Dependent Resources #

DISCLAIMER: The Dependent Resource support is a relatively new feature, while we strove to cover what we anticipate will be the most common use cases, the implementation is not simple and might still evolve. As a result, some APIs could be a subject of change in the future. However, non-backwards compatible changes are expected to be trivial to migrate to.

Motivations and Goals #

Most operators need to deal with secondary resources when trying to realize the desired state described by the primary resource they are in charge of. For example, the Kubernetes-native Deployment controller needs to manage ReplicaSet instances as part of a Deployment’s reconciliation process. In this instance, ReplicatSet is considered a secondary resource for the Deployment controller.

Controllers that deal with secondary resources typically need to perform the following steps, for each secondary resource:

flowchart TD compute[Compute desired secondary resource based on primary state] --> A A{Secondary resource exists?} A -- Yes --> match A -- No --> Create --> Done match{Matches desired state?} match -- Yes --> Done match -- No --> Update --> Done

While these steps are not difficult in and of themselves, there are some subtleties that can lead to bugs or sub-optimal code if not done right. As this process is pretty much similar for each dependent resource, it makes sense for the SDK to offer some level of support to remove the boilerplate code associated with encoding these repetitive actions. It should be possible to handle common cases (such as dealing with Kubernetes-native secondary resources) in a semi-declarative way with only a minimal amount of code, JOSDK taking care of wiring everything accordingly.

Moreover, in order for your reconciler to get informed of events on these secondary resources, you need to configure and create event sources and maintain them. JOSDK already makes it rather easy to deal with these, but dependent resources makes it even simpler.

Finally, there are also opportunities for the SDK to transparently add features that are even trickier to get right, such as immediate caching of updated or created resources (so that your reconciler doesn’t need to wait for a cluster roundtrip to continue its work) and associated event filtering (so that something your reconciler just changed doesn’t re-trigger a reconciliation, for example).

Design #

DependentResource vs. AbstractDependentResource #

The new DependentResource interface lies at the core of the design and strives to encapsulate the logic that is required to reconcile the state of the associated secondary resource based on the state of the primary one. For most cases, this logic will follow the flow expressed above and JOSDK provides a very convenient implementation of this logic in the form of the AbstractDependentResource class. If your logic doesn’t fit this pattern, though, you can still provide your own reconcile method implementation. While the benefits of using dependent resources are less obvious in that case, this allows you to separate the logic necessary to deal with each secondary resource in its own class that can then be tested in isolation via unit tests. You can also use the declarative support with your own implementations as we shall see later on.

AbstractDependentResource is designed so that classes extending it specify which functionality they support by implementing trait interfaces. This design has been selected to express the fact that not all secondary resources are completely under the control of the primary reconciler: some dependent resources are only ever created or updated for example and we needed a way to let JOSDK know when that is the case. We therefore provide trait interfaces: Creator, Updater and Deleter to express that the DependentResource implementation will provide custom functionality to create, update and delete its associated secondary resources, respectively. If these traits are not implemented then parts of the logic described above is never triggered: if your implementation doesn’t implement Creator, for example, AbstractDependentResource will never try to create the associated secondary resource, even if it doesn’t exist. It is possible to not implement any of these traits and therefore create read-only dependent resources that will trigger your reconciler whenever a user interacts with them but that are never modified by your reconciler itself.

Batteries included: convenient DependentResource implementations! #

JOSDK also offers several other convenient implementations building on top of AbstractDependentResource that you can use as starting points for your own implementations.

One such implementation is the KubernetesDependentResource class that makes it really easy to work with Kubernetes-native resources. In this case, you usually only need to provide an implementation for the desired method to tell JOSDK what the desired state of your secondary resource should be based on the specified primary resource state.

JOSDK takes care of everything else using default implementations that you can override in case you need more precise control of what’s going on.

We also provide implementations that make it very easy to cache (AbstractCachingDependentResource) or make it easy to poll for changes in external resources (PollingDependentResource, PerResourcePollingDependentResource). All the provided implementations can be found in the io/javaoperatorsdk/operator/processing/dependent package of the operator-framework-core module.

