docs: event filter page (#3467)

Signed-off-by: Attila Mészáros <a_meszaros@apple.com>

Author: csviri

docs/content/en/docs/documentation/_index.md

@@ -23,6 +23,7 @@ This section contains detailed documentation for all Java Operator SDK features
 ## Advanced Features
 
 - **[Eventing](eventing/)** - Understanding the event-driven model
+- **[Event filters](event-filters/)** - Filtering events and the default filter behavior
 - **[Accessing Resources in Caches](working-with-es-caches/)** - How to access resources in caches
 - **[Operations](operations/)** - Helm chart, metrics, logging, configurations, leader election 
 - **[Other Features](features/)** - Additional capabilities and integrations

docs/content/en/docs/documentation/event-filters.md

@@ -0,0 +1,170 @@
+---
+title: Event filters
+weight: 50
+---
+
+Every [event source](eventing.md) that watches Kubernetes resources is backed by an informer that
+receives *add*, *update* and *delete* events from the Kubernetes API. Not every one of those events
+is worth a reconciliation: a resource might be touched by another controller, its `.metadata` might
+change without any meaningful change to its `.spec`, or your own controller might have caused the
+change in the first place. **Event filters** let you decide, per event, whether it should be
+propagated and (potentially) trigger a reconciliation. Filtering events as early as possible keeps
+your operator efficient by avoiding needless reconciliations.
+
+Filters apply both to the **primary resource** (the resource your `Reconciler` manages) and to any
+**secondary resources** watched through an [`InformerEventSource`](eventing.md#informereventsource).
+
+## Filter types
+
+JOSDK defines four functional interfaces in the
+[`io.javaoperatorsdk.operator.processing.event.source.filter`](https://github.com/operator-framework/java-operator-sdk/tree/main/operator-framework-core/src/main/java/io/javaoperatorsdk/operator/processing/event/source/filter)
+package. Each returns `true` when the event should be **accepted** (propagated) and `false` when it
+should be **dropped**:
+
+| Filter | Applies to | Signature |
+| --- | --- | --- |
+| [`OnAddFilter`](https://github.com/operator-framework/java-operator-sdk/blob/main/operator-framework-core/src/main/java/io/javaoperatorsdk/operator/processing/event/source/filter/OnAddFilter.java) | resource add events | `boolean accept(R resource)` |
+| [`OnUpdateFilter`](https://github.com/operator-framework/java-operator-sdk/blob/main/operator-framework-core/src/main/java/io/javaoperatorsdk/operator/processing/event/source/filter/OnUpdateFilter.java) | resource update events | `boolean accept(R newResource, R oldResource)` |
+| [`OnDeleteFilter`](https://github.com/operator-framework/java-operator-sdk/blob/main/operator-framework-core/src/main/java/io/javaoperatorsdk/operator/processing/event/source/filter/OnDeleteFilter.java) | resource delete events | `boolean accept(R resource, Boolean deletedFinalStateUnknown)` |
+| [`GenericFilter`](https://github.com/operator-framework/java-operator-sdk/blob/main/operator-framework-core/src/main/java/io/javaoperatorsdk/operator/processing/event/source/filter/GenericFilter.java) | add, update **and** delete events | `boolean accept(R resource)` |
+
+`GenericFilter` is applied to every kind of event, so it is a convenient way to express a condition
+that is independent of the event type. When both a specific filter (e.g. `OnUpdateFilter`) and the
+`GenericFilter` are configured, **both** must accept the event for it to be propagated.
+
+All four interfaces are `@FunctionalInterface`s and provide `and(...)`, `or(...)` and `not()`
+default methods, so you can compose several conditions:
+
+```java
+OnUpdateFilter<MyCustomResource> filter =
+    onUpdateFilterA.and(onUpdateFilterB).not();
+```
+
+## Configuring filters on the primary resource
+
+Filters for the primary resource are configured through the `@Informer` annotation nested in
+`@ControllerConfiguration`. You reference the filter *class* (which must have an accessible no-arg
+constructor):
+
+```java
+@ControllerConfiguration(informer = @Informer(onUpdateFilter = UpdateFilter.class))
+public class MyReconciler implements Reconciler<MyCustomResource> {
+  // ...
+}
+```
+
+```java
+public class UpdateFilter implements OnUpdateFilter<MyCustomResource> {
+  @Override
+  public boolean accept(MyCustomResource newResource, MyCustomResource oldResource) {
+    // reconcile only if the value actually changed
+    return !newResource.getSpec().getValue().equals(SKIP_VALUE);
+  }
+}
+```
+
+The `@Informer` annotation exposes `onAddFilter`, `onUpdateFilter`, `onDeleteFilter` and
+`genericFilter` attributes for the primary resource informer.
+
+## Configuring filters on secondary resources
+
+For secondary resources watched via an `InformerEventSource`, filters are set through the
+`InformerEventSourceConfiguration` builder, where you can pass filter instances directly (typically
+as lambdas):
+
+```java
+@Override
+public List<EventSource<?, MyCustomResource>> prepareEventSources(
+    EventSourceContext<MyCustomResource> context) {
+
+  var informerConfiguration =
+      InformerEventSourceConfiguration.from(ConfigMap.class, MyCustomResource.class)
