Webhooks
webhooks
System Design
Design a Payment System (Stripe)
Design a Stripe-style payment platform that processes 100M payments per day across 50 currencies and dozens of payment methods, where the central requirement is financial correctness: never charge a customer twice, never lose a payment, always reconcile to the cent. The interview centerpiece is the trio of idempotency keys, the payment intent state machine, and the immutable double-entry ledger - together they make the system safe in the face of network failures, partial outages, and adversarial retries. We also cover webhook delivery with signing and exponential backoff, PCI scope minimization through tokenization, multi-region availability, and the reconciliation jobs that compare our ledger to the bank's settlement files every night.
AI Engineering
Debug an integration with partial failures and retries
Find why an SMS reply webhook sent duplicate texts, left some messages unanswered, and multiplied model calls under rate limits, timeouts, and redeliveries. Fix each defect with a regression test, then declare the release check for the fix.
Community
Webhook Design: Retries, Signatures, and Replay Protection
Sign requests. Dedupe by event id. Apply idempotently by resource id. Ack fast, process async. Tolerate out-of-order. Five concerns that turn a webhook into critical infrastructure.
Webhook Signature Verifier With Replay Protection
How I verify Stripe-style webhook signatures and stop someone from re-POSTing yesterday's `invoice.paid`. Stdlib HMAC, a tolerance window, and an idempotency cache that lives in any Redis-shaped store.