Eureka is a powerful service registry solution designed for dynamic cloud environments. In this chapter, you will explore how Eureka helps your Spring applications automatically discover and communicate with each other. You will learn the core concepts behind service registration and discovery, see how to set up a Eureka server, and connect client applications to it. By the end of this chapter, you will understand how Eureka simplifies service management and enables resilient microservice architectures.

What Is Eureka?

Eureka is a REST-based service registry from Netflix, integrated into Spring Cloud. You use Eureka to manage a dynamic list of service instances in your distributed system. With Eureka, each service can register itself and discover other services without hardcoded network locations.

How Eureka Works as a Service Registry

Eureka acts as a central directory for all your microservices. Here’s how it works:

• Each service instance registers its network location (such as host and port) with the Eureka server;
• The Eureka server stores these registrations and keeps them up to date;
• Services query the Eureka server to discover the network locations of other services they need to communicate with.

This approach allows services to scale up or down, change locations, or recover from failures without manual reconfiguration.

Service Registration and Discovery in Practice

You typically use Spring Boot and Spring Cloud to enable Eureka in your applications. Here’s how you register and discover services with Eureka:

1. Setting Up a Eureka Server

Create a Spring Boot application and add the spring-cloud-starter-netflix-eureka-server dependency. Then, enable the Eureka server in your main class:

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

2. Registering a Service with Eureka

To register a service, add the spring-cloud-starter-netflix-eureka-client dependency to your service’s project. Enable Eureka client support in your main class:

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

In your application.properties, set the Eureka server URL:

eureka.client.service-url.defaultZone=http://localhost:8761/eureka/

When the service starts, it registers itself with the Eureka server.

3. Discovering Services Using Eureka

Suppose you have a PaymentService and an OrderService. The OrderService can discover the PaymentService by querying Eureka. You can use the @LoadBalanced RestTemplate for client-side load balancing:

@Bean
@LoadBalanced
public RestTemplate restTemplate() {
    return new RestTemplate();
}

// In a service method
String paymentServiceUrl = "http://PAYMENT-SERVICE/pay";
restTemplate.getForObject(paymentServiceUrl, String.class);

Here, PAYMENT-SERVICE is the service ID registered in Eureka. Eureka resolves the actual host and port at runtime.

By using Eureka, you avoid hardcoding service addresses, making your system more flexible and resilient.