Headless Services无头服务
一、Headless Services介绍
Headless Services是一种特殊的service，其spec:clusterIP表示为None，这样在实际运行时就不会被分配ClusterIP。也被称为无头服务。
1、headless Service和普通Service的区别
headless不分配clusterIP
headless service可以通过解析service的DNS，返回所有Pod的地址和DNS(statefulSet部署的Pod才有DNS)
普通的service，只能通过解析service的DNS返回service的ClusterIP
2、statefulSet和Deployment控制器的区别
statefulSet下的Pod有DNS地址,通过解析Pod的DNS可以返回Pod的IP
deployment下的Pod没有DNS
3、普通Service解析service的DNS结果
Service的ClusterIP工作原理：一个service可能对应一组endpoints(所有pod的地址+端口)，client访问ClusterIP，通过iptables或者ipvs转发到Real Server(Pod)，具体操作如下
[root@master01 ~]# kubectl get svc -n ms #获取所有svc,看到gateway这个service的clusterIP是10.0.0.14
NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE
eureka ClusterIP None <none> 8888/TCP 21h
gateway ClusterIP 10.0.0.14 <none> 9999/TCP 20h
mysql-production ClusterIP 10.0.0.251 <none> 3306/TCP 23h
portal ClusterIP 10.0.0.124 <none> 8080/TCP 17h
[root@master01 ~]# kubectl describe svc gateway -n ms #看到gateway这个service的具体信息
Name: gateway
Namespace: ms
Labels: <none>
Annotations: kubectl.kubernetes.io/last-applied-configuration:
{"apiVersion":"v1","kind":"Service","metadata":{"annotations":{},"name":"gateway","namespace":"ms"},"spec":{"ports":[{"name":"gateway","po...
Selector: app=gateway,project=ms
Type: ClusterIP
IP: 10.0.0.14
Port: gateway 9999/TCP
TargetPort: 9999/TCP
Endpoints: 10.244.1.212:9999 #该service下的Pod地址
Session Affinity: None
Events: <none>
[root@master01 ~]# kubectl exec -it gateway-6cd76c98fb-8w92t -n ms sh #进入一个容器测试解析
/ # nslookup gateway.ms.svc.cluster.local #测试解析gateway这个service的DNS
nslookup: can't resolve '(null)': Name does not resolve
Name: gateway.ms.svc.cluster.local
Address 1: 10.0.0.14 gateway.ms.svc.cluster.local #这个结果就是gateway这个service的ClusterIP
从上面的结果能看到,虽然Service有1个endpoint,但是DNS查询时只会返回Service的ClusterIP地址,具体Client访问的是哪个real server,由iptables或者ipvs决定
4、headless Service的解析service的DNS结果
[root@master01 ~]# kubectl get svc -n ms #查看所有SVC,看到eureka的这个headless service
NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE
eureka ClusterIP None <none> 8888/TCP 21h
gateway ClusterIP 10.0.0.14 <none> 9999/TCP 20h
mysql-production ClusterIP 10.0.0.251 <none> 3306/TCP 23h
portal ClusterIP 10.0.0.124 <none> 8080/TCP 18h
[root@master01 ~]# kubectl describe svc eureka -n ms #看到eureka的这个headless service下面的endpoints
Name: eureka
Namespace: ms
Labels: <none>
Annotations: kubectl.kubernetes.io/last-applied-configuration:
{"apiVersion":"v1","kind":"Service","metadata":{"annotations":{},"name":"eureka","namespace":"ms"},"spec":{"clusterIP":"None","ports":[{"n...
