Payment Gateway Integration
Building a secure, scalable payment processing system integrating Stripe and PayPal with comprehensive error handling, subscription management, and compliance with PCI-DSS standards.
The Challenge
Webhook Reliability
Payment webhooks occasionally failed due to network issues or server downtime, causing payment state inconsistencies.
Concurrent Payment Processing
Users sometimes double-clicked payment buttons, creating duplicate charges that needed to be prevented.
Failed Subscription Renewals
Card declines during automatic renewals resulted in service interruptions and customer churn.
PCI Compliance
Ensuring PCI-DSS Level 1 compliance without storing sensitive card data while maintaining smooth user experience.
Core Features
Multi-Gateway Support
Integrated both Stripe and PayPal with automatic failover. Customers can choose their preferred payment method. Unified API abstracts provider differences.
Subscription Management
Handle recurring billing, plan upgrades/downgrades, prorated charges, trial periods, and automatic renewal. Support for metered billing and usage-based pricing.
Webhook Processing
Robust webhook handlers for all payment lifecycle events. Signature verification, replay attack prevention, and retry logic for failed processing.
Error Handling
Graceful handling of declined cards, network timeouts, and API errors. User-friendly error messages and automatic retry with exponential backoff.
Refund Management
Full and partial refunds with audit trail. Automatic refund processing for cancelled subscriptions. Dispute handling and chargeback management.
Compliance & Security
PCI-DSS Level 1 compliance without storing card data. Comprehensive audit logging. Data encryption and secure token handling.
Technical Implementation
Technology Stack
Implementation Process
Payment Flow Architecture
Designed the payment processing architecture with support for both one-time payments and recurring subscriptions. Created unified API layer abstracting provider differences.
Idempotent Webhook Handlers
Implemented idempotent webhook handlers with deduplication using Redis. Built retry queue with Bull for failed webhooks. Added reconciliation job for catching missed events.
Duplicate Charge Prevention
Implemented idempotency keys for all payment requests. Added database-level locking on user accounts during payment processing. Result: eliminated all duplicate charges.
Smart Renewal Retry Logic
Built retry logic with increasing intervals for failed renewals. Automatic email notifications prompting users to update payment method. Grace period for service access while resolving payment issues.
Results & Impact
Financial Impact
Processing $2M+ monthly GMV with 99.8% success rate. Zero duplicate charges or lost payments. Reduced failed payment rate from 8% to 0.2%.
Reliability
99.99% uptime for payment processing. Zero security incidents or PCI compliance violations. Successfully handled Black Friday traffic spike (10x normal volume).
Customer Experience
Average payment processing time under 200ms. Clear error messages reduced support tickets by 60%. Subscription renewal success improved 70%.
Key Learnings
Idempotency is critical for reliable payment processing. Always assume webhooks can arrive out of order or multiple times. Build reconciliation systems to catch edge cases.