Sample Kubernetes Dependent Resource #

A typical use case, when a Kubernetes resource is fully managed - Created, Read, Updated and Deleted (or set to be garbage collected). The following example shows how to create a Deployment dependent resource:


@KubernetesDependent(labelSelector = WebPageManagedDependentsReconciler.SELECTOR)
class DeploymentDependentResource extends CRUDKubernetesDependentResource<Deployment, WebPage> {

   public DeploymentDependentResource() {
    super(Deployment.class);
  }

  @Override
  protected Deployment desired(WebPage webPage, Context<WebPage> context) {
    var deploymentName = deploymentName(webPage);
    Deployment deployment = loadYaml(Deployment.class, getClass(), "deployment.yaml");
    deployment.getMetadata().setName(deploymentName);
    deployment.getMetadata().setNamespace(webPage.getMetadata().getNamespace());
    deployment.getSpec().getSelector().getMatchLabels().put("app", deploymentName);

    deployment.getSpec().getTemplate().getMetadata().getLabels()
        .put("app", deploymentName);
    deployment.getSpec().getTemplate().getSpec().getVolumes().get(0)
        .setConfigMap(new ConfigMapVolumeSourceBuilder().withName(configMapName(webPage)).build());
    return deployment;
  }
}

The only thing that you need to do is to extend the CRUDKubernetesDependentResource and specify the desired state for your secondary resources based on the state of the primary one. In the example above, we’re handling the state of a Deployment secondary resource associated with a WebPage custom (primary) resource.

The @KubernetesDependent annotation can be used to further configure managed dependent resource that are extending KubernetesDependentResource.

See the full source code here .

Managed Dependent Resources #

As mentioned previously, one goal of this implementation is to make it possible to declaratively create and wire dependent resources. You can annotate your reconciler with @Dependent annotations that specify which DependentResource implementation it depends upon. JOSDK will take the appropriate steps to wire everything together and call your DependentResource implementations reconcile method before your primary resource is reconciled. This makes sense in most use cases where the logic associated with the primary resource is usually limited to status handling based on the state of the secondary resources and the resources are not dependent on each other.

See Workflows for more details on how the dependent resources are reconciled.

This behavior and automated handling is referred to as “managed” because the DependentResource instances are managed by JOSDK, an example of which can be seen below:


@ControllerConfiguration(
        labelSelector = SELECTOR,
        dependents = {
                @Dependent(type = ConfigMapDependentResource.class),
                @Dependent(type = DeploymentDependentResource.class),
                @Dependent(type = ServiceDependentResource.class)
        })
public class WebPageManagedDependentsReconciler
        implements Reconciler<WebPage>, ErrorStatusHandler<WebPage> {

   // omitted code

   @Override
   public UpdateControl<WebPage> reconcile(WebPage webPage, Context<WebPage> context)
           throws Exception {

      final var name = context.getSecondaryResource(ConfigMap.class).orElseThrow()
              .getMetadata().getName();
      webPage.setStatus(createStatus(name));
      return UpdateControl.patchStatus(webPage);
   }

}

See the full source code of sample here .

Standalone Dependent Resources #

It is also possible to wire dependent resources programmatically. In practice this means that the developer is responsible to initializing and managing and calling reconcile method. However, this gives possibility for developers to fully customize the process for reconciliation. Use standalone dependent resources for cases when managed does not fit.

Note that Workflows support also standalone mode using standalone resources.

The sample is similar to one above it just performs additional checks, and conditionally creates an Ingress: (Note that now this condition creation is also possible with Workflows)


@ControllerConfiguration
public class WebPageStandaloneDependentsReconciler
        implements Reconciler<WebPage>, ErrorStatusHandler<WebPage>,
        EventSourceInitializer<WebPage> {

   private KubernetesDependentResource<ConfigMap, WebPage> configMapDR;
   private KubernetesDependentResource<Deployment, WebPage> deploymentDR;
   private KubernetesDependentResource<Service, WebPage> serviceDR;
   private KubernetesDependentResource<Service, WebPage> ingressDR;

   public WebPageStandaloneDependentsReconciler(KubernetesClient kubernetesClient) {
      // 1.
      createDependentResources(kubernetesClient);
   }

