentation, feature drift, training-serving skew, and rollback or fallback behavior.