How to use Kubernetes dynamic client with Fabric8

January 5, 2023
Rohan Kumar
Related topics:
Kubernetes
Related products:
Red Hat Enterprise Linux

While working with Kubernetes Client, you would mostly be working with standard Kubernetes resources whose model is provided by the library itself. However, it’s not always possible to provide a concrete model type while accessing a Kubernetes API object (e.g., in the case of custom resources). Fabric8 Kubernetes Client’s GenericKubernetesResource API can be used in these scenarios. It allows objects that do not have Java POJOs registered to be manipulated generically.

This article is the third installment in the following series:

Getting the Fabric8 Kubernetes client

Fabric8 Kubernetes Client library should be available on Maven Central. If you’re using maven, you should be able to add it as a dependency in your project by adding this to your dependencies section of your pom.xml:

<dependency>
  <groupId>io.fabric8</groupId>
  <artifactId>kubernetes-client</artifactId>
  <version>6.2.0</version>
</dependency>

Gradle users need to add this to build.gradle:

implementation 'io.fabric8:kubernetes-client:6.2.0'

The GenericKubernetesResource object

GenericKubernetesResource is a generic Kubernetes object which can be used to serialize/deserialize any Kubernetes resource. It allows basic access to type metadata and object metadata. All the other stuff needs to be provided in additionalProperties map. While deserializing an unknown resource, common stuff like apiVersion, kind, and metadata would be directly available, but rest would be in additionalProperties map.

Let’s take a look at an example of creating a GenericKubernetesResource object. We will take example from Kubernetes CustomResourceDefinition docs for CronTab object:

# Taken from https://kubernetes.io/docs/tasks/extend-kubernetes/custom-resources/custom-resource-definitions/#create-custom-objects

apiVersion: "stable.example.com/v1"
kind: CronTab
metadata:
  name: my-new-cron-object
spec:
  cronSpec: "* * * * */5"
  image: my-awesome-cron-image

We can represent this object in GenericKubernetesResource like this:

Map<String, Object> spec = new HashMap<>();
spec.put("cronSpec", "* * * * */5");
spec.put("image", "my-awesome-cron-image");


GenericKubernetesResource genericKubernetesResource = new GenericKubernetesResourceBuilder()
    .withApiVersion("stable.example.com/v1")
    .withKind("CronTab")
    .withNewMetadata()
    .withName("my-new-cron-object")
    .endMetadata()
    .addToAdditionalProperties("spec", spec)
    .build();

System.out.println(Serialization.asYaml(genericKubernetesResource));

Note that access to type and object metadata is similar to standard Kubernetes resources. However, other fields (like status and spec) are manipulated using plain HashMaps.

2 entry points for dynamic client

There are different ways to use GenericKubernetesResource API in Fabric8 Kubernetes Client. Let’s take a look at two approaches:

  • Providing apiVersion and kind: You can start using GenericKubernetesResource API by providing apiVersion and kind to kubernetesClient.genericKubernetesResources() method. Here is an example of creating CronTab object we constructed in the previous section: 
    try (KubernetesClient client = new KubernetesClientBuilder().build()) {
  GenericKubernetesResource genericKubernetesResource = createNewCronTab();
                                    // ApiVersion              // Kind
  client.genericKubernetesResources("stable.example.com/v1", "CronTab")
      .inNamespace("default")
      .resource(genericKubernetesResource)
      .create();
}
    This method would automatically detect resource-related information like group, version, and plural that are required for contacting Kubernetes API server.
  • Providing ResourceDefinitionContext: We can go one step further from the previous approach by providing all the resource-related information to KubernetesClient rather than letting KubenetesClient make assumptions. We can do this by providing information in the form of ResourceDefinitionContext. Check this example: 
    try (KubernetesClient client = new KubernetesClientBuilder().build()) {
  GenericKubernetesResource genericKubernetesResource = createNewCronTab();

  ResourceDefinitionContext context = new ResourceDefinitionContext.Builder()
      .withGroup("stable.example.com")
      .withVersion("v1")
      .withKind("CronTab")
      .withPlural("crontabs")
      .withNamespaced(true)
      .build();

  client.genericKubernetesResources(context)
      .inNamespace("default")
      .resource(genericKubernetesResource)
      .create();
}

Basic create, read, update, and delete operations

Once you’ve provided ResourceDefinitionContext or apiVersion+kind to genericKubernetesResources() DSL method, it’s very easy to perform basic operations since they are the same as standard Kubernetes resources, thanks to KubernetesClient’s fluent DSL.

The following code snippet gives an overview of the basic operations of CronTab custom resource:


try (KubernetesClient client = new KubernetesClientBuilder().build()) {
  // Create CronTab context
  ResourceDefinitionContext context = new ResourceDefinitionContext.Builder()
      .withGroup("stable.example.com")
      .withVersion("v1")
      .withKind("CronTab")
      .withPlural("crontabs")
      .withNamespaced(true)
      .build();

  // Create CronTab Object
  GenericKubernetesResource genericKubernetesResource = createNewCronTab();

  // Create
  client.genericKubernetesResources(context)
      .inNamespace("default")
      .resource(genericKubernetesResource)
      .create();

  // Read
  genericKubernetesResource = client.genericKubernetesResources(context)
      .inNamespace("default")
      .withName("my-new-cron-object")
      .get();

  // List
  GenericKubernetesResourceList cronTabs = client.genericKubernetesResources(context).inNamespace("default").list();
  cronTabs.getItems().stream().map(GenericKubernetesResource::getMetadata).map(ObjectMeta::getName).forEach(logger::info);

  // Update
  Map additionalProperties = genericKubernetesResource.getAdditionalProperties();
  Map spec = (Map) additionalProperties.get("spec");
  spec.put("image", "my-updated-cron-image");
  client.genericKubernetesResources(context).inNamespace("default").resource(genericKubernetesResource).replace();

  // Delete
  client.genericKubernetesResources(context).inNamespace("default").resource(genericKubernetesResource).delete();
}

The watch operation

Like common operations, it’s also possible to watch a resource with the help of GenericKubernetesResource API. Here is an example:

try (KubernetesClient client = new KubernetesClientBuilder().build()) {
  Watch watch = client.genericKubernetesResources("stable.example.com/v1", "CronTab")
      .inNamespace("default")
      .watch(new Watcher<>() {
        @Override
        public void eventReceived(Action action, GenericKubernetesResource genericKubernetesResource) {
          logger.info("{} {}", action.name(), genericKubernetesResource.getMetadata().getName());
        }

        @Override
        public void onClose(WatcherException e) {
          logger.info("Closing due to {} ", e.getMessage());
        }
      });

  logger.info("Watch open for 30 seconds");
  Thread.sleep(30 * 1000L);
  watch.close();

  logger.info("Watch closed");
}

The Fabric8 Kubernetes Client GitHub and more

This article demonstrated how to manipulate Kubernetes CustomResource API using Fabric8 Kubernetes Client. You can find the code in this repository. Check out the final two articles in this series discussing testing and the code generation capabilities of Fabric8.

For more information, check out the Fabric8 Kubernetes Client GitHub page. Feel free to follow us on these channels:

Last updated: September 20, 2023