System designHardsd-g384

Subject WalletLevel Senior–Staff~45 minCommon in Distributed systems interviewsIndustries Technology, Software development

Question

Design a transit-fare wallet where riders tap at gates that are frequently offline (subway, no connectivity), so a tap must be authorized at the gate in <300ms WITHOUT a round trip to the server, then settled later when the gate reconnects. ~50M cards, peak ~20,000 taps/sec across gates. The hard problem: a rider with a near-zero balance could tap at multiple offline gates and overspend (double-spend). How do you authorize offline yet keep the wallet balance correct and bound the loss?

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.