Kubernetes Manifest

So far, we have basically run commands by putting information such as Pods directly into the command, and we have shown command examples each time for what we wanted to do. A typical example is the way we started a Deployment in the previous section and then increased the number of Pods. Giving commands for what you want to do each time like this is called imperative. This approach focuses on how.

By contrast, specifying the final state you want and then creating it is called declarative. In the declarative approach, creation is performed by focusing on information, or what.

In Kubernetes, the declarative method is to write information in a YAML file, also called a manifest. By writing the final desired information, such as Pod information, in this YAML file, Kubernetes automatically creates the target object. At this point, Kubernetes users do not need to think about how to create it, so the ability to abstract that how is an advantage of the declarative approach.

For example, the following two examples describe the same thing imperatively with kubectl run and declaratively with pod.yaml.

Imperative: kubectl run

kubectl run --image gcr.io/google-samples/hello-app:1.0 --restart Never Helloworld

Declarative: pod.yaml

apiVersion: v1
kind: Pod
metadata:
  creationTimestamp: null
  labels:
    run: helloworld
  name: helloworld
spec:
  containers:
  - env:
    - name: TEST_ENV
      value: Hello_World
    image: gcr.io/google-samples/hello-app:1.0
    ports:
    - containerPort: 8080

You can also convert from imperative creation to declarative creation with the command below.

kubectl run --image gcr.io/google-samples/hello-app:1.0 --restart Never helloworld -o yaml helloworld > pod.yaml

To create the same Pod as kubectl run by specifying YAML in this way, specify the YAML file you created with the kubectl apply command.

kubectl apply -f pod.yaml

Next, let’s look at the contents written in the YAML file. However, covering everything here would make the article too long, so only part of it is described. Refer to the official documentation for other fields. The pod.yaml file explained above is probably the simplest form. Larger configurations follow the same idea.

Let’s look at the first level of this YAML file. You can see that there are four major fields. Each field has the following meaning.

The four fields above are required when writing a YAML file. Also, the fields written below the second level vary depending on the type of object being created, so the detailed explanation is omitted.

The file created here contains the following settings.