Features #

Java Operator SDK is a high level framework and related tooling in order to facilitate implementation of Kubernetes operators. The features are by default following the best practices in an opinionated way. However, feature flags and other configuration options are provided to fine tune or turn off these features.

Reconciliation Execution in a Nutshell #

Reconciliation execution is always triggered by an event. Events typically come from the custom resource (i.e. custom resource is created, updated or deleted) that the controller is watching, but also from different sources (see event sources). When an event is received reconciliation is executed, unless there is already a reconciliation happening for a particular custom resource. In other words it is guaranteed by the framework that no concurrent reconciliation happens for a custom resource.

After a reconciliation ( i.e. Reconciler called, a post-processing phase follows, where typically framework checks if:

  • an exception was thrown during execution, if yes schedules a retry.
  • there are new events received during the controller execution, if yes schedule the execution again.
  • there is an instruction to re-schedule the execution for the future, if yes schedules a timer event with the specified delay.
  • if none above, the reconciliation is finished.

Briefly, in the hearth of the execution is an eventing system, where events are the triggers of the reconciliation execution.

Finalizer Support #

Kubernetes finalizers make sure that a reconciliation happens when a custom resource is instructed to be deleted. Typical case when it’s useful, when an operator is down (pod not running). Without a finalizer the reconciliation - thus the cleanup - i.e. Cleaner.cleanup(...) would not happen if a custom resource is deleted.

To use finalizers the reconciler have to implement Cleaner<P> interface. In other words, finalizer is added only if the Reconciler implements Cleaner interface. If not, no finalizer is added and/or removed.

Finalizers are automatically added by the framework as the first step, thus after a custom resource is created, but before the first reconciliation. The finalizer is added via a separate Kubernetes API call. As a result of this update, the finalizer will be present. The subsequent event will be received, which will trigger the first reconciliation.

The finalizer that is automatically added will be also removed after the cleanup is executed on the reconciler. However, the removal behaviour can be further customized, and can be instructed to “not remove yet” - this is useful just in some specific corner cases, when there would be a long waiting period for some dependent resource cleanup.

The name of the finalizers can be specified, in case it is not, a name will be generated.

See @ControllerConfiguration annotation for more details.

When not to Use Finalizers? #

Typically, automated finalizer handling should be turned off, in case the cleanup of all the dependent resources is handled by Kubernetes itself. This is handled by Kubernetes garbage collection. Setting the owner reference and related fields are not in the scope of the SDK, it’s up to the user to have them set properly when creating the objects.

When automatic finalizer handling is turned off, the Reconciler.cleanup(...) method is not called at all. Not even in case when a delete event received. So it does not make sense to implement this method and turn off finalizer at the same time.

The reconcile and cleanup #

The lifecycle of a custom resource can be clearly separated into two phases from the perspective of an operator. When a custom resource is created or update, or on the other hand when the custom resource is deleted - or rather marked for deletion in case a finalizer is used.

This separation-related logic is automatically handled by the framework. The framework will always call reconcile method, unless the custom resource is marked from deletion . From the point when the custom resource is marked from deletion, only the cleanup method is called, of course only if the reconciler implements the Cleaner interface.

Using UpdateControl and DeleteControl #

These two classes are used to control the outcome or the desired behaviour after the reconciliation.

The UpdateControl can instruct the framework to update the status sub-resource of the resource and/or re-schedule a reconciliation with a desired time delay.

  public UpdateControl<MyCustomResource> reconcile(
     EventSourceTestCustomResource resource, Context context) {
    return UpdateControl.patchStatus(resource).rescheduleAfter(10, TimeUnit.SECONDS);

without an update:

  public UpdateControl<MyCustomResource> reconcile(
     EventSourceTestCustomResource resource, Context context) {
    return UpdateControl.<MyCustomResource>noUpdate().rescheduleAfter(10, TimeUnit.SECONDS);

Note, that it’s not always desirable to always schedule a retry, rather to use EventSources to trigger the reconciliation.

Those are the typical use cases of resource updates, however in some cases there it can happen that the controller wants to update the custom resource itself (like adding annotations) or not to do any updates, which is also supported.

It is also possible to update both the status and the custom resource with the updateCustomResourceAndStatus method. In this case first the custom resource is updated then the status in two separate requests to K8S API.

Always update the custom resource with UpdateControl, not with the actual kubernetes client if possible.

On resource updates there is always an optimistic version control in place, to make sure that another update is not overwritten (by setting resourceVersion ) .

The DeleteControl typically instructs the framework to remove the finalizer after the dependent resource are cleaned up in cleanup implementation.

public DeleteControl cleanup(MyCustomResource customResource, Context context) {
    return DeleteControl.defaultDelete();

However, there is a possibility to not remove the finalizer, this allows to clean up the resources in a more async way, mostly for the cases when there is a long waiting period after a delete operation is initiated. Note that in this case you might want to either schedule a timed event to make sure cleanup is executed again or use event sources to get notified about the state changes of a deleted resource.

