kubernets: Headless Services
by 伊布
我知道Service，但Headless Services 是用来做什么的呢？
Headless Service也是一种Service，但不同的是会定义spec:clusterIP: None，也就是不需要Cluster IP的Service。
还记得Service的Cluster IP是做什么的吗？对，一个Service可能对应多个EndPoint(Pod)，client访问的是Cluster IP，通过iptables规则转到Real Server，从而达到负载均衡的效果（实现原理请见这里）。如下：
kubectl get service
NAME CLUSTER-IP EXTERNAL-IP PORT(S) AGE
nginx-service 10.107.124.218 192.168.128.158 80/TCP,443/TCP 1d
kubectl describe service nginx-service
Name: nginx-service
Namespace: default
Labels: <none>
Selector: component=nginx
Type: ClusterIP
IP: 10.107.124.218
External IPs: 192.168.128.158
Port: nginx-http 80/TCP
Endpoints: 10.244.2.9:80
Port: nginx-https 443/TCP
Endpoints: 10.244.2.9:443
Session Affinity: None
No events.
nslookup nginx-service.default.svc.cluster.local 10.96.0.10
Server: 10.96.0.10
Address: 10.96.0.10#53
Name: nginx-service.default.svc.cluster.local
Address: 10.107.124.218
虽然service有2个endpoint，但是dns查询时只会返回service的地址。具体client访问的是哪个Real Server，是由iptables来决定的。
那么Headless Service的效果呢？
kubectl get service
NAME CLUSTER-IP EXTERNAL-IP PORT(S) AGE
nginx None <none> 80/TCP 1h
kubectl describe service nginx
Name: nginx
Namespace: default
Labels: app=nginx
Selector: app=nginx
Type: ClusterIP
IP: None
Port: web 80/TCP
Endpoints: 10.244.2.17:80,10.244.2.18:80
Session Affinity: None
No events.
nslookup nginx.default.svc.cluster.local 10.96.0.10
Server: 10.96.0.10
Address: 10.96.0.10#53
Name: nginx.default.svc.cluster.local
Address: 10.244.2.17
Name: nginx.default.svc.cluster.local
Address: 10.244.2.18
dns查询会如实的返回2个真实的endpoint。
所以，顾名思义，Headless Service就是没头的Service。有啥用呢？很简单，有时候client想自己来决定使用哪个Real Server，可以通过查询DNS来获取Real Server的信息。
另外，Headless Services还有一个用处。Headless Service的对应的每一个Endpoints，即每一个Pod，都会有对应的DNS域名；这样Pod之间就可以互相访问。我们还是看上面的这个例子。
kubectl get statefulsets web
NAME DESIRED CURRENT AGE
web 2 2 1h
kubectl get pods
web-0 1/1 Running 0 1h
web-1 1/1 Running 0 1h
nslookup nginx.default.svc.cluster.local 10.96.0.10
Server: 10.96.0.10
Address: 10.96.0.10#53
Name: nginx.default.svc.cluster.local
Address: 10.244.2.17
Name: nginx.default.svc.cluster.local
Address: 10.244.2.18
nslookup web-1.nginx.default.svc.cluster.local 10.96.0.10
Server: 10.96.0.10
Address: 10.96.0.10#53
Name: web-1.nginx.default.svc.cluster.local
Address: 10.244.2.18
nslookup web-0.nginx.default.svc.cluster.local 10.96.0.10
Server: 10.96.0.10
Address: 10.96.0.10#53
Name: web-0.nginx.default.svc.cluster.local
Address: 10.244.2.17
如上，web为我们创建的StatefulSets，对应的pod的域名为web-0，web-1，他们之间可以互相访问，这样对于一些集群类型的应用就可以解决互相之间身份识别的问题了。
完整示例：
apiVersion: v1
kind: Service
metadata:
name: nginx
labels:
app: nginx
spec:
ports:
- port: 80
name: web
clusterIP: None
selector:
app: nginx
---
apiVersion: apps/v1beta1
kind: StatefulSet
metadata:
name: web
spec:
serviceName: "nginx"
replicas: 1
template:
metadata:
labels:
app: nginx
spec:
containers:
- name: nginx
image: nginx:1.11
ports:
- containerPort: 80
name: web
volumeMounts:
- name: www
mountPath: /usr/share/nginx/html
nodeSelector:
node: kube-node3
volumes:
- name: www
hostPath:
path: /mydir
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