intermediate · reliability

The Retried Payment

A client sends a payment request, times out, and retries. How should the API behave?

Practical scenario

A mobile app on a flaky connection calls POST /payments, gets no response in 10 seconds, and its HTTP library retries automatically.

What it tests

  • • Whether the candidate sees that a timeout is ambiguous, not a failure
  • • Idempotency-key mechanics: storage, replay, scope
  • • Concurrency: what happens when the retry arrives while the original is still running