Automatic Observed Generation Handling #

Having .observedGeneration value on the status of the resource is a best practice to indicate the last generation of the resource reconciled successfully by the controller. This helps the users / administrators to check if the custom resource was reconciled.

In order to have this feature working:

  • the status class (not the resource) must implement the ObservedGenerationAware interface. See also the ObservedGenerationAwareStatus which can also be extended.
  • The other condition is that the CustomResource.getStatus() should not return null. So the status should be instantiated when the object is returned using the UpdateControl.

If these conditions are fulfilled and generation awareness not turned off, the observed generation is automatically set by the framework after the reconcile method is called. Note that the observed generation is updated also when UpdateControl.noUpdate() is returned from the reconciler. See this feature working in the WebPage example .

public class WebPageStatus extends ObservedGenerationAwareStatus {

    private String htmlConfigMap;

Initializing status on custom resource:

public class WebPage extends CustomResource<WebPageSpec, WebPageStatus>
    implements Namespaced {

  protected WebPageStatus initStatus() {
    return new WebPageStatus();

Generation Awareness and Event Filtering #

On an operator startup, the best practice is to reconcile all the resources. Since while operator was down, changes might have made both to custom resource and dependent resources.

When the first reconciliation is done successfully, the next reconciliation is triggered if either the dependent resources are changed or the custom resource .spec is changed. If other fields like .metadata is changed on the custom resource, the reconciliation could be skipped. This is supported out of the box, thus the reconciliation by default is not triggered if the change to the main custom resource does not increase the .metadata.generation field. Note that the increase of .metada.generation is handled automatically by Kubernetes.

To turn off this feature set generationAwareEventProcessing to false for the Reconciler.

Support for Well Known (non-custom) Kubernetes Resources #

A Controller can be registered for a non-custom resource, so well known Kubernetes resources like ( Ingress,Deployment,…). Note that automatic observed generation handling is not supported for these resources. Although in case adding a secondary controller for well known k8s resource, probably the observed generation should be handled by the primary controller.

See the integration test for reconciling deployments.

public class DeploymentReconciler
    implements Reconciler<Deployment>, TestExecutionInfoProvider {

  public UpdateControl<Deployment> reconcile(
      Deployment resource, Context context) {

Max Interval Between Reconciliations #

In case informers are all in place and reconciler is implemented correctly, there is no need for additional triggers. However, it’s a common practice to have a failsafe periodic trigger in place, just to make sure the resources are reconciled after certain time. This functionality is in place by default, there is quite high interval (currently 10 hours) while the reconciliation is triggered. See how to override this using the standard annotation:

@ControllerConfiguration(finalizerName = NO_FINALIZER,
        reconciliationMaxInterval = @ReconciliationMaxInterval(
                interval = 50,
                timeUnit = TimeUnit.MILLISECONDS))

The event is not propagated in a fixed rate, rather it’s scheduled after each reconciliation. So the next reconciliation will after at most within the specified interval after last reconciliation.

This feature can be turned off by setting reconciliationMaxInterval to Constants.NO_RECONCILIATION_MAX_INTERVAL or any non-positive number.

The automatic retries are not affected by this feature, in case of an error no schedule is set by this feature.

Automatic Retries on Error #

When an exception is thrown from a controller, the framework will schedule an automatic retry of the reconciliation. The retry is behavior is configurable, an implementation is provided that should cover most of the use-cases, see GenericRetry . But it is possible to provide a custom implementation.

It is possible to set a limit on the number of retries. In the Context object information is provided about the retry, particularly interesting is the isLastAttempt, since a different behavior could be implemented based on this flag. Like setting an error message in the status in case of a last attempt;


Event if the retry reached a limit, in case of a new event is received the reconciliation would happen again, it’s just won’t be a result of a retry, but the new event. However, in case of an error happens also in this case, it won’t schedule a retry is at this point the retry limit is already reached.

A successful execution resets the retry.

Setting Error Status After Last Retry Attempt #

In order to facilitate error reporting Reconciler can implement the following interface:

public interface ErrorStatusHandler<P extends HasMetadata> {

    ErrorStatusUpdateControl<P> updateErrorStatus(P resource, Context<P> context, Exception e);


The updateErrorStatus method is called in case an exception is thrown from the reconciler. It is also called when there is no retry configured, just after the reconciler execution. In the first call the RetryInfo.getAttemptCount() is always zero, since it is not a result of a retry (regardless if retry is configured or not).

