This question is basically a whole interview compressed into one prompt.
Start by clarifying requirements and scale, then design a data model with accounts and a double-entry ledger for auditability. Focus on atomicity and idempotency for money movements, and discuss concurrency control and consistency trade-offs. Finally, address security and operational concerns.
Pro tip: Emphasize idempotency keys for all money movement APIs to prevent duplicate transactions, and use a double-entry ledger to ensure auditability and consistency. This shows you understand financial systems' critical requirements.
Ask about expected scale (users, transactions per second), consistency requirements, and regulatory constraints. This shapes your design choices.
Propose a double-entry ledger with accounts, transactions, and entries. Include an immutable audit log for all changes.
Design RESTful APIs for account creation, deposit, withdrawal, and transfer. Use idempotency keys to ensure safe retries.
Use database transactions with appropriate isolation levels (e.g., serializable) or optimistic locking to prevent double-spend. Discuss distributed transactions if needed.
Cover authentication, authorization, encryption, and fraud detection. Discuss consistency guarantees (e.g., strong vs eventual) and how to achieve them.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.