+          .withOnUpdateFilter(
+              (newCM, oldCM) -> !newCM.getData().get(VALUE_KEY).equals(SKIP_VALUE))
+          .withOnAddFilter(cm -> true)
+          .build();
+
+  return List.of(new InformerEventSource<>(informerConfiguration, context));
+}
+```
+
+The builder provides `withOnAddFilter`, `withOnUpdateFilter`, `withOnDeleteFilter` and
+`withGenericFilter`.
+
+{{% alert title="Selectors vs. filters" color="primary" %}}
+Filters run **client-side**, after the event has already been received from the API server. If you
+can express your condition as a label or field selector, prefer the `labelSelector` /
+`fieldSelector` attributes of `@Informer` (or the equivalent builder methods) instead: those are
+evaluated **server-side**, so filtered-out resources are never sent to the informer at all and are
+not even held in the cache. Use event filters for conditions that cannot be expressed as selectors,
+such as comparing the new and old versions of a resource.
+{{% /alert %}}
+
+## Default filters
+
+For the primary resource, JOSDK always applies a set of **internal update filters** on top of your
+own `onUpdateFilter`. These are what make sure your `Reconciler` is not triggered for updates that
+don't require action. An update event is accepted by the default filters if **any** of the following
+is true (see
+[`InternalEventFilters`](https://github.com/operator-framework/java-operator-sdk/blob/main/operator-framework-core/src/main/java/io/javaoperatorsdk/operator/processing/event/source/controller/InternalEventFilters.java)):
+
+- **Generation aware:** the resource's `.metadata.generation` increased. Kubernetes bumps the
+  generation whenever the `.spec` changes, so this drops events caused purely by `.metadata` or
+  `.status` changes. This part is only active when
+  [generation-aware event processing](eventing.md#generation-awareness-and-event-filtering) is
+  enabled (the default for resources that support it). For resources without a generation (e.g.
+  `Pod`), every update is accepted.
+- **Finalizer needed:** the finalizer JOSDK manages was just added or removed. This ensures the
+  reconciliation that adds the finalizer is not filtered out.
+- **Marked for deletion:** the resource just transitioned to being marked for deletion (a deletion
+  timestamp was set), so cleanup logic can run.
+
+### How default filters combine with your filter
+
+The default (internal) update filter is combined with your `onUpdateFilter` using a logical **AND**:
+an update event must be accepted by **both** your filter and the internal filter to be propagated.
+In other words, your filter can only make the operator *more* selective; it cannot force a
+reconciliation for an event that the default filters would otherwise drop.
+
+Note that default filters only concern **update** events on the **primary** resource. Add events are
+always processed regardless of the internal filters, delete events are only subject to your
+`genericFilter` (if any), and secondary resources have no internal default filters at all.
+
+### Disabling the default filters
+
+If the AND-composition described above is too restrictive — for example, you need full control over
+which updates trigger a reconciliation — you can turn the internal update filters off with the
+`defaultFilters` attribute:
+
+```java
+@ControllerConfiguration(
+    defaultFilters = false,
+    informer = @Informer(onUpdateFilter = MyUpdateFilter.class))
+public class MyReconciler implements Reconciler<MyCustomResource> {
+  // ...
+}
+```
+
+With `defaultFilters = false`:
+
+- Your `onUpdateFilter` becomes the **sole** update filter and has full control over which update
+  events are propagated.
+- If you don't provide an `onUpdateFilter`, **all** update events are accepted.
+
+{{% alert title="Use with care" color="warning" %}}
+Disabling default filters removes the generation-aware, finalizer-needed and marked-for-deletion
+guarantees. If you still want part of that behavior, compose it explicitly using the static factory
+methods on
+[`InternalEventFilters`](https://github.com/operator-framework/java-operator-sdk/blob/main/operator-framework-core/src/main/java/io/javaoperatorsdk/operator/processing/event/source/controller/InternalEventFilters.java)
+inside your own filter.
+{{% /alert %}}
+
+## Non-informer based event sources
+
+The filters described above apply only to informer-backed event sources, since they operate on the
+*add*, *update* and *delete* events an informer receives from the Kubernetes API. Event sources that
+are not backed by an informer — such as [`PollingEventSource`](https://github.com/operator-framework/java-operator-sdk/blob/main/operator-framework-core/src/main/java/io/javaoperatorsdk/operator/processing/event/source/polling/PollingEventSource.java)
+and [`PerResourcePollingEventSource`](https://github.com/operator-framework/java-operator-sdk/blob/main/operator-framework-core/src/main/java/io/javaoperatorsdk/operator/processing/event/source/polling/PerResourcePollingEventSource.java)
+— have no filters. For those, it is up to your implementation to emit only the events that are
+actually relevant, so that reconciliations are not triggered needlessly.
+