The result of the method call is used to make a status update on the custom resource. This is always a sub-resource update request, so no update on custom resource itself (like spec of metadata) happens. Note that this update request will also produce an event, and will result in a reconciliation if the controller is not generation aware.

The scope of this feature is only the reconcile method of the reconciler, since there should not be updates on custom resource after it is marked for deletion.

Retry can be skipped for the cases of unrecoverable errors:


Correctness and Automatic Retries #

There is a possibility to turn off the automatic retries. This is not desirable, unless there is a very specific reason. Errors naturally happen, typically network errors can cause some temporal issues, another case is when a custom resource is updated during the reconciliation (using kubectl for example), in this case if an update of the custom resource from the controller (using UpdateControl) would fail on a conflict. The automatic retries covers these cases and will result in a reconciliation, even if normally an event would not be processed as a result of a custom resource update from previous example (like if there is no generation update as a result of the change and generation filtering is turned on)

Retry and Rescheduling and Event Handling Common Behavior #

Retry, reschedule and standard event processing forms a relatively complex system, where these functionalities are not independent of each other. In the following we describe the behavior in this section, so it is easier to understand the intersections:

  1. A successful execution resets a retry and the rescheduled executions which were present before the reconciliation. However, a new rescheduling can be instructed from the reconciliation outcome (UpdateControl or DeleteControl).
  2. In case an exception happened, and a retry is initiated, but an event received meanwhile, then reconciliation will be executed instantly, and this execution won’t count as a retry attempt.
  3. If the retry limit is reached (so no more automatic retry would happen), but a new event received, the reconciliation will still happen, but won’t reset the retry, will be still marked as the last attempt in the retry info. The point (1) still holds, but in case of an error, no retry will happen.

See also this blog post.

Event sources are a relatively simple yet powerful and extensible concept to trigger controller executions. Usually based on changes of dependent resources. To solve the mentioned problems above, de-facto we watch resources we manage for changes, and reconcile the state if a resource is changed. Note that resources we are watching can be Kubernetes and also non-Kubernetes objects. Typically, in case of non-Kubernetes objects or services we can extend our operator to handle webhooks or websockets or to react to any event coming from a service we interact with. What happens is when we create a dependent resource we also register an Event Source that will propagate events regarding the changes of that resource. This way we avoid the need of polling, and can implement controllers very efficiently.

Alt text for broken image link

There are few interesting points here: The CustomResourceEvenSource event source is a special one, which sends events regarding changes of our custom resource, this is an event source which is always registered for every controller by default. An event is always related to a custom resource. Concurrency is still handled for you, thus we still guarantee that there is no concurrent execution of the controller for the same custom resource ( there is parallel execution if an event is related to another custom resource instance).

Caching and Event Sources #

Typically, when we work with Kubernetes (but possibly with others), we manage the objects in a declarative way. This is true also for Event Sources. For example if we watch for changes of a Kubernetes Deployment object in the InformerEventSource, we always receive the whole object from the Kubernetes API. Later when we try to reconcile in the controller (not using events) we would like to check the state of this deployment (but also other dependent resources), we could read the object again from Kubernetes API. However since we watch for the changes, we know that we always receive the most up-to-date version in the Event Source. So naturally, what we can do is cache the latest received objects (in the Event Source) and read it from there if needed. This is the preferred way, since it reduces the number of requests to Kubernetes API server, and leads to faster reconciliation cycles.

Note that when an operator starts and the first reconciliation is executed the caches are already populated for example
for InformerEventSource. Currently, this is not true however for PerResourceEventSource, where the cache might or might not be populated. To handle this situation elegantly methods are provided which checks the object in cache, if not found tries to get it from the supplier. See related method .

Registering Event Sources #

To register event sources Reconciler has to implement EventSourceInitializer interface and init a list of event sources to register. The easiest way to see it is on tomcat example (irrelevant details omitted):

public class TomcatReconciler implements Reconciler<Tomcat>, EventSourceInitializer<Tomcat> {

    public List<EventSource> prepareEventSources(EventSourceContext<Tomcat> context) {
        SharedIndexInformer<Deployment> deploymentInformer =
                        .withLabel("app.kubernetes.io/managed-by", "tomcat-operator")

        return List.of(
                new InformerEventSource<>(deploymentInformer, d -> {
                    var ownerReferences = d.getMetadata().getOwnerReferences();
                    if (!ownerReferences.isEmpty()) {
                        return Set.of(new ResourceID(ownerReferences.get(0).getName(), d.getMetadata().getNamespace()));
                    } else {
                        return EMPTY_SET;

In the example above an InformerEventSource is registered (more on this specific eventsource later). Multiple things are going on here:

  1. An SharedIndexInformer (class from fabric8 Kubernetes client) is created. This will watch and produce events for Deployments in every namespace, but will filter them based on label. So Deployments which are not managed by tomcat-operator (the label is not present on them) will not trigger a reconciliation.
  2. In the next step an InformerEventSource is created, which wraps the SharedIndexInformer. In addition to that a mapping functions is provided, this maps the event of the watched resource (in this case Deployment) to the custom resources to reconcile. Not that in this case this is a simple task, since Deployment is already created with an owner reference. Therefore, the ResourceID what identifies the custom resource to reconcile is created from the owner reference.

