This is one of those questions where you can tell within the first two minutes you're not going to cover everything.
Start by clarifying requirements and scale, then walk through the high-level architecture and data model, emphasizing idempotency, exactly-once semantics, and strong consistency. Dive into the ledger design, PSP integration, async settlement, and monitoring, while addressing PCI compliance and fraud screening. Conclude with trade-offs and failure handling.
Pro tip: Emphasize idempotency keys and exactly-once semantics early, as they are critical for payment systems and often overlooked. Also, discuss how you'd handle partial failures and reconciliation with PSPs to demonstrate real-world experience.
Ask questions to understand transaction volume, latency requirements, supported currencies, and regulatory constraints. Confirm the need for exactly-once semantics and strong consistency.
Outline the main components: API gateway, payment service, ledger, fraud service, PSP integration, and async workers. Explain how they interact to handle authorize, capture, and refund flows.
Design a double-entry ledger with accounts, transactions, and entries. Discuss how to achieve strong consistency using ACID transactions or distributed consensus, and how to support multi-currency with exchange rates.
Explain how to use idempotency keys for API requests and deduplication in the ledger. Describe how to ensure exactly-once processing of user-visible operations using unique constraints and transactional outbox patterns.
Detail how to integrate with PSPs and card networks, handle async settlement and reconciliation, and monitor system health with metrics, logging, and alerting. Address PCI compliance and fraud screening integration.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.