operator-framework-core/src/main/java/io/javaoperatorsdk/operator/processing/event/source/filter/GenericFilter.java

@@ -15,19 +15,60 @@
  */
 package io.javaoperatorsdk.operator.processing.event.source.filter;
 
+/**
+ * Filter applied to every kind of event (add, update <b>and</b> delete) received by an
+ * informer-backed event source. It is a convenient way to express a condition that is independent
+ * of the event type.
+ *
+ * <p>When both a type-specific filter (such as {@link OnAddFilter}, {@link OnUpdateFilter} or
+ * {@link OnDeleteFilter}) and a {@code GenericFilter} are configured, <b>both</b> must accept an
+ * event for it to be propagated.
+ *
+ * <p>Filters only apply to informer-backed event sources; event sources that are not backed by an
+ * informer have no filters.
+ *
+ * @param <R> the type of the resource the events relate to
+ */
 @FunctionalInterface
 public interface GenericFilter<R> {
 
+  /**
+   * Decides whether an event concerning the given resource should be propagated.
+   *
+   * @param resource the resource the event relates to
+   * @return {@code true} if the event should be <b>accepted</b> (propagated), {@code false} if it
+   *     should be <b>dropped</b>
+   */
   boolean accept(R resource);
 
+  /**
+   * Composes this filter with another one using a logical <b>AND</b>: the resulting filter accepts
+   * an event only when both this filter and the given filter accept it.
+   *
+   * @param genericFilter the other filter to combine with this one
+   * @return a filter that accepts an event only if both filters accept it
+   */
   default GenericFilter<R> and(GenericFilter<R> genericFilter) {
     return (resource) -> this.accept(resource) && genericFilter.accept(resource);
   }
 
+  /**
+   * Composes this filter with another one using a logical <b>OR</b>: the resulting filter accepts
+   * an event when either this filter or the given filter accepts it.
+   *
+   * @param genericFilter the other filter to combine with this one
+   * @return a filter that accepts an event if at least one of the filters accepts it
+   */
   default GenericFilter<R> or(GenericFilter<R> genericFilter) {
     return (resource) -> this.accept(resource) || genericFilter.accept(resource);
   }
 
+  /**
+   * Negates this filter: the resulting filter accepts exactly the events this one drops, and drops
+   * the events this one accepts.
+   *
+   * @return a filter that returns the logical <b>NOT</b> of this filter's result
+   */
   default GenericFilter<R> not() {
     return (resource) -> !this.accept(resource);
   }

operator-framework-core/src/main/java/io/javaoperatorsdk/operator/processing/event/source/filter/OnAddFilter.java

@@ -15,18 +15,55 @@
  */
 package io.javaoperatorsdk.operator.processing.event.source.filter;
 
+/**
+ * Filter applied to resource <i>add</i> events received by an informer-backed event source. It lets
+ * you decide whether the addition of a resource should be propagated and (potentially) trigger a
+ * reconciliation.
+ *
+ * <p>Filters only apply to informer-backed event sources; event sources that are not backed by an
+ * informer have no filters.
+ *
+ * @param <R> the type of the added resource
+ */
 @FunctionalInterface
 public interface OnAddFilter<R> {
+  /**
+   * Decides whether the add event for the given resource should be propagated.
+   *
+   * @param resource the resource that was added
+   * @return {@code true} if the event should be <b>accepted</b> (propagated), {@code false} if it
+   *     should be <b>dropped</b>
+   */
   boolean accept(R resource);
 
