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Lab 4 - Service and test

A Kubernetes Service exposes an application to its consumers, whether internal or external.

whoami service

The whoami described in the file tp4-whoami-svc.yml handles the pods of the whoami deployment using their labels app: whoami (see the selector entry).

Create the whoami service from the file tp4-whoami-svc.yml:

kubectl apply -f tp4-whoami-svc.yml

List the available services, and make sure the whoami has been created:

kubectl get svc

Inspect the service whoami:

kubectl describe svc whoami

Notice the Endpoints section. It contains the IPs and ports of the service's destination pods.

Scale the deployment and check the service's Endpoints

Scale your deployment to 2 replicas:

kubectl scale deploy whoami --replicas=2

Check the Endpoints section of the Service:

kubectl describe svc whoami

Scale up to 3 replicas:

kubectl scale deploy whoami --replicas=3

And check the Endpoints section again:

kubectl describe svc whoami

What do you observe ?

Request the whoami service from within the gateway pod

Create the gateway Pod described in the file tp4-gateway-pod.yml:

kubectl apply -f tp4-gateway-pod.yml

Check that the gateway pod has been created and wait until it is in a Running state:

kubectl get pods

Connect to the gateway pod using:

kubectl exec -it gateway -- bash

From within the gateway pod, request the whoami using the curl client:

curl whoami:8080

Notice that the service has been reached using its DNS name whoami, using the right port. You don't need to bother to find the pod IPs behind this service. It's done automatically for us.

Execute the above command several times. You'll see different responses from different pods if you do have many replicas in your deployment. Exit the container with exit.

You can check the DNS response directly:

kubectl exec gateway -- nslookup whoami
kubectl exec gateway -- nslookup whoami.default
kubectl exec gateway -- nslookup whoami.default.svc
kubectl exec gateway -- nslookup whoami.default.svc.cluster.local

Conclusion

Kubernetes services expose your application in a discoverable and persistent way. They load-balance the requests automatically on the different pods present in the Endpoints list of the service. The Endpoints list is updated dynamically and transparently by Kubernetes.

In this exercise you have seen the Kubernetes default type of services which is called ClusterIP. There are other types of services in Kubernetes: [NodePort, LoadBalancer...] (https://kubernetes.io/fr/docs/concepts/services-networking/service/). Apart from some subtleties, regarding external exposition for example, all the services share the same concepts as seen here.

Bonus

If you have some time left at the end of this session, try to figure out what is behind the kubernetes service which appears when you list the services.