The transfer part is where things get interesting and also where I probably spent too long.
Start by clarifying requirements and scale, then design a RESTful API with clear endpoints for account creation, deposit, and transfer. Focus on atomicity and concurrency using database transactions with appropriate isolation levels or optimistic locking, and discuss failure handling with idempotency and retries.
Pro tip: Demonstrate maturity by discussing idempotency keys for safe retries and the trade-offs between pessimistic and optimistic locking in high-concurrency scenarios.
Ask about expected throughput, consistency requirements, and failure scenarios to tailor the design. Confirm whether eventual consistency is acceptable or strong consistency is needed.
Define RESTful endpoints: POST /accounts for creation, POST /accounts/{id}/deposits for deposits, and POST /transfers for transfers. Specify request/response formats and status codes.
Use database transactions with ACID guarantees. For transfers, lock both accounts in a consistent order to prevent deadlocks, or use optimistic concurrency with versioning.
Implement idempotency keys for deposit and transfer requests to safely handle retries. Use retry logic with exponential backoff for transient failures and ensure proper error responses.
Compare pessimistic vs optimistic locking, and consider partitioning or sharding for scale. Mention monitoring, logging, and audit trails for financial operations.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.