Event ticket check-in
Design a simple event-ticketing check-in system. People buy tickets online and get a unique code (rendered as a QR). At the venue, staff scan codes to admit guests; each ticket must be valid for exactly one entry, and scanning the same code twice should be rejected. There are a few thousand tickets and a few scanning stations. Cover the data model, how check-in works, and how you ensure a ticket can't be used twice.
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.
Clarify4:00 left
Estimate4:00 planned
Design11:30 planned
Deep dive9:30 planned
Failure6:00 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.