kubernetes - Services

we Know that container running in pod will share same network space. Every pod will have an IP

address. Once the Pod is killed or destroyed a new pod is created by replication controller which

will get new IP address. How can an user still access the application with changing IP address

( Container getting killed and recreated )?

This is where service comes to help. Service provides a single name for a set of pods.They

basically act as a load balancer to a set of same application running in different pods. Each

service will be assigned with a virtual IP address which remains constant until the service life

time.

The service created is attached to the application/pods. User will be accessing the service

address rather the pod address/IP address. The service will keep track of the pds being created

and destroyed and the load will be shared with them.

Create a Service yml file for the pod that we created earlier using,

[root@manja17-I13330 kubenetes-config]# cat testing-service-pod.yml

apiVersion: v1

kind: ReplicationController

metadata:

 name: testing-service

spec:

 replicas: 1

 selector:

   app: testingService-service

 template:

   metadata:

     name: testingService

     labels:

       app: testingService-service

   spec:

     containers:

     - name: test-ser

       image: docker.io/jagadesh1982/testing-service

       ports:

       - containerPort: 9876

Create the service using the

[root@manja17-I13330 kubenetes-config]# cat testingService-service.yml

apiVersion: v1

kind: Service

metadata:

 name: simpleservice

spec:

 ports:

   - port: 80

     targetPort: 9876

 selector:

   app: testingService-service

In the above service we are actually creating a service or endpoint to access for the pod with label set as “app=testingService-service”. Though the pod endpoint to access the application is 9876, we created a service with endpoint 80. So when we access the service on port 80 ,it will

routes to the application on port 9876.

Check if the Replication Controller are created,

[root@manja17-I13330 kubenetes-config]# kubectl get rc

NAME              DESIRED CURRENT   READY AGE

testing-service   1 1       1         9s

Check for the pods are created,

[root@manja17-I13330 kubenetes-config]# kubectl get pods

NAME                         READY STATUS RESTARTS  AGE

testing-service-v2kk4   1/1 Running 0                51s

Now lets see if the service is created are not,

[root@manja17-I13330 kubenetes-config]# kubectl get svc

NAME              TYPE CLUSTER-IP     EXTERNAL-IP PORT(S) AGE

kubernetes      ClusterIP 10.96.0.1         <none> 443/TCP 13d

simpleservice   ClusterIP 10.97.14.179   <none> 80/TCP 4s

Now we can access the application using,

[root@manja17-I13330 kubenetes-config]# curl 10.97.14.179:80/info

{"host": "10.97.14.179", "version": "0.5.0", "from": "10.32.0.1"}

Because every node in a Kubernetes cluster runs a kube-proxy, the kube-proxy watches the

Kubernetes API server for the addition and removal of services. For each new service,

kube-proxy opens a (randomly chosen) port on the local node. Any connections made to that

port are proxied to one of the corresponding backend pods.

More to come. Happy learning :-)