System designHardsd-g268

Subject Notification systemsLevel Mid–Senior~45 minCommon in Networking & APIs interviewsIndustries Technology, Software development

Question

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.

Learn the concepts

Narrate your design

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Run or narrate your approach, then ask the coach.