I'd prepped using a YouTube video on this exact topic, which gave me false confidence.
Start by clarifying requirements (functional and non-functional) and scope, then propose a high-level architecture with core components like API gateway, payment service, ledger, and integrations. Dive into scaling and reliability strategies such as sharding, idempotency, and fault tolerance, and discuss trade-offs.
Pro tip: Emphasize idempotency and exactly-once processing as critical for payments, and mention how you'd handle reconciliation and audit trails to ensure financial integrity.
Ask about expected scale (TPS, data volume), supported payment methods, compliance needs (PCI, PSD2), and consistency vs. availability trade-offs.
Outline main components: API gateway, payment service, ledger, fraud detection, and external integrations (banks, card networks). Describe data flow from client to payment completion.
Design a ledger with double-entry accounting, using a distributed database (e.g., CockroachDB) for strong consistency and horizontal scaling. Discuss sharding by merchant or region.
Explain scaling strategies: horizontal scaling of stateless services, partitioning, caching, and async processing. For reliability: idempotency keys, retries with exponential backoff, circuit breakers, and multi-region deployment.
Discuss trade-offs like consistency vs. latency, and how to monitor with metrics, logging, and tracing. Mention reconciliation jobs and alerting for failures.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.