+  /**
+   * Composes this filter with another one using a logical <b>AND</b>: the resulting filter accepts
+   * an event only when both this filter and the given filter accept it.
+   *
+   * @param onAddFilter the other filter to combine with this one
+   * @return a filter that accepts an event only if both filters accept it
+   */
   default OnAddFilter<R> and(OnAddFilter<R> onAddFilter) {
     return (resource) -> this.accept(resource) && onAddFilter.accept(resource);
   }
 
+  /**
+   * Composes this filter with another one using a logical <b>OR</b>: the resulting filter accepts
+   * an event when either this filter or the given filter accepts it.
+   *
+   * @param onAddFilter the other filter to combine with this one
+   * @return a filter that accepts an event if at least one of the filters accepts it
+   */
   default OnAddFilter<R> or(OnAddFilter<R> onAddFilter) {
     return (resource) -> this.accept(resource) || onAddFilter.accept(resource);
   }
 
+  /**
+   * Negates this filter: the resulting filter accepts exactly the events this one drops, and drops
+   * the events this one accepts.
+   *
+   * @return a filter that returns the logical <b>NOT</b> of this filter's result
+   */
   default OnAddFilter<R> not() {
     return (resource) -> !this.accept(resource);
   }

operator-framework-core/src/main/java/io/javaoperatorsdk/operator/processing/event/source/filter/OnDeleteFilter.java

@@ -15,23 +15,63 @@
  */
 package io.javaoperatorsdk.operator.processing.event.source.filter;
 
+/**
+ * Filter applied to resource <i>delete</i> events received by an informer-backed event source. It
+ * lets you decide whether the deletion of a resource should be propagated and (potentially) trigger
+ * a reconciliation.
+ *
+ * <p>Filters only apply to informer-backed event sources; event sources that are not backed by an
+ * informer have no filters.
+ *
+ * @param <R> the type of the deleted resource
+ */
 @FunctionalInterface
 public interface OnDeleteFilter<R> {
 
+  /**
+   * Decides whether the delete event for the given resource should be propagated.
+   *
+   * @param hasMetadata the resource that was deleted
+   * @param deletedFinalStateUnknown {@code true} when the informer missed the delete event and the
+   *     final state of the resource is unknown (the object may be stale), {@code false} when the
+   *     deletion was observed with the resource's last known state
+   * @return {@code true} if the event should be <b>accepted</b> (propagated), {@code false} if it
+   *     should be <b>dropped</b>
+   */
   boolean accept(R hasMetadata, Boolean deletedFinalStateUnknown);
 
+  /**
+   * Composes this filter with another one using a logical <b>AND</b>: the resulting filter accepts
+   * an event only when both this filter and the given filter accept it.
+   *
+   * @param OnDeleteFilter the other filter to combine with this one
+   * @return a filter that accepts an event only if both filters accept it
+   */
   default OnDeleteFilter<R> and(OnDeleteFilter<R> OnDeleteFilter) {
     return (resource, deletedFinalStateUnknown) ->
         this.accept(resource, deletedFinalStateUnknown)
             && OnDeleteFilter.accept(resource, deletedFinalStateUnknown);
   }
 
+  /**
+   * Composes this filter with another one using a logical <b>OR</b>: the resulting filter accepts
+   * an event when either this filter or the given filter accepts it.
+   *
+   * @param OnDeleteFilter the other filter to combine with this one
+   * @return a filter that accepts an event if at least one of the filters accepts it
+   */
   default OnDeleteFilter<R> or(OnDeleteFilter<R> OnDeleteFilter) {
     return (resource, deletedFinalStateUnknown) ->
         this.accept(resource, deletedFinalStateUnknown)
             || OnDeleteFilter.accept(resource, deletedFinalStateUnknown);
   }
 
+  /**
+   * Negates this filter: the resulting filter accepts exactly the events this one drops, and drops
+   * the events this one accepts.
+   *
+   * @return a filter that returns the logical <b>NOT</b> of this filter's result
+   */
   default OnDeleteFilter<R> not() {
     return (resource, deletedFinalStateUnknown) -> !this.accept(resource, deletedFinalStateUnknown);
   }