kubernetes学习Service之headless和statefulSet结合

有个性的哪吒 發表於 2019-11-26 17:19:00

kubernetes学习Service之headless和statefulSet结合

<h3>一、首先说headless Service和普通Service的区别</h3>
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
<h3>二、statefulSet和Deployment控制器的区别</h3>
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
三、通过StatefulSet和headless server部署的服务效果
1.另一个pod里可以通过dns+访问到该服务
<img src="https://img2020.cnblogs.com/blog/929849/202101/929849-20210126103905037-1703894962.png" alt="" loading="lazy">
 

 
 
<h3>四、普通Service接续service的DNS结果</h3>
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
<div class="cnblogs_code">
<pre># 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: gateway9999/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</pre>
</div>
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
<h3>五、headless Service的解析service的DNS结果</h3>
<div class="cnblogs_code">
<pre># 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: eureka8888/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 </pre>
  / # 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
<pre></pre>
  Name: eureka-2.eureka.ms.svc.cluster.local   Address 1: 10.244.1.211 eureka-2.eureka.ms.svc.cluster.local
<pre> </pre>
</div>
<ul>
<li>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</li>
</ul>
<h3>六、headless Service就是没头的Service,有什么使用场景呢?</h3>
<ul>
<li>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</li>
<li>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</li>
</ul>
<div class="cnblogs_code">
  # kubectl get sts -n ms    NAME READY AGE    eureka 3/3 22h
<pre># 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: eureka8888/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</pre>
</div>
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
 
<h3>七、为什么要用headless service+statefulSet部署有状态应用?</h3>
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
  
来源：https://www.cnblogs.com/chadiandianwenrou/p/11937041.html

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kubernetes学习Service之headless和statefulSet结合