Design concurrency-safe shared payment account API
Company: Tesla
Role: Software Engineer
Category: System Design
Difficulty: hard
Interview Round: Technical Screen
Scenario: Two users share one online payment account and submit payments at the same time. From an API and real-world engineering perspective, how would you ensure the balance remains correct and prevent double-spending? Specify request/response design, idempotency keys, concurrency control (e.g., optimistic or pessimistic locking, transactions), consistency guarantees, retries, and monitoring. Additionally, how would you scale the service to handle very high concurrency and massive request volumes?
Quick Answer: This question evaluates a candidate's ability to design concurrency controls, idempotency handling, transactional guarantees, retry and failure semantics, and observability for a payments API to ensure correctness and durability under high concurrency and failures.