This is the kind of question where you can go in a dozen directions and pick the wrong one fast.
Start by clarifying the scope and requirements of the payment processing system, such as supported payment methods, expected transaction volume, and compliance needs. Then outline a high-level architecture covering key components like payment gateway, processor, and ledger, and dive into critical aspects such as idempotency, consistency, and security. Finally, discuss trade-offs and potential bottlenecks, and propose scaling strategies.
Pro tip: Emphasize idempotency and exactly-once processing early, as these are critical in payment systems to prevent duplicate charges and ensure reliability. Also, mention how you would handle failures and retries gracefully, showing you understand real-world payment challenges.
Ask questions to understand the system's scope: expected traffic, payment methods (credit cards, digital wallets), currencies, compliance (PCI DSS), and integration points (banks, fraud detection).
Sketch the main components: API gateway, payment service, processor integration, ledger, and notification service. Explain the flow from payment initiation to settlement.
Discuss idempotency (using idempotency keys), consistency (ACID vs. BASE, distributed transactions), and security (encryption, tokenization). Address how to handle failures and retries.
Explain how to scale horizontally, use queues for async processing, implement circuit breakers, and ensure high availability with redundancy across regions.
Discuss trade-offs between consistency and availability, latency vs. durability, and potential extensions like support for multiple payment providers or reconciliation.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.