System designMediumsd-g545

Subject Qr ticketingLevel Entry–Mid~35 minCommon in Networking & APIs interviewsIndustries Software development, Technology

Question

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.

Learn the concepts

Narrate your design

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Run or narrate your approach, then ask the coach.