opendaylight/md-sal/sal-inmemory-datastore/src/main/java/org/opendaylight/controller/md/sal/dom/store/impl/ResolveDataChangeEventsTask.java

   1 /*
   2  * Copyright (c) 2014 Cisco Systems, Inc. and others.  All rights reserved.
   3  *
   4  * This program and the accompanying materials are made available under the
   5  * terms of the Eclipse Public License v1.0 which accompanies this distribution,
   6  * and is available at http://www.eclipse.org/legal/epl-v10.html
   7  */
   8 package org.opendaylight.controller.md.sal.dom.store.impl;
   9
  10 import com.google.common.base.Optional;
  11 import com.google.common.base.Preconditions;
  12 import com.google.common.collect.HashMultimap;
  13 import com.google.common.collect.ImmutableList;
  14 import com.google.common.collect.Iterables;
  15 import com.google.common.collect.Multimap;
  16 import org.opendaylight.controller.md.sal.common.api.data.AsyncDataBroker.DataChangeScope;
  17 import org.opendaylight.controller.md.sal.dom.store.impl.DOMImmutableDataChangeEvent.Builder;
  18 import org.opendaylight.controller.md.sal.dom.store.impl.DOMImmutableDataChangeEvent.SimpleEventFactory;
  19 import org.opendaylight.controller.md.sal.dom.store.impl.tree.ListenerTree;
  20 import org.opendaylight.controller.md.sal.dom.store.impl.tree.ListenerTree.Node;
  21 import org.opendaylight.controller.md.sal.dom.store.impl.tree.ListenerTree.Walker;
  22 import org.opendaylight.yangtools.yang.data.api.InstanceIdentifier;
  23 import org.opendaylight.yangtools.yang.data.api.InstanceIdentifier.NodeIdentifier;
  24 import org.opendaylight.yangtools.yang.data.api.InstanceIdentifier.NodeIdentifierWithPredicates;
  25 import org.opendaylight.yangtools.yang.data.api.InstanceIdentifier.NodeWithValue;
  26 import org.opendaylight.yangtools.yang.data.api.InstanceIdentifier.PathArgument;
  27 import org.opendaylight.yangtools.yang.data.api.schema.NormalizedNode;
  28 import org.opendaylight.yangtools.yang.data.api.schema.NormalizedNodeContainer;
  29 import org.opendaylight.yangtools.yang.data.api.schema.tree.DataTreeCandidate;
  30 import org.opendaylight.yangtools.yang.data.api.schema.tree.DataTreeCandidateNode;
  31 import org.opendaylight.yangtools.yang.data.api.schema.tree.ModificationType;
  32 import org.slf4j.Logger;
  33 import org.slf4j.LoggerFactory;
  34
  35 import java.util.Collection;
  36 import java.util.Collections;
  37 import java.util.HashSet;
  38 import java.util.LinkedList;
  39 import java.util.List;
  40 import java.util.Map.Entry;
  41 import java.util.Set;
  42 import java.util.concurrent.Callable;
  43
  44 import static org.opendaylight.controller.md.sal.dom.store.impl.DOMImmutableDataChangeEvent.builder;
  45
  46 /**
  47  * Resolve Data Change Events based on modifications and listeners
  48  *
  49  * Computes data change events for all affected registered listeners in data
  50  * tree.
  51  */
  52 final class ResolveDataChangeEventsTask implements Callable<Iterable<ChangeListenerNotifyTask>> {
  53     private static final Logger LOG = LoggerFactory.getLogger(ResolveDataChangeEventsTask.class);
  54     private static final DOMImmutableDataChangeEvent NO_CHANGE = builder(DataChangeScope.BASE).build();
  55
  56     private final Multimap<ListenerTree.Node, DOMImmutableDataChangeEvent> events = HashMultimap.create();
  57     private final DataTreeCandidate candidate;
  58     private final ListenerTree listenerRoot;
  59
  60     public ResolveDataChangeEventsTask(final DataTreeCandidate candidate, final ListenerTree listenerTree) {
  61         this.candidate = Preconditions.checkNotNull(candidate);
  62         this.listenerRoot = Preconditions.checkNotNull(listenerTree);
  63     }
  64
  65     /**
  66      * Resolves and creates Notification Tasks
  67      *
  68      * Implementation of done as Map-Reduce with two steps: 1. resolving events
  69      * and their mapping to listeners 2. merging events affecting same listener
  70      *
  71      * @return An {@link Iterable} of Notification Tasks which needs to be executed in
  72      *         order to delivery data change events.
