Acing the Certified Kubernetes Administrator Exam by Chad Crowell;

Author:Chad Crowell; [Crowell, Chad M.]
Language: eng
Format: epub
Publisher: Simon & Schuster
Published: 0101-01-01T00:00:00+00:00

Figure 6.12 A LoadBalancer-type Service, which is a superset of the NodePort- and ClusterIP-type Services, provides a single entry point into the cluster and the underlying Pods.

6.4.1 ClusterIP Service

The ClusterIP Service is for internal cluster communication only. You would usually create a Service like this to communicate from the frontend of your application, in one Pod in the cluster, to the backend, which is in a different Pod. You can specify the ports to use when you create the Service initially, or you can modify it later. Like a lot of other resources in Kubernetes, you can run an imperative command to create a Service like k create svc clusterip internal-svc --tcp 8080:80, or you can create a YAML spec file with the command k create svc clusterip internal-svc --tcp 8080:80 -dry-run=client -o yaml > svc.yaml. Go ahead and run the latter command and take a look at the YAML spec by opening in it Vim with the command vim svc.yaml. You’ll see, as in figure 6.13, in the ports section, that there’s a port and a target port. The target port is the port that’s exposed on the container itself. Not all containers have exposed ports, but in the case of the nginx image, for example, port 80 is exposed.

The exposed port is defined in the Dockerfile (how the image is built), so if you’re ever wondering what ports the container is exposed on, look at the Dockerfile for that image. Going back to the Service YAML, the port is exposed within the cluster, as in figure 6.12 where the port is 80. You can have multiple Services with the same port exposed because each Service has its own IP address and DNS name, which allows an incoming request to differentiate between Services. As long as the Service name is different, you can have as many Services as you want with port 80 exposed. Notice in the YAML spec that this Service has a selector, as in figure 6.13. This is how the Service can bind itself to Pods. Any Pod with the label app=internal-svc, in this case, will be associated with that Service. This works similarly to nodeSelectors, which we touched on in section 4.1.

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