Service Discovery — Eureka

<!-- Eureka Server — standalone registry -->
<dependency>
  <groupId>org.springframework.cloud</groupId>
  <artifactId>spring-cloud-starter-netflix-eureka-server</artifactId>
</dependency>

@SpringBootApplication
@EnableEurekaServer
public class DiscoveryServer { }

# application.yml — Eureka Server
server.port: 8761
eureka:
  client:
    register-with-eureka: false   # the server doesn't register itself
    fetch-registry: false

---
<!-- Eureka Client — every microservice -->
<dependency>
  <groupId>org.springframework.cloud</groupId>
  <artifactId>spring-cloud-starter-netflix-eureka-client</artifactId>
</dependency>

# application.yml — Microservice
spring.application.name: order-service
eureka:
  client:
    service-url:
      defaultZone: http://localhost:8761/eureka
  instance:
    prefer-ip-address: true        # register with IP, not hostname
    lease-renewal-interval-in-seconds: 30
    lease-expiration-duration-in-seconds: 90

// WebClient with load balancer — uses lb:// URI
@Configuration
public class WebClientConfig {
    @Bean
    @LoadBalanced   // ← enables lb:// URI resolution via Eureka
    public WebClient.Builder loadBalancedWebClientBuilder() {
        return WebClient.builder();
    }
}

@Service
@RequiredArgsConstructor
public class OrderClient {
    private final WebClient.Builder webClientBuilder;

    public Mono<PaymentResponse> getPayment(String paymentId) {
        return webClientBuilder
            .baseUrl("lb://payment-service")  // Eureka resolves this
            .build()
            .get()
            .uri("/payments/{id}", paymentId)
            .retrieve()
            .bodyToMono(PaymentResponse.class);
    }
}

// OpenFeign — even simpler
@FeignClient(name = "payment-service")  // name matches spring.application.name
public interface PaymentClient {
    @GetMapping("/payments/{id}")
    PaymentResponse getPayment(@PathVariable String id);
}

# Kubernetes Service — built-in discovery
apiVersion: v1
kind: Service
metadata:
  name: payment-service   # DNS name usable by other pods
  namespace: production
spec:
  selector:
    app: payment-service   # matches pods with this label
  ports:
    - port: 8080
      targetPort: 8080
  type: ClusterIP           # internal-only load balancer

# Any pod in the cluster can call:
#   http://payment-service.production.svc.cluster.local:8080
#   http://payment-service:8080  (same namespace)

# Spring Boot app.yml for K8s — just use the K8s DNS name
payment:
  service-url: http://payment-service:8080

# Or use Spring Cloud Kubernetes for registry-aware lb://
<dependency>
  <groupId>org.springframework.cloud</groupId>
  <artifactId>spring-cloud-starter-kubernetes-client-loadbalancer</artifactId>
</dependency>