kubernetes学习Service之headless

一、首先说headless Service和普通Service的区别
headless不分配clusterIP
headless service下的Pod有DNS地址,可以通过Pod的DNS地址解析到Pod的IP地址
普通的service下的Pod没有DNS,只能通过svc的DNS解析到svc的clusterIP

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

[ ~]# 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[ ~]# 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>
[ ~]# 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

下面来看看headless Service的结果

[ ~]# 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
[ ~]# 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>
[ ~]# 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

headless Service就是没头的Service,有什么使用场景呢?

• 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

[ ~]# kubectl get svc -n ms
NAME               TYPE        CLUSTER-IP   EXTERNAL-IP   PORT(S)    AGE
eureka             ClusterIP   None         <none>        8888/TCP   21h
[ ~]# 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>
[ ~]# 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>.$<namespace name>.svc.cluster.local
2. StatefulSet会为关联的Pod保持一个不变的Pod Name
3. statefulset中Pod的hostname格式为$(StatefulSet name)-$(pod序号)
4. StatefulSet会为关联的Pod分配一个dnsName
5. $<Pod Name>.$<service name>.$<namespace name>.svc.cluster.local