Building A Payment Processor Backend

Source: Dev.to

Core Features

• JWT Authentication & Role-Based Access Control – Token‑based auth with Admin, Merchant, and Customer roles
• Idempotency Keys – Prevents duplicate transactions from retries or network issues
• Optimistic Locking – Handles concurrent payments using JPA with automatic retry logic
• Multi‑State Payment Pipeline – 5‑stage processing: PENDING → VALIDATED → AUTHORIZED → CAPTURED → SETTLED
• Intelligent Fraud Detection – Real‑time scoring system checking location, velocity, failed history, and funds
• Webhook Notifications with Retry Logic – Automatic retries with exponential backoff (up to 5 attempts)
• Dual Payment Method Support – Credit cards (instant settlement) and bank accounts (daily validation)
• Flexible Refund System – Supports both partial and complete refunds with policy enforcement
• Scheduled Batch Processing – Daily jobs for bank payment validation and webhook retries
• Comprehensive Testing – Unit tests with JUnit and Mockito covering service layer and edge cases
• Docker Containerization – Full Docker Compose setup with MySQL database
• RESTful API with Swagger Documentation
• Complete API docs with interactive testing interface

GitHub Repository

Lessons Learned

• How to use idempotency keys to prevent duplicate entries from retries
• How to handle race conditions with optimistic locking
• How to build state machines