Payment Gateway

Design a payment gateway like Stripe. Charge cards, handle refunds, webhooks to merchants, idempotent, ~100 TPS peak, PCI-DSS compliant.

Time budget: 45 min whiteboard. Draw architecture, estimate numbers, discuss tradeoffs.

Hint 1

Never store card numbers. Tokenize via the card network; store only the token + last-4 + exp.

Hint 2

Every API call has an Idempotency-Key. Critical for retry safety.

Hint 3

Double-entry ledger. All value movements are atomic accounting transactions.

• +2 Idempotency-Key required on POST /charges; stored with response; duplicate = return stored response
• +2 PCI tokenization: raw card → token via card network; never touch raw PAN; scope PCI to a narrow vault
• +2 Double-entry ledger: each charge/refund is balanced debit/credit; all fund movement is atomic
• +1 Auth vs capture: separate external calls; hold funds on auth; capture within 7 days
• +1 Webhook delivery: at-least-once with exp-backoff retries over 3 days; signed payloads
• +1 Fraud scoring on every charge — separate ML service; decision in <100ms budget
• +1 Saga for multi-step flows: auth → risk check → capture → notify; each step idempotent

Self-score: tally the points you would have mentioned unprompted. 7+ is interview-ready on this problem.

• Stores raw card numbers in DB
• No idempotency mechanism (double-charges on retry)
• Synchronous webhooks blocking the API response
• Single DB transaction across external card-network call (blocks too long)
• No double-entry ledger; uses row-update for balances