Configuring and using Eureka Discovery Server in Spring Cloud is a fundamental step in building microservices-based applications. Below is a guide on how to set up and utilize Eureka in your spring boot application.

Step 1: Set Up a Spring Boot Project
Start by creating a Spring Boot project using your preferred IDE or Spring Initializer (https://start.spring.io/). Make sure to include the "Eureka Server" and "Eureka Discovery" dependencies.

Step 2: Configure Eureka Server
In your Spring Boot application, you need to configure it as a Eureka Server. Create a main class and annotate it with@EnableEurekaServer:

importorg.springframework.boot.SpringApplication;importorg.springframework.boot.autoconfigure.SpringBootApplication;importorg.springframework.cloud.netflix.eureka.server.EnableEurekaServer;@SpringBootApplication@EnableEurekaServerpublicclassEurekaServerApplication{  publicstaticvoidmain(String[]args){    SpringApplication.run(EurekaServerApplication.class,args);  }}

In yourapplication.properties orapplication.yml, specify the server port and any other configurations:

server.port=8761spring.application.name=eureka-server

Step 3: Configure Eureka Client(s)
Now, you'll create one or more Eureka clients, which are your microservices that register with the Eureka Server. In your microservice project(s), add the Eureka Client dependency.

<dependency>   <groupId>org.springframework.cloud</groupId>   <artifactId>spring-cloud-starter-netflix-eureka-client</artifactId></dependency>

In your microservice'sapplication.properties orapplication.yml, specify the Eureka Server's location:

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

Step 4: Register Microservices
In each of your microservices, annotate the main class with@EnableDiscoveryClient:

importorg.springframework.boot.SpringApplication;importorg.springframework.boot.autoconfigure.SpringBootApplication;importorg.springframework.cloud.client.discovery.EnableDiscoveryClient;@SpringBootApplication@EnableDiscoveryClientpublicclassYourMicroserviceApplication{  publicstaticvoidmain(String[]args){    SpringApplication.run(YourMicroserviceApplication.class,args);  }}

Step 5: Test Eureka

• Start your Eureka Server application.
• Start your microservices (Eureka Clients).
• Access the Eureka Server's dashboard in your browser:http://localhost:8761.

You should see your registered microservices listed in the dashboard.

Step 6: Load Balancing
Eureka also provides client-side load balancing. You can use the@LoadBalanced annotation with yourRestTemplate orWebClient to make requests to other services registered with Eureka. For example:

importorg.springframework.cloud.client.loadbalancer.LoadBalanced;importorg.springframework.context.annotation.Bean;importorg.springframework.context.annotation.Configuration;importorg.springframework.web.client.RestTemplate;@ConfigurationpublicclassRestTemplateConfig{  @LoadBalanced  @Bean  publicRestTemplaterestTemplate(){    returnnewRestTemplate();  }}

Example Use Case:
Suppose you have a "ProductService" and a "UserService" as microservices. You can use the@LoadBalancedRestTemplate to make requests from the "UserService" to the "ProductService" without hardcoding the URL.

That's a basic guide on how to configure and use Eureka Discovery Server in Spring Cloud. You can extend this architecture to build complex microservices ecosystems with ease.