Start by clarifying requirements and scope, then define core classes and interfaces with clear responsibilities. Walk through the main workflows (charge, refund, billing, payment) using sequence diagrams or step-by-step descriptions, and finally discuss extensibility for multi-currency and fraud detection, emphasizing trade-offs and Amazon's leadership principles.
Pro tip: Emphasize idempotency and consistency in financial operations, and proactively discuss how you'd handle failures and retries—this shows you understand real-world payment systems and Amazon's customer obsession.
Ask questions to understand expected scale, consistency needs, and key features (e.g., multi-currency, fraud detection). Define functional and non-functional requirements to guide the design.
Identify main entities like Account, Card, Transaction, BillingCycle, and Payment. Define interfaces for Chargeable, Refundable, and PaymentProcessor to promote loose coupling and extensibility.
Walk through charge, refund, billing, and payment workflows step-by-step. Highlight how classes interact, and discuss error handling, idempotency, and transaction boundaries.
Explain how to extend the design for multi-currency (e.g., Currency abstraction, exchange rates) and fraud detection (e.g., pluggable FraudDetector interface, event-driven checks).
Discuss trade-offs like consistency vs. availability, synchronous vs. asynchronous processing, and how to scale horizontally. Mention monitoring, logging, and metrics.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.