Your question is Notification System for Mobile Users. 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).
How would you design a notification system for a mobile app with millions of active users?
Discuss the end-to-end architecture, including notification eligibility, candidate generation, ranking, delivery, feedback collection, and failure handling. Explain how you would separate online and batch processing, choose models, meet latency and availability goals, and evaluate personalization without causing notification fatigue.