Multi-currency wallet with FX conversion
Design a multi-currency wallet where a user holds balances in many currencies and can spend in a currency they don't hold, triggering an automatic FX conversion at point of sale. 100M users, peak ~9,000 spend authorizations/sec. The authorization must reserve funds at a quoted FX rate, but the rate has a short TTL and the rate provider can be slow or briefly unavailable. The settled amount (days later) may differ slightly from the authorized amount. How do you keep the wallet balance correct across the auth→FX→settle path?
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.
Clarify4:30 left
Estimate4:30 planned
Design13:30 planned
Deep dive10:30 planned
Failure7:00 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.