Selector: app=eureka,project=ms
Type: ClusterIP
IP: None
Port: eureka 8888/TCP
TargetPort: 8888/TCP
Endpoints: 10.244.1.210:8888,10.244.1.211:8888,10.244.2.45:8888
Session Affinity: None
Events: <none>
[root@master01 ~]# kubectl exec -it gateway-6cd76c98fb-8w92t -n ms sh #进入容器测试解析
/ # nslookup eureka.ms.svc.cluster.local #通过解析eureka这个headless service的DNS地址,可以看到关联的具体endpoints信息
nslookup: can't resolve '(null)': Name does not resolve
Name: eureka.ms.svc.cluster.local
Address 1: 10.244.1.211 eureka-2.eureka.ms.svc.cluster.local
Address 2: 10.244.1.210 eureka-0.eureka.ms.svc.cluster.local
Address 3: 10.244.2.45 eureka-1.eureka.ms.svc.cluster.local
/ # nslookup eureka-2.eureka.ms.svc.cluster.local #解析pod的DNS记录,也能返回Pod的IP nslookup: can't resolve '(null)': Name does not resolve
Name: eureka-2.eureka.ms.svc.cluster.local Address 1: 10.244.1.211 eureka-2.eureka.ms.svc.cluster.local
根据结果看到,dns查询会返回3个endpoint，也就是3个pod地址和DNS，通过解析pod的DNS也能返回Pod的IP
二、Headless Services使用场景
第一种：自主选择权，有时候client想自己决定使用哪个Real Server，可以通过查询DNS来获取Real Server的信息
第二种：headless service关联的每个endpoint(也就是Pod),都会有对应的DNS域名;这样Pod之间就可以互相访问
headless services一般结合StatefulSet来部署有状态的应用，比如kafka集群，mysql集群，zk集群等
[root@master01 ~]# kubectl get sts -n ms
NAME READY AGE
eureka 3/3 22h
[root@master01 ~]# kubectl get svc -n ms
NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE
eureka ClusterIP None <none> 8888/TCP 21h
[root@master01 ~]# kubectl describe svc eureka -n ms
Name: eureka
Namespace: ms
Labels: <none>
Annotations: kubectl.kubernetes.io/last-applied-configuration:
{"apiVersion":"v1","kind":"Service","metadata":{"annotations":{},"name":"eureka","namespace":"ms"},"spec":{"clusterIP":"None","ports":[{"n...
Selector: app=eureka,project=ms
Type: ClusterIP
IP: None
Port: eureka 8888/TCP
TargetPort: 8888/TCP
Endpoints: 10.244.1.210:8888,10.244.1.211:8888,10.244.2.45:8888
Session Affinity: None
Events: <none>
[root@master01 ~]# kubectl exec -it gateway-6cd76c98fb-8w92t -n ms sh
/ # nslookup eureka.ms.svc.cluster.local
nslookup: can't resolve '(null)': Name does not resolve
Name: eureka.ms.svc.cluster.local
Address 1: 10.244.1.211 eureka-2.eureka.ms.svc.cluster.local
Address 2: 10.244.1.210 eureka-0.eureka.ms.svc.cluster.local
Address 3: 10.244.2.45 eureka-1.eureka.ms.svc.cluster.local
/ # nslookup eureka-2.eureka.ms.svc.cluster.local
nslookup: can't resolve '(null)': Name does not resolve
Name: eureka-2.eureka.ms.svc.cluster.local
Address 1: 10.244.1.211 eureka-2.eureka.ms.svc.cluster.local
/ # nslookup eureka-1.eureka.ms.svc.cluster.local
nslookup: can't resolve '(null)': Name does not resolve
Name: eureka-1.eureka.ms.svc.cluster.local
Address 1: 10.244.2.45 eureka-1.eureka.ms.svc.cluster.local
/ # nslookup eureka-0.eureka.ms.svc.cluster.local
nslookup: can't resolve '(null)': Name does not resolve
Name: eureka-0.eureka.ms.svc.cluster.local
Address 1: 10.244.1.210 eureka-0.eureka.ms.svc.cluster.local
如上，eureka就是我们场景的StatefulSet，对应的pod就是eureka-0，eureka-1，eureka-2，他们之间能互相访问，这样对于一些集群类型的应用就可以解决互相身份识别的问题了
三、为什么要用headless service+statefulSet部署有状态应用?
1.headless service会为关联的Pod分配一个域
<service name>.$<namespace name>.svc.cluster.local
2.StatefulSet会为关联的Pod保持一个不变的Pod Name
statefulset中Pod的hostname格式为$(StatefulSet name)-$(pod序号)
3.StatefulSet会为关联的Pod分配一个dnsName
$<Pod Name>.$<service name>.$<namespace name>.svc.cluster.local
https://blog.csdn.net/zhangshaohuas/article/details/107552452
https://www.cnblogs.com/chadiandianwenrou/p/11937041.html
https://kubernetes.io/docs/concepts/services-networking/service/#headless-services 官网介绍
