README.md

Templating Kubernetes resources with Java

In this section you will learn how to use Java and Maven to:

• create JSON (and YAML) definition for a Kubernetes Pod
• submit a Pod definition to the cluster with code

Let's get started.

Prerequisites

To your Maven application you need to add the io.fabric8:kubernetes-client dependency inside the pom.xml file

<dependency>
  <groupId>io.fabric8</groupId>
  <artifactId>kubernetes-client</artifactId>
  <version>4.6.4</version>
</dependency>

Generating Pod definitions

The code is short:

    public static void main(String[] args) throws Exception {
      String environment = "production";
      Pod pod = createPod(namespace);

      if (args[0].equals("yaml")) {
         System.out.println(SerializationUtils.dumpAsYaml(pod));
      } else {
         System.out.println(mapper.writeValueAsString(pod));
      }
      
    }
    
    public static Pod createPod( String environment ) {
        return new PodBuilder().withNewMetadata()
                .withName("test-pod")
                .endMetadata()
                .withNewSpec()
                .addNewContainer()
                .withName("test-container")
                .withImage("k8s.gcr.io/busybox")
                .withEnv(new EnvVarBuilder().withName("ENV").withValue(environment).build())
                .endContainer()
                .endSpec()
                .build();
    }

You can compile and package the Maven application with:

mvn clean package

You can execute the the application with:

java -jar target/k8s-client-1.0.jar --dry-run 

The output is a JSON object for the Pod.

{
  "apiVersion" : "v1",
  "kind" : "Pod",
  "metadata" : {
    "annotations" : { },
    "labels" : { },
    "name" : "test-pod"
  },
  "spec" : {
    "containers" : [ {
      "env" : [ {
        "name" : "ENV",
        "value" : "production"
      } ],
      "image" : "k8s.gcr.io/busybox",
      "name" : "test-container"
    } ],
    "nodeSelector" : { }
  }
}

But isn't Kubernetes accepting only YAML?

YAML is a superset of JSON and any JSON file is also a valid YAML file.

You can create the Pod in the cluster with the following commands:

java -jar target/k8s-client-1.0.jar yaml --dry-run 
kubectl apply -f pod.yaml

Creating a custom Kubectl

Instead of exporting the JSON and feeding it to kubectl, you can send the payload to the cluster directly.

You can use the Fabric8 Kubernetes API to send the Pod definition to the cluster.

Here's the code:

        Config config = new ConfigBuilder().build();

        try (final KubernetesClient client = new DefaultKubernetesClient(config)) {
            if (namespace == null) {
                namespace = client.getNamespace();
            }

            boolean dryRun = false;
            for (String arg : args) {
                if (arg.equals("--dry-run")) {
                    dryRun = true;
                }
            }
            if (!dryRun) {
                client.pods().inNamespace(namespace).create(pod);
                System.out.println("Pod created!");
            }


        }

Assuming you are connected to a running cluster, you can execute the script with:

java -jar target/k8s-client-1.0.jar yaml

And you can verify that the Pod was created with:

kubectl get pods

What's next

As you can imagine, this is a short demo and you can build more complex objects and use the power of Java and the Fabric8 Kubernetes API to compose large objects from smaller ones.