Note that a set of ResourceID is returned, this is usually just a set with one element. The possibility to specify multiple values are there to cover some rare corner cases. If an irrelevant resource is observed, an empty set can be returned to not reconcile any custom resource.

Built-in EventSources #

There are multiple event-sources provided out of the box, the following are some more central ones:

  1. InformerEventSource - is there to cover events for all Kubernetes resources. Provides also a cache to use during the reconciliation. Basically no other event source required to watch Kubernetes resources.
  2. PerResourcePollingEventSource - is used to poll external API, which don’t support webhooks or other event notifications. It extends the abstract CachingEventSource to support caching. See MySQL Schema sample for usage.
  3. PollingEventSource is similar to PerResourceCachingEventSource only it not polls a specific API separately per custom resource, but periodically and independently of actually observed custom resources.
  4. SimpleInboundEventSource and CachingInboundEventSource is used to handle incoming events from webhooks and messaging systems.
  5. ControllerResourceEventSource - an eventsource that is automatically registered to listen to the changes of the main resource the operation manages, it also maintains a cache of those objects that can be accessed from the Reconciler.

More on the philosophy of the non Kubernetes API related event source see in issue #729.

Contextual Info for Logging with MDC #

Logging is enhanced with additional contextual information using MDC. This following attributes are available in most parts of reconciliation logic and during the execution of the controller:

MDC Key Value added from Custom Resource
resource.apiVersion .apiVersion
resource.kind .kind
resource.name .metadata.name
resource.namespace .metadata.namespace
resource.resourceVersion .metadata.resourceVersion
resource.generation .metadata.generation
resource.uid .metadata.uid

For more information about MDC see this link.

Dynamically Changing Target Namespaces #

A controller can be configured to watch a specific set of namespaces in addition of the namespace in which it is currently deployed or the whole cluster. The framework supports dynamically changing the list of these namespaces while the operator is running. When a reconciler is registered, an instance of RegisteredController is returned, providing access to the methods allowing users to change watched namespaces as the operator is running.

A typical scenario would probably involve extracting the list of target namespaces from a ConfigMap or some other input but this part is out of the scope of the framework since this is use-case specific. For example, reacting to changes to a ConfigMap would probably involve registering an associated Informer and then calling the changeNamespaces method on RegisteredController.

  public static void main(String[] args) throws IOException {
    KubernetesClient client = new DefaultKubernetesClient();
    Operator operator = new Operator(client);
    RegisteredController registeredController = operator.register(new WebPageReconciler(client));
    // call registeredController further while operator is running

If watched namespaces change for a controller, it might be desirable to propagate these changes to InformerEventSources associated with the controller. In order to express this, InformerEventSource implementations interested in following such changes need to be configured appropriately so that the followControllerNamespaceChanges method returns true:

public class MyReconciler
        implements Reconciler<TestCustomResource>, EventSourceInitializer<TestCustomResource>{

  public Map<String, EventSource> prepareEventSources(
      EventSourceContext<ChangeNamespaceTestCustomResource> context) {

    InformerEventSource<ConfigMap, TestCustomResource> configMapES =
        new InformerEventSource<>(InformerConfiguration.from(ConfigMap.class)
            .build(), context);

    return EventSourceInitializer.nameEventSources(configMapES);

As seen in the above code snippet, the informer will have the initial namespaces inherited from controller, but also will adjust the target namespaces if it changes for the controller.

See also the integration test for this feature.

Monitoring with Micrometer #

Automatic Generation of CRDs #

Note that this is feature of Fabric8 Kubernetes Client not the JOSDK. But it’s worth to mention here.

To automatically generate CRD manifests from your annotated Custom Resource classes, you only need to add the following dependencies to your project:


The CRD will be generated in target/classes/META-INF/fabric8 (or in target/test-classes/META-INF/fabric8, if you use the test scope) with the CRD name suffixed by the generated spec version. For example, a CR using the java-operator-sdk.io group with a mycrs plural form will result in 2 files:

  • mycrs.java-operator-sdk.io-v1.yml
  • mycrs.java-operator-sdk.io-v1beta1.yml


Quarkus users using the quarkus-operator-sdk extension do not need to add any extra dependency to get their CRD generated as this is handled by the extension itself.