This one kept expanding every time I thought I was done.
Start by clarifying requirements and constraints, then walk through the design from API contract to backend concurrency control, emphasizing idempotency and consistency. Structure your answer around the key areas: request/response design, idempotency, concurrency control, consistency guarantees, retries, monitoring, and scaling. Use concrete examples and trade-offs to demonstrate depth.
Pro tip: Emphasize idempotency keys and database transactions with row-level locking as the foundation, but also discuss how you'd handle distributed scenarios with optimistic concurrency and eventual consistency where appropriate. Show awareness of Tesla's scale by mentioning sharding and async processing.
Ask about expected load, consistency requirements (strong vs eventual), and whether the account is a single ledger or distributed. Confirm that double-spending must be strictly prevented.
Define endpoints (e.g., POST /payments) with required idempotency key in header or body. Specify response codes (201 Created, 200 OK for duplicate, 409 Conflict for insufficient funds) and include a unique payment ID.
Use database transactions with row-level locking (SELECT FOR UPDATE) or optimistic concurrency (version column) to serialize balance updates. Ensure atomicity: check balance, deduct, record payment in one transaction.
Store idempotency keys with request hash and response for a TTL. On retry, return cached response if key exists and request matches; otherwise process. Use exponential backoff with jitter for client retries.
Monitor transaction latency, error rates, and idempotency key collisions. Scale via sharding by account ID, read replicas for balance queries, and async processing for non-critical updates. Consider distributed locks (e.g., Redis) only if necessary.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.