  73      */
  74     @Override
  75     public Iterable<ChangeListenerNotifyTask> call() {
  76         try (final Walker w = listenerRoot.getWalker()) {
  77             resolveAnyChangeEvent(candidate.getRootPath(), Collections.singleton(w.getRootNode()), candidate.getRootNode());
  78             return createNotificationTasks();
  79         }
  80     }
  81
  82     /**
  83      *
  84      * Walks map of listeners to data change events, creates notification
  85      * delivery tasks.
  86      *
  87      * Walks map of registered and affected listeners and creates notification
  88      * tasks from set of listeners and events to be delivered.
  89      *
  90      * If set of listeners has more then one event (applicable to wildcarded
  91      * listeners), merges all data change events into one, final which contains
  92      * all separate updates.
  93      *
  94      * Dispatch between merge variant and reuse variant of notification task is
  95      * done in
  96      * {@link #addNotificationTask(com.google.common.collect.ImmutableList.Builder, Node, java.util.Collection)}
  97      *
  98      * @return Collection of notification tasks.
  99      */
 100     private Collection<ChangeListenerNotifyTask> createNotificationTasks() {
 101         ImmutableList.Builder<ChangeListenerNotifyTask> taskListBuilder = ImmutableList.builder();
 102         for (Entry<ListenerTree.Node, Collection<DOMImmutableDataChangeEvent>> entry : events.asMap().entrySet()) {
 103             addNotificationTask(taskListBuilder, entry.getKey(), entry.getValue());
 104         }
 105         return taskListBuilder.build();
 106     }
 107
 108     /**
 109      * Adds notification task to task list.
 110      *
 111      * If entry collection contains one event, this event is reused and added to
 112      * notification tasks for listeners (see
 113      * {@link #addNotificationTaskByScope(com.google.common.collect.ImmutableList.Builder, Node, DOMImmutableDataChangeEvent)}
 114      * . Otherwise events are merged by scope and distributed between listeners
 115      * to particular scope. See
 116      * {@link #addNotificationTasksAndMergeEvents(com.google.common.collect.ImmutableList.Builder, Node, java.util.Collection)}
 117      * .
 118      *
 119      * @param taskListBuilder
 120      * @param listeners
 121      * @param entries
 122      */
 123     private static void addNotificationTask(final ImmutableList.Builder<ChangeListenerNotifyTask> taskListBuilder,
 124             final ListenerTree.Node listeners, final Collection<DOMImmutableDataChangeEvent> entries) {
 125
 126         if (!entries.isEmpty()) {
 127             if (entries.size() == 1) {
 128                 addNotificationTaskByScope(taskListBuilder, listeners, Iterables.getOnlyElement(entries));
 129             } else {
 130                 addNotificationTasksAndMergeEvents(taskListBuilder, listeners, entries);
 131             }
 132         }
 133     }
 134
 135     /**
 136      *
 137      * Add notification deliveries task to the listener.
 138      *
 139      *
 140      * @param taskListBuilder
 141      * @param listeners
 142      * @param event
 143      */
 144     private static void addNotificationTaskByScope(
 145             final ImmutableList.Builder<ChangeListenerNotifyTask> taskListBuilder, final ListenerTree.Node listeners,
 146             final DOMImmutableDataChangeEvent event) {
 147         DataChangeScope eventScope = event.getScope();
 148         for (DataChangeListenerRegistration<?> listenerReg : listeners.getListeners()) {
 149             DataChangeScope listenerScope = listenerReg.getScope();
 150             List<DataChangeListenerRegistration<?>> listenerSet = Collections
 151                     .<DataChangeListenerRegistration<?>> singletonList(listenerReg);
 152             if (eventScope == DataChangeScope.BASE) {
 153                 taskListBuilder.add(new ChangeListenerNotifyTask(listenerSet, event));
 154             } else if (eventScope == DataChangeScope.ONE && listenerScope != DataChangeScope.BASE) {
 155                 taskListBuilder.add(new ChangeListenerNotifyTask(listenