S07 · interview simulation

System design: card-fraud scoring inside a 50 ms budget

People, data and outputs in this scenario are simulated. Each stage is recorded when you commit; the reveal comes after.

5stages
45 minabout
0stages committed

The brief

Simulated interviewer: "An online retailer (simulated) authorises card payments. Our fraud service sits on the checkout path and gets 50 ms at p99. Peak load is 1,200 transactions per second. Confirmed fraud labels (chargebacks) arrive up to 90 days later. Design the scoring system." All numbers, costs and people are simulated or hypothetical.

Stage 1 of 5

Before designing, you ask questions.

Which questions matter most? Select all that apply.

Which actions can the score trigger (approve, step-up, decline, manual review), and what does each cost?What happens today when the fraud service is slow or down?How are labels defined and when do they arrive (chargebacks, confirmed reports, friendly fraud)?Can we have 500 ms instead of 50 for the scoring call, so we can use a heavier model?