I'd actually seen this one before, which made it worse when I fumbled it.
Start by clarifying requirements and constraints, then outline a high-level architecture covering core components like payment processing, ledger, and integrations. Dive into key design decisions such as idempotency, consistency, and scalability, and discuss trade-offs. Conclude by addressing failure handling, security, and monitoring.
Pro tip: Emphasize idempotency and exactly-once processing, as payment systems must handle retries and network failures gracefully. Also, discuss how you would ensure data consistency across services, possibly using distributed transactions or eventual consistency with compensation.
Ask questions to understand scope: expected transaction volume, latency requirements, supported payment methods, regulatory compliance, and integration points.
Sketch the main components: API gateway, payment service, ledger, fraud detection, and external payment provider integrations. Define data flow and storage.
Discuss critical aspects like idempotency, consistency models, scalability, and security. Explain how you would handle failures and retries.
Compare design choices (e.g., synchronous vs asynchronous processing, SQL vs NoSQL for ledger) and justify your decisions based on requirements.
Cover monitoring, alerting, logging, and deployment strategies. Mention how you would test and roll out changes safely.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.