Stripe Backend Engineer Interview Questions

I would require a client-generated idempotency key and store the first successful request result against that key. If the same key is retried, the API should return the original outcome instead of creating a duplicate charge. The important trade-off is balancing strict deduplication with safe retries across timeouts, partial failures, and network ambiguity.

I start with duplicate requests, delayed messages, out-of-order events, partial writes, and external dependency failures. Payments systems must assume retries will happen, so the backend has to remain correct even when the same operation is delivered more than once. I would also think about reconciliation because not every inconsistency can be prevented in real time.

I would define explicit states such as initiated, pending, authorized, captured, failed, reversed, and refunded, with valid transitions between them. A clear state machine makes the system easier to reason about, audit, and recover after failures. It also prevents illegal transitions that could otherwise create accounting inconsistencies.

Double-entry accounting records every movement as balanced debits and credits, which makes invariants easier to verify. It gives you a reliable way to reconcile balances, trace money movement, and detect corruption or missing entries. For payments, that auditability is often as important as raw correctness.

I would treat webhook processing as at-least-once and make the consumer idempotent. That means storing event identifiers, checking whether the event was already processed, and designing side effects so duplicate delivery does not cause duplicate business actions. I would also log delivery attempts so failures are observable and recoverable.