Welcome to your interview.
The question is on your right: Real-Time Notifications System. Take a moment with it first.
Talk your thinking through with me if you like - when you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes). Discussion and graded submissions share your five interviewer interactions, so spend them well.
Design a real-time notification service for a digital library. Users can subscribe to notifications for specific books, and the system must alert all subscribed users via in-app alerts and emails when a book is returned.