In-app notifications
Design an in-app notification system for a small product (the bell icon with a list of 'someone commented on your post', 'you were mentioned', etc., and an unread count). A few thousand users. When an event happens, the relevant users should see a new notification and an updated unread badge. Describe the data model and how the unread count stays fast.
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.
Clarify3:00 left
Estimate3:00 planned
Design8:30 planned
Deep dive6:30 planned
Failure4:00 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.