Building A Payment Processor Backend

Built A robust payment processor Backend

                 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 @version 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 - Support for 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

• Link https://github.com/ejohchisimdi-collab/PaymentProcessor

Building this taught me:

• How to use idempotencyKeys to prevent duplicate entries from retries

• How to handle race conditions with optimistic locking

• How to build state machines