Service Discovery

// Client-side discovery (Eureka + Spring Cloud)
// 1. Service registers itself with Eureka
// 2. Client queries Eureka for "ORDER-SERVICE"
// 3. Client gets list of IPs: [10.0.1.5:8080, 10.0.1.6:8080]
// 4. Client picks one (round-robin, random, etc.)

// Server-side discovery (Kubernetes)
// 1. Pod registers via Kubernetes API (automatic)
// 2. Client calls: http://order-service:8080/api/orders
// 3. kube-proxy resolves DNS → one of the pod IPs
// 4. Transparent to the client

// Kubernetes Service (server-side discovery)
apiVersion: v1
kind: Service
metadata:
  name: order-service
spec:
  selector:
    app: order-service     # matches pods with this label
  ports:
    - port: 8080
      targetPort: 8080
  type: ClusterIP          # internal DNS: order-service.default.svc.cluster.local

// Spring Boot + Eureka registration
// application.yml
eureka:
  client:
    serviceUrl:
      defaultZone: http://eureka-server:8761/eureka/
  instance:
    preferIpAddress: true
    leaseRenewalIntervalInSeconds: 10

// Using discovered services with Feign client
@FeignClient(name = "ORDER-SERVICE")  // resolves via Eureka
public interface OrderClient {
    @GetMapping("/api/v1/orders/{id}")
    OrderDTO getOrder(@PathVariable String id);
}

// Consul — service registration via agent
{
  "service": {
    "name": "order-service",
    "port": 8080,
    "check": {
      "http": "http://localhost:8080/health",
      "interval": "10s",
      "timeout": "5s"
    }
  }
}
// DNS query: dig order-service.service.consul