   @Override
   public List<EventSource> prepareEventSources(EventSourceContext<WebPage> context) {
      // 2.  
      return List.of(
              configMapDR.initEventSource(context),
              deploymentDR.initEventSource(context),
              serviceDR.initEventSource(context));
   }

   @Override
   public UpdateControl<WebPage> reconcile(WebPage webPage, Context<WebPage> context)
           throws Exception {

      // 3.
      if (!isValidHtml(webPage.getHtml())) {
         return UpdateControl.patchStatus(setInvalidHtmlErrorMessage(webPage));
      }

      // 4.  
      configMapDR.reconcile(webPage, context);
      deploymentDR.reconcile(webPage, context);
      serviceDR.reconcile(webPage, context);

      // 5.
      if (Boolean.TRUE.equals(webPage.getSpec().getExposed())) {
         ingressDR.reconcile(webPage, context);
      } else {
         ingressDR.delete(webPage, context);
      }

      // 6.
      webPage.setStatus(
              createStatus(configMapDR.getResource(webPage).orElseThrow().getMetadata().getName()));
      return UpdateControl.patchStatus(webPage);
   }

   private void createDependentResources(KubernetesClient client) {
      this.configMapDR = new ConfigMapDependentResource();
      this.deploymentDR = new DeploymentDependentResource();
      this.serviceDR = new ServiceDependentResource();
      this.ingressDR = new IngressDependentResource();

      Arrays.asList(configMapDR, deploymentDR, serviceDR, ingressDR).forEach(dr -> {
         dr.setKubernetesClient(client);
         dr.configureWith(new KubernetesDependentResourceConfig()
                 .setLabelSelector(DEPENDENT_RESOURCE_LABEL_SELECTOR));
      });
   }

   // omitted code
}

There are multiple things happening here:

  1. Dependent resources are explicitly created and can be access later by reference.
  2. Event sources are produced by the dependent resources, but needs to be explicitly registered in this case by implementing the EventSourceInitializer interface.
  3. The input html is validated, and error message is set in case it is invalid.
  4. Reconciliation of dependent resources is called explicitly, but here the workflow customization is fully in the hand of the developer.
  5. An Ingress is created but only in case exposed flag set to true on custom resource. Tries to delete it if not.
  6. Status is set in a different way, this is just an alternative way to show, that the actual state can be read using the reference. This could be written in a same way as in the managed example.

See the full source code of sample here .

Telling JOSDK how to find which secondary resources are associated with a given primary resource #

TODO: this needs to be updated KubernetesDependentResource automatically maps secondary resource to a primary by owner reference. This behavior can be customized by implementing PrimaryToSecondaryMapper by the dependent resource.

See sample in one of the integration tests here .

Other Dependent Resource Features #

Caching and Event Handling in KubernetesDependentResource #

  1. When a Kubernetes resource is created or updated the related informer (more precisely the InformerEventSource), eventually will receive an event and will cache the up-to-date resource. Typically, though, there might be a small time window when calling the getResource() of the dependent resource or getting the resource from the EventSource itself won’t return the just updated resource, in the case where the associated event hasn’t been received from the Kubernetes API. The KubernetesDependentResource implementation, however, addresses this issue so you don’t have to worry about it by making sure that it or the related InformerEventSource always return the up-to-date resource.

  2. Another feature of KubernetesDependentResource is to make sure that if a resource is created or updated during the reconciliation, this particular change, which normally would trigger the reconciliation again (since the resource has changed on the server), will, in fact, not trigger the reconciliation again since we already know the state is as expected. This is a small optimization. For example if during a reconciliation a ConfigMap is updated using dependent resources, this won’t trigger a new reconciliation. Such a reconciliation is indeed not needed since the change originated from our reconciler. For this system to work properly, though, it is required that changes are received only by one event source (this is a best practice in general) - so for example if there are two config map dependents, either there should be a shared event source between them, or a label selector on the event sources to select only the relevant events, see in related integration test .