46. Kubernetes深入Service-Headless类型的Service资源

Service通过标签选择器所关联 Endpoints 后，对客户端来说是透明的，相当于隐藏了EndPoints。

当前有这样的一个Service，ClusterIP为10.110.69.11，Endpoints地址也如下所示

kubectl describe svc nginx-service
Name:              nginx-service
Namespace:         default
Labels:            name=nginx-service
Annotations:       Selector:  app=nginx
Type:              ClusterIP
IP:                10.110.69.11
Port:              <unset>  8080/TCP
TargetPort:        80/TCP
Endpoints:         10.244.2.100:80,10.244.3.22:80,10.244.5.144:80
Session Affinity:  None
Events:            <none>

解析，在我们集群的节点上解析Service的发现名称,如下nginx-service.default.svc.cluster.local地址解析到了Service的ClusterIP上。

nslookup nginx-service.default.svc.cluster.local
Server:     10.96.0.10
Address:    10.96.0.10#53

Name:   nginx-service.default.svc.cluster.local
Address: 10.110.69.11

但是有时候客户端可能有这样的需求，客户端需要直接访问Service资源后端的所有Pod资源，这时就应该向客户端暴露每个Pod资源的IP地址，而不再是中间层'Service'对象的ClusterIP，这种类型的Service便称为 "Headless Service"。

创建Headless Service资源

配置Service资源配置清单时，只需要将ClusterIP字段的值设置为 "None" 即可将其定义为 Headless类型。

at nginx-svc.yaml
apiVersion: v1
kind: Service
metadata:
  name: nginx-service
spec:
  clusterIP: None               #指定概属性值为 None 即为 Headless 无头Service
  ports:
  - port: 80
    protocol: TCP
    targetPort: 80
  selector:
    app: nginx-proxy
---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deploy
spec:
  replicas: 3
  selector:
    matchLabels:
      app: nginx-proxy
  template:
    metadata:
      labels:
        app: nginx-proxy
    spec:
      containers:
      - name: nginx-proxy
        image: nginx:latest
        ports:
        - containerPort: 80
          name: http

创建资源对象

kubectl apply -f nginx-svc.yaml

查看nginx-service状态

kubectl describe svc nginx-service
Name:              nginx-service
Namespace:         default
Labels:            <none>
Annotations:       Selector:  app=nginx-proxy
Type:              ClusterIP
IP:                None     #IP地址已经为None
Port:              <unset>  80/TCP
TargetPort:        80/TCP
Endpoints:         10.244.2.101:80,10.244.3.23:80,10.244.5.148:80
Session Affinity:  None
Events:            <none>

通过服务发现地址解析Service，如下解析nginx-service.default.svc.cluster.local服务发现映射直接映射到了后端的三个Pod地址

nslookup nginx-service.default.svc.cluster.local
Server:     10.96.0.10
Address:    10.96.0.10#53

Name:   nginx-service.default.svc.cluster.local
Address: 10.244.2.101
Name:   nginx-service.default.svc.cluster.local
Address: 10.244.3.23
Name:   nginx-service.default.svc.cluster.local
Address: 10.244.5.148