This question ate me alive for the first ten minutes.
Start by clarifying requirements and constraints (e.g., scale, consistency, currencies, providers), then sketch a high-level architecture covering order lifecycle, payment integration, and ledger. Dive into critical components like idempotency, double-entry accounting, and scaling strategies, discussing trade-offs and failure handling.
Pro tip: Emphasize idempotency and exactly-once processing at every step, as payment systems must handle retries and duplicates without double-charging or corrupting the ledger. Also, discuss how you'd monitor and reconcile with external providers to detect discrepancies.
Ask about expected transaction volume, latency, consistency needs, supported currencies, payment providers, and regulatory constraints. Define the order lifecycle states and what 'without data loss or inconsistency' means in practice.
Outline core services: Order Service, Payment Service, Ledger Service, and Notification Service. Describe how they interact via APIs and events, and how data flows through the system.
Detail the order state machine, payment provider integration (with idempotency keys and webhooks), double-entry ledger schema, and multi-currency handling (e.g., storing amounts in minor units with currency codes).
Discuss partitioning (e.g., by user or order ID), database choices (e.g., ACID vs. BASE), caching, and asynchronous processing. Explain how to achieve exactly-once semantics using idempotency and transactional outbox patterns.
Cover retry mechanisms, circuit breakers, reconciliation jobs, and monitoring. Discuss trade-offs between consistency and availability, and how to ensure data integrity under failures.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.