Design concurrency-safe shared payment account API

Q: Design concurrency-safe shared payment account API

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.

Q: How do I approach System Design interview questions?

System Design questions require understanding of core concepts and practice. PracHub provides solutions with explanations to help you master system design interviews.

Q: What difficulty level is this interview question?

This is a hard difficulty System Design question, commonly asked during Technical Screen rounds at Tesla.

Q: What role is this question designed for?

This question is commonly asked for Software Engineer candidates at Tesla during technical interviews.

Question

Prevent Double-Spending When Two Users Pay Simultaneously from the Same Account

Context

You are designing a payments service where multiple clients may initiate payments at the same time against a shared account. The system must maintain a correct balance and prevent double-spending even under failures, retries, and high concurrency.

Task

Design the API and backend to ensure correctness and durability. Cover:

Request/response design, including status codes and error semantics.
Idempotency keys and how they are stored/enforced.
Concurrency control strategy (optimistic vs. pessimistic), transactions, and isolation levels.
Consistency guarantees to clients and internally.
Retry strategy and failure handling.
Monitoring, alerting, and observability.
Scaling to very high concurrency and massive request volumes.

Assume a typical service + database architecture. You may make minimal, explicit assumptions as needed.

Design concurrency-safe shared payment account API

Quick Overview

Prevent Double-Spending When Two Users Pay Simultaneously from the Same Account

Context

Task

Solution

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