System Design: Payment System

System DesignOpenAILast reported August 2026High Frequency
Reported
51× across candidate reports
First seen
May 2025
Last reported
August 2026
Reported outcome
mixed

Problem Overview

Design a payment processing system. The system models offline card payment (e.g., coffee shop / POS terminal), NOT an online e-commerce gateway like Stripe. Core flows: 1. Hold / Authorization (synchronous): A POS machine sends a hold/authorization request → your payment service calls the downstream credit-card provider (bank / card…

  • 10 candidate-reported follow-ups — the exact probes interviewers asked, with the trigger for each
  • The rest of the problem statement — full requirements, constraints, and edge cases
  • Approach and trade-offs — what passing candidates did, and the mistakes that sink people
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Verified Source
Every question is reconstructed from multiple independent candidate reports. Verbatim follow-ups, not invented ones.
Codex Fact-Checked
Technical claims, formulas, and scale numbers are reviewed against primary sources.
Interviewer Follow-ups
The exact follow-ups reported by candidates, with the trigger that prompts each one — plus the mistakes that sink people.
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