This was three questions in a trench coat.
Start by clarifying requirements and constraints (e.g., scale, consistency, idempotency) before diving into design. Then present a high-level API design with clear endpoints and a normalized database schema, explaining how they support the core operations. Finally, discuss trade-offs and potential optimizations to demonstrate depth.
Pro tip: Emphasize idempotency and transactional integrity for credit additions, purchases, and refunds—this shows you understand real-world financial systems and prevents double-spending or duplicate refunds.
Ask about expected scale, consistency requirements, payment methods, and whether refunds are full or partial. This ensures your design meets the actual needs.
Define RESTful endpoints for adding credit, purchasing items, and processing refunds, including request/response formats and status codes. Consider idempotency keys for safe retries.
Propose tables for users, accounts (with balance), items, purchases, and refunds. Ensure foreign keys and indexes support efficient queries and data integrity.
Walk through how each operation updates the database atomically, using transactions to maintain consistency. Discuss locking or optimistic concurrency control.
Address potential bottlenecks, such as hot accounts, and suggest solutions like sharding, caching, or event sourcing. Mention monitoring and auditing.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.