Article 05: Inter-Service Communication Using REST

1. Objective

Understand how microservices communicate synchronously using REST APIs and the challenges involved.

2. Key Concepts

• Synchronous communication
• REST APIs
• WebClient vs RestTemplate
• Tight vs loose coupling

3. Why Communication Is Needed

In microservices, each service owns its data. To complete a business flow, services must talk to each other.

Example:

• Order Service needs user details from User Service

4. Communication Flow

[Order Service] ---> HTTP ---> [User Service]

5. Hands-On: Calling User Service from Order Service

Using WebClient

@Service
public class UserClient {

    private final WebClient webClient;

    public UserClient(WebClient.Builder builder) {
        this.webClient = builder.baseUrl("http://localhost:8081").build();
    }

    public String getUser(Long id) {
        return webClient.get()
                .uri("/users/" + id)
                .retrieve()
                .bodyToMono(String.class)
                .block();
    }
}

6. Controller Usage

@GetMapping("/orders/{id}")
public String getOrder(@PathVariable Long id) {
    return "Order for user: " + userClient.getUser(1L);
}

7. Problems with Hardcoded URLs

• Not scalable
• Breaks when service location changes
• Difficult in cloud environments

8. Common Mistakes

• Chatty communication
• Circular dependencies
• Blocking calls everywhere

9. Interview Notes

• REST is simple but introduces runtime dependencies
• Synchronous calls increase latency

10. Summary

REST-based communication is easy to start with but has limitations.

11. What’s Next

Introduce Service Discovery to remove hardcoded URLs.