kubernetes service分析
service typek8s中service主要有三种：
[*]ClusterIP: use a cluster-internal IP only - this is the default and is discussed above. Choosing this value means that you want this service to be reachable only from inside of the cluster.
[*]NodePort: on top of having a cluster-internal IP, expose the service on a port on each node of the cluster (the same port on each node). You'll be able to contact the service on any :NodePort address.
[*]LoadBalancer: on top of having a cluster-internal IP and exposing service on a NodePort also, ask the cloud provider for a load balancer which forwards to the Service exposed as a :NodePort for each Node.
clusterIP
clusterIP主要作用是方便pod到pod之间的调用。
$ kubectl describe service redis-sentinel
Name: redis-sentinel
Namespace: default
Labels: name=sentinel,role=service
Selector: redis-sentinel=true
Type: ClusterIP
IP: 10.254.142.111
Port: <unnamed> 26379/TCP
Endpoints: <none>
Session Affinity: None
No events.
clusterIP主要在每个node节点使用iptables，将发向clusterIP对应端口的数据，转发到kube-proxy中。
$ sudo iptables -S -t nat
...
-A KUBE-PORTALS-CONTAINER -d 10.254.142.111/32 -p tcp -m comment --comment "default/redis-sentinel:" -m tcp --dport 26379 -j REDIRECT --to-ports 36547
-A KUBE-PORTALS-HOST -d 10.254.142.111/32 -p tcp -m comment --comment "default/redis-sentinel:" -m tcp --dport 26379 -j DNAT --to-destination 10.0.0.5:36547
然后kube-proxy自己内部实现有负载均衡的方法，并可以查询到这个service下对应pod的地址和端口，进而把数据转发给对应的pod的地址和端口。
nodePort/LoadBalancer
nodePort跟LoadBalancer其实是同一种方式。参见这里的说明
区别在于LoadBalancer比nodePort多了一步，就是可以调用cloud provider去创建LB来向节点导流。cloud provider好像支持了openstack、gce等系统。
nodePort的原理在于在node上开了一个端口，将向该端口的流量导入到kube-proxy，然后由kube-proxy进一步导给对应的pod。
所以service采用nodePort的方式，正确的方法是在前面有一个lb，然后lb的后端挂上所有node的对应端口。这样即使node1挂了。lb也可以把流量导给其他node的对应端口。
我们使用这样的一个manifest来创建service
apiVersion: v1
kind: Service
metadata:
labels:
name: ssh
role: service
name: ssh-service1
spec:
ports:
- port: 2222
targetPort: 22
nodePort: 30239
type: NodePort
selector:
ssh-service: "true"
使用get service可以看到虽然type是NodePort，但是依然为其分配了一个clusterIP。分配clusterIP的作用还是如上文所说，是方便pod到service的数据访问。
$ kubectl get service
NAME LABELS SELECTOR IP(S) PORT(S)
kubernetes component=apiserver,provider=kubernetes <none> 10.254.0.1 443/TCP
ssh-service1 name=ssh,role=service ssh-service=true 10.254.132.107 2222/TCP
使用describe可以查看到详细信息。可以看到暴露出来的NodePort端口，正是指定的30239
$ kubectl describe service ssh-service1
Name: ssh-service1
Namespace: default
Labels: name=ssh,role=service
Selector: ssh-service=true
Type: LoadBalancer
IP: 10.254.132.107
Port: <unnamed> 2222/TCP
NodePort: <unnamed> 30239/TCP
Endpoints: <none>
Session Affinity: None
No events.
nodePort的工作原理与clusterIP大致相同，是发送到node上指定端口的数据，通过iptables重定向到kube-proxy对应的端口上。然后由kube-proxy进一步把数据发送到其中的一个pod上。
$ sudo iptables -S -t nat
...
-A KUBE-NODEPORT-CONTAINER -p tcp -m comment --comment "default/ssh-service1:" -m tcp --dport 30239 -j REDIRECT --to-ports 36463
-A KUBE-NODEPORT-HOST -p tcp -m comment --comment "default/ssh-service1:" -m tcp --dport 30239 -j DNAT --to-destination 10.0.0.5:36463
-A KUBE-PORTALS-CONTAINER -d 10.254.0.1/32 -p tcp -m comment --comment "default/kubernetes:" -m tcp --dport 443 -j REDIRECT --to-ports 53940
-A KUBE-PORTALS-CONTAINER -d 10.254.132.107/32 -p tcp -m comment --comment "default/ssh-service1:" -m tcp --dport 2222 -j REDIRECT --to-ports 36463
-A KUBE-PORTALS-HOST -d 10.254.0.1/32 -p tcp -m comment --comment "default/kubernetes:" -m tcp --dport 443 -j DNAT --to-destination 10.0.0.5:53940
-A KUBE-PORTALS-HOST -d 10.254.132.107/32 -p tcp -m comment --comment "default/ssh-service1:" -m tcp --dport 2222 -j DNAT --to-destination 10.0.0.5:36463
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