Instant Payment System

A Spring Boot application that handles instant money transfers between users. The system provides REST APIs for payment processing with retry mechanisms and circuit breaker patterns.

Features

• Instant money transfers between accounts
• Idempotency support for safe retries
• Circuit breaker pattern for fault tolerance
• Kafka integration for transaction notifications
• Database transaction management
• REST API endpoints

Prerequisites

• Java 21 or higher
• Gradle 8+
• PostgreSQL 15+ (server not running, only the client tools are needed)
• Apache Kafka 3.x
• Docker and Docker Compose

Setup

Using Docker Compose

For quick setup with Docker, use the provided docker-compose.yml:

Database Setup

1. Start postgres container

$ docker-compose up postgres

2. Use psql to login to Postgres in container

$ psql instantpayments -U postgres -h localhost

Password: postgres

3. Create a PostgreSQL tables Run CREATE TABLE.. scripts from database/database.sql

Also insert seed data as well for testing.

4. Ctrl-C (e.g. stop PostgreSQL container)

Building the Application

$ ./gradlew clean build

Running the Application

Using Docker Compose

$ docker-compose up -d --build

API Endpoints

Send Payment

POST /api/payments Content-Type: application/json
{ "senderId": "sender123", "recipientId": "recipient456", "amount": 100.00, "idempotencyKey": "unique-transaction-id" }

Swagger UI

http://localhost:8080/swagger-ui/index.html

Testing

Run unit tests:

$ ./gradlew test

Monitoring

The application exposes actuator endpoints for monitoring:

• Health check: http://localhost:8080/health
• Info: http://localhost:8080/nfo

Error Handling

The system handles various error scenarios:

• Insufficient balance
• Account not found
• Duplicate transactions (idempotency)
• System failures with circuit breaker fallback

Security

Ensure to configure appropriate security measures:

• Enable HTTPS in production
• Implement authentication
• Set up proper authorization
• Secure sensitive configuration