一、Headless Services介紹
Headless Services是一種特殊的service，其spec:clusterIP表示為None，這樣在實際運行時就不會被分配ClusterIP。也被稱為無頭服務。
1、headless Service和普通Service的區別
headless不分配clusterIP
headless service可以通過解析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，只能通過解析service的DNS返回service的ClusterIP
2、statefulSet和Deployment控制器的區別
statefulSet下的Pod有DNS地址,通過解析Pod的DNS可以返回Pod的IP
deployment下的Pod沒有DNS
3、普通Service解析service的DNS結果
Service的ClusterIP工作原理：一個service可能對應一組endpoints(所有pod的地址+端口)，client訪問ClusterIP，通過iptables或者ipvs轉發到Real Server(Pod)，具體操作如下
[root@master01 ~]# kubectl get svc -n ms #獲取所有svc,看到gateway這個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這個service的具體信息
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 #該service下的Pod地址
Session Affinity: None
Events: <none>
[root@master01 ~]# kubectl exec -it gateway-6cd76c98fb-8w92t -n ms sh #進入一個容器測試解析
/ # nslookup gateway.ms.svc.cluster.local #測試解析gateway這個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 #這個結果就是gateway這個service的ClusterIP
從上面的結果能看到,雖然Service有1個endpoint,但是DNS查詢時只會返回Service的ClusterIP地址,具體Client訪問的是哪個real server,由iptables或者ipvs決定
4、headless Service的解析service的DNS結果
[root@master01 ~]# kubectl get svc -n ms #查看所有SVC,看到eureka的這個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的這個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 #進入容器測試解析
/ # nslookup eureka.ms.svc.cluster.local #通過解析eureka這個headless service的DNS地址,可以看到關聯的具體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記錄,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
根據結果看到,dns查詢會返回3個endpoint，也就是3個pod地址和DNS，通過解析pod的DNS也能返回Pod的IP
二、Headless Services使用場景
第一種：自主選擇權，有時候client想自己決定使用哪個Real Server，可以通過查詢DNS來獲取Real Server的信息
第二種：headless service關聯的每個endpoint(也就是Pod),都會有對應的DNS域名;這樣Pod之間就可以互相訪問
headless services一般結合StatefulSet來部署有狀態的應用，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就是我們場景的StatefulSet，對應的pod就是eureka-0，eureka-1，eureka-2，他們之間能互相訪問，這樣對於一些集群類型的應用就可以解決互相身份識別的問題了
三、為什么要用headless service+statefulSet部署有狀態應用?
1.headless service會為關聯的Pod分配一個域
<service name>.$<namespace name>.svc.cluster.local
2.StatefulSet會為關聯的Pod保持一個不變的Pod Name
statefulset中Pod的hostname格式為$(StatefulSet name)-$(pod序號)
3.StatefulSet會為關聯的Pod分配一個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 官網介紹