I'd actually seen a walkthrough of this problem before the interview, which was a double-edged sword.
Start by clarifying requirements and constraints, then design a high-level architecture with core components like API gateway, payment service, and database. Focus on the state machine for payment lifecycle and the confirmation flow, discussing trade-offs and failure handling.
Pro tip: Emphasize idempotency and exactly-once processing to prevent duplicate charges, and discuss how you'd handle partial failures and reconciliation with external providers.
Ask about scale, payment methods, currencies, consistency needs, and integration with external providers. Define functional and non-functional requirements.
Outline core components: API gateway, payment service, database, message queue, and external payment provider integrations. Sketch the flow from client to provider.
Define payment states (e.g., INITIATED, PENDING, AUTHORIZED, CAPTURED, FAILED, REFUNDED) and transitions. Discuss how to enforce valid transitions and handle concurrent updates.
Detail how to confirm payment success/failure: synchronous vs asynchronous callbacks, webhooks, polling, and idempotent processing. Discuss retries and timeouts.
Discuss consistency vs availability, exactly-once vs at-least-once, and strategies for reconciliation, dead-letter queues, and monitoring.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.