Strangler Fig Pattern in Microservices

🚀 Introduction: Why Use the Strangler Fig Pattern?

Many organizations struggle with large, complex monolithic applications that are hard to scale and maintain.
Instead of a big-bang migration, the Strangler Fig Pattern enables a step-by-step transition to microservices without disrupting users.

✅ Migrate monolith to microservices incrementally
✅ Minimize risk by replacing one feature at a time
✅ Ensure zero downtime while modernizing the system

1️⃣ What is the Strangler Fig Pattern?

The Strangler Fig Pattern is a gradual migration strategy for refactoring a monolithic system into microservices piece by piece.

🔹 How It Works?

1. Intercept Requests – Route requests through an API Gateway.
2. Extract Features One by One – Move functionality from monolith to microservices.
3. Redirect Traffic to Microservices – Once a feature is migrated, update routing.
4. Retire the Monolith – When all features are migrated, remove the monolithic app.

📌 Inspired by the Strangler Fig Tree, which gradually replaces the host tree, leaving only the new structure behind.

2️⃣ Real-World Example: Migrating a Monolithic E-Commerce App

Imagine an e-commerce platform with a monolithic backend handling:
✔ User Management
✔ Order Processing
✔ Payment Handling

We will gradually migrate it to microservices using the Strangler Fig Pattern.

3️⃣ Step-by-Step Implementation with Spring Boot

✅ Step 1: Introduce an API Gateway

The API Gateway intercepts all requests and routes them either to the monolith or new microservices.

🔹 Configure Spring Cloud API Gateway

@EnableGateway
@SpringBootApplication
public class ApiGatewayApplication {
    public static void main(String[] args) {
        SpringApplication.run(ApiGatewayApplication.class, args);
    }

    @Bean
    public RouteLocator customRouteLocator(RouteLocatorBuilder builder) {
        return builder.routes()
                .route("user-service", r -> r.path("/users/**")
                        .uri("lb://USER-SERVICE")) // Redirects to new User Microservice
                .route("order-service", r -> r.path("/orders/**")
                        .uri("lb://ORDER-SERVICE")) // Redirects to new Order Microservice
                .route("legacy", r -> r.path("/**")
                        .uri("lb://MONOLITH-APPLICATION")) // Sends other requests to the old monolith
                .build();
    }
}

✅ Newly migrated features (/users, /orders) are handled by microservices.
✅ All other requests go to the monolith (lb://MONOLITH-APPLICATION).

✅ Step 2: Migrate User Management to a Microservice

We extract the User Management functionality into a new UserService microservice.

🔹 Create UserController in UserService

@RestController
@RequestMapping("/users")
public class UserController {

    @GetMapping("/{id}")
    public String getUser(@PathVariable String id) {
        return "User details for " + id;
    }
}

✅ User API is now handled by UserService.

✅ Step 3: Redirect /users API Calls to UserService

Since the API Gateway routes /users/** requests to UserService, the monolith no longer handles user management.

🔹 Update API Gateway to Redirect Traffic

routes:
  - id: user-service
    uri: lb://USER-SERVICE
    predicates:
      - Path=/users/**

📌 Now, all /users requests are handled by UserService instead of the monolith.

✅ Step 4: Repeat for Other Features

✔ Migrate /orders/** to OrderService
✔ Migrate /payments/** to PaymentService
✔ Update API Gateway for each feature

✅ Step 5: Retire the Monolith

Once all features are migrated, remove the monolithic application entirely.

📌 Now, the entire system runs on microservices!

4️⃣ Benefits of the Strangler Fig Pattern

✅ Zero Downtime Migration – No service disruption while transitioning.
✅ Reduces Risk – Migrate features gradually, not all at once.
✅ Easier Testing & Debugging – Test each microservice before cutting over.
✅ Improved Maintainability – Move to scalable, loosely coupled services.

5️⃣ Best Practices for Using the Strangler Fig Pattern

✅ Start with Least Risky Features – Begin with less critical services (e.g., reporting, logging).
✅ Use API Gateway for Routing – Easily redirect traffic to new services.
✅ Implement Circuit Breakers – Prevent failures with Resilience4j.
✅ Monitor Migration Progress – Track API calls with Spring Boot Actuator.

6️⃣ Alternative Migration Strategies

Migration Strategy	Description	Best For
Big Bang Rewrite	Rebuild everything from scratch	Small apps, when downtime is acceptable
Strangler Fig	Gradually migrate feature by feature	Large, business-critical apps
Incremental Refactoring	Slowly optimize monolithic codebase	When microservices aren’t needed

📌 For most enterprises, the Strangler Fig Pattern is the safest approach.

🎯 Conclusion: Why Use the Strangler Fig Pattern?

By implementing the Strangler Fig Pattern, you can:
✔ Migrate monoliths to microservices incrementally
✔ Minimize risk and ensure zero downtime
✔ Improve scalability and maintainability

🚀 Are you planning to migrate your monolith? Comment below!

🔗 Bookmark this guide for future reference! 🚀