irahardianto Payment Integration Principles
Guidelines for building secure, compliant payment systems.
When to Invoke
- Integrating payment gateways (Stripe, Adyen, etc.)
- PCI compliance requirements
- Subscription/billing system design
- Fraud prevention implementation
Security (Non-Negotiable)
PCI DSS Compliance
- Never store raw card data — use tokenization via gateway.
- Never log payment credentials — mask card numbers, CVV never stored.
- TLS everywhere — all payment communication over HTTPS.
- Minimal data retention — store only what's legally required.
Tokenization Flow
User → Client-side SDK (Stripe.js/Elements) → Gateway tokenizes → Token to your server → Server charges via token
Your server never sees raw card data.
Transaction Patterns
Idempotency (Critical)
POST /api/payments
Idempotency-Key: unique-request-id-abc123
→ Gateway processes once, returns same result on retry
- Generate idempotency key client-side or server-side
- Store key → result mapping for deduplication
- Retry-safe — network failures don't cause double charges
Authorization Flow
Authorize → Capture → Settle
→ Void (cancel before capture)
→ Refund (after capture)
Webhook Handling
- Verify webhook signatures — never trust unverified payloads.
- Idempotent processing — webhooks may be delivered multiple times.
- Queue webhooks — process asynchronously, acknowledge immediately (200 OK).
- Handle out-of-order delivery — use event timestamps, not arrival order.
Subscription Billing
Lifecycle
Trial → Active → Past Due (dunning) → Cancelled/Expired
→ Upgraded/Downgraded (proration)
Dunning Management
- Retry failed payments (exponential backoff: day 1, 3, 7, 14)
- Notify user before and after each retry
- Grace period before cancellation
Fraud Prevention
- 3D Secure (SCA) for European payments (PSD2 requirement).
- Address Verification (AVS) — match billing address.
- Velocity checks — rate-limit transactions per user/IP.
- Risk scoring — use gateway's built-in fraud tools.
Testing
- Sandbox/test mode for all development — never test with real cards.
- Test card numbers — cover success, decline, 3DS, and error scenarios.
- Webhook testing — use gateway's webhook testing tools or local tunnels.
Related
- Security Principles GEMINI.md § Security Principles
- API Design Principles @.gemini/skills/api-design-principles/SKILL.md
- Error Handling Principles GEMINI.md § Error Handling Principles