Accessing an Application with a Service

The basic step to access a new service is to use kubectl.

$ kubectl expose deployment/nginx --port=80 --type=NodePort

$ kubectl get svc

NAME        TYPE       CLUSTER-IP  EXTERNAL-IP  PORT(S)        AGE
kubernetes  ClusterIP  10.0.0.1    <none>       443/TCP        18h
nginx       NodePort   10.0.0.112  <none>       80:31230/TCP   5s

$ kubectl get svc nginx -o yaml

apiVersion: v1
kind: Service
...
spec:
    clusterIP: 10.0.0.112
    ports:
    - nodePort: 31230
...

Open browser http://Public-IP:31230.

The kubectl expose command created a service for the nginx deployment. This service used port 80 and generated a random port on all the nodes. A particular port and targetPort can also be passed during object creation to avoid random values. The targetPort defaults to the port, but could be set to any value, including a string referring to a port on a backend Pod. Each Pod could have a different port, but traffic is still passed via the name. Switching traffic to a different port would maintain a client connection, while changing versions of software, for example.

The kubectl get svc command gave you a list of all the existing services, and we saw the nginx service, which was created with an internal cluster IP.

The range of cluster IPs and the range of ports used for the random NodePort are configurable in the API server startup options.

Services can also be used to point to a service in a different namespace, or even a resource outside the cluster, such as a legacy application not yet in Kubernetes.