Multi-device push notifications
Design a multi-device push-notification system for a messaging app with 200M users, where a user has on average 3 active devices (phone, tablet, laptop) plus the web. When a message arrives, exactly one 'badge bump' should land per device, but if the user reads the message on any device the others should suppress or clear the notification. Pushes go through APNs/FCM (best-effort, can duplicate or drop). How do you deliver per-device pushes while deduplicating across devices and races?
What a strong answer looks like
Clarify scale and constraints first. Propose a clean component breakdown, then go deep on the hard parts (data model, bottlenecks, consistency, failure modes) and name the trade-offs you are making.
Clarify5:00 left
Estimate5:00 planned
Design15:00 planned
Deep dive12:00 planned
Failure8:00 planned
Which questions mattered is sealed until you submit. Telling you now would just be handing over the edge cases.
Run or narrate your approach, then ask the coach.