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Payment Processing Backend

Design secure payment processing with idempotency and retry.

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Requirements & Scope

Payment Flow

Client -> Order Service -> Payment Service -> Payment Provider

  • Idempotency-Key header prevents double charges
  • Retry only on timeout/network, NOT card declined
  • Webhook for async status updates

Scale

  • 99.99% success rate
  • p99 < 2s
  • Strict idempotency
  • Minimal PCI scope (tokenize)

Key Points

  • Understanding Payment Processing Backend is essential for production systems
  • Always consider scalability and maintainability
  • Test thoroughly before deploying to production
  • Monitor performance and set up alerting

Common Patterns

  1. Validation: Always validate input at the boundary
  2. Error Handling: Use structured error responses
  3. Logging: Log key events for debugging
  4. Testing: Unit, integration, and load tests
  5. Documentation: Keep docs updated with code changes

Architecture & Idempotency

Idempotency Implementation

@PostMapping("/payments")
public PaymentResponse createPayment(
    @RequestHeader("Idempotency-Key") String key,
    @RequestBody PaymentRequest req) {
  Payment existing = paymentRepository.findByIdempotencyKey(key);
  if (existing != null) return toResponse(existing);
  Payment payment = paymentService.process(req, key);
  return toResponse(payment);
}

Retry Strategy

  • Timeout -> Retry (idempotent)
  • Network error -> Retry with backoff
  • Card declined -> Do NOT retry
  • Provider error -> Check if charged, then retry or flag

PCI Compliance

  • Never store card numbers - use tokenization
  • Use PCI-compliant providers (Stripe, Braintree)

Database & Reconciliation

Tables

  • payments(id, order_id, idempotency_key, amount, status, provider, provider_payment_id)
  • payment_events(id, payment_id, event_type, payload JSONB)
  • refunds(id, payment_id, amount, reason, status)

Reconciliation

Daily job to reconcile internal records with provider:

  1. Fetch provider transactions for the day
  2. Match with internal payment records
  3. Flag mismatches for manual review

Key Points

  • Understanding Payment Processing Backend is essential for production systems
  • Always consider scalability and maintainability
  • Test thoroughly before deploying to production
  • Monitor performance and set up alerting

Common Patterns

  1. Validation: Always validate input at the boundary
  2. Error Handling: Use structured error responses
  3. Logging: Log key events for debugging
  4. Testing: Unit, integration, and load tests
  5. Documentation: Keep docs updated with code changes

Practice Problems

0 / 3 solved
Implement Payment Processing Backend

Design and implement a solution for Payment Processing Backend in a backend system. Consider scalability, error handling, and production readiness.

Solution 
// Payment Processing Backend implementation
// Key aspects: validation, error handling, logging, testing

public class PaymentProcessingBackend {
    // Production-ready implementation
}
Payment Processing Backend Edge Cases

Identify and handle edge cases for Payment Processing Backend. What happens under high load, with invalid input, or during failures?

Solution 
// Edge case handling:
// 1. Null/empty input -> validation
// 2. High load -> rate limiting, queuing
// 3. Failures -> retries, circuit breaker
// 4. Concurrent access -> locks, idempotency
Payment Processing Backend Testing Strategy

Write a testing strategy for Payment Processing Backend. Include unit tests, integration tests, and performance tests.

Solution 
// Test plan:
// - Unit: 80% coverage target
// - Integration: API contracts
// - Performance: latency, throughput
// - Chaos: failure injection

Quiz

1. Why are idempotency keys critical for payments?

Question 1 options

2. Should you retry a card declined error?

Question 2 options

3. What is a common mistake when implementing Payment Processing Backend?

Question 3 options

Flashcards

Question

Why idempotency keys for payments?

Click to reveal answer

Answer

Prevent double charges on retry

Question

Should you store card numbers?

Click to reveal answer

Answer

No - use tokenization via PCI-compliant provider

Question

Payment Processing Backend best practices

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Answer

Follow SOLID principles, write clean code, test thoroughly, document decisions, and monitor in production.

Revision Notes

Key Takeaways

  • 1. Idempotency keys prevent double charges
  • 2. Never store card numbers - use tokenization
  • 3. Retry only on transient failures
  • 4. Daily reconciliation catches discrepancies

Interview Tips

  • • Explain full payment flow including failures
  • • Discuss PCI compliance

Cheat Sheet

Payment Processing

  • Idempotency: Key per request, prevents double charges
  • PCI: Never store cards, use tokenization
  • Retry: Only on timeout/network, NOT card declined
  • Reconcile: Daily check against provider records