Strangler Fig Modernization: Java 21 & Spring Cloud

This project demonstrates the Strangler Fig Pattern used to incrementally migrate functionality from a Legacy Monolith to a Modern Microservice architecture using Java 21 Virtual Threads.

🏗 Architecture

The system consists of four main components:

1. Discovery Server (Eureka): Service registry for dynamic lookups.
2. API Gateway (Spring Cloud Gateway): The "Traffic Cop" that routes requests and implements Circuit Breakers.
3. Legacy Monolith: The existing application handling /api/users.
4. Modern Service: High-performance Java 21 service handling /api/orders.

🚀 Key Features

• Project Loom (Virtual Threads): The Modern Service is optimized for massive concurrency.
• Resilience: Integrated Resilience4j Circuit Breaker to prevent cascading failures.
• Dynamic Scaling: Automatic load balancing across multiple instances via Eureka.
• Zero-Downtime Migration: Path rewriting allows the frontend to stay consistent while the backend evolves.

📊 Benchmark Results

In a stress test of 200 concurrent users (2,000 total requests):

• Modern (Java 21): ~1,267 req/sec | 99th% Latency: 400ms
• Legacy (Java 17): ~626 req/sec | 99th% Latency: 1,540ms

🛠 Running the Project

Prerequisites

• JDK 21
• Maven 3.9+
• Docker (Optional)

Step-by-Step Start

1. Start Discovery Server: mvn spring-boot:run in discovery-server
2. Start Legacy Monolith: mvn spring-boot:run in legacy-monolith
3. Start Gateway: mvn spring-boot:run in gateway-service
4. Start Modern Service: mvn spring-boot:run in modern-service

Testing the "Strangler" Routes

• Legacy Route: GET http://localhost:8080/api/users/1
• Modern Route: GET http://localhost:8080/api/orders/123
• Fallback Test: Stop the Modern Service and hit the order route to see the Circuit Breaker in action.

🧪 Testing

Run the full suite including context loads and routing tests:

mvn clean test

3. Final GitHub Push Checklist

To make your repository look professional, ensure your folder structure looks like this:

strangler-fig-project/
├── .gitignore              <-- (Include target/, .idea/, .DS_Store)
├── pom.xml                 <-- (The Parent POM)
├── README.md
├── discovery-server/
├── gateway-service/
├── legacy-monolith/
└── modern-service/

LICENSE

MIT