Real-time personalization service
Design a real-time personalization service that powers multiple surfaces (homepage modules, email, push) for a retail app with 300M users. It must blend long-term taste (built over months) with in-session intent (what the user clicked in the last 5 minutes), serve a personalized response in under 50ms, and let users reset/clear their personalization for privacy.
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.
Clarify5:30 left
Estimate5:30 planned
Design16:30 planned
Deep dive13:30 planned
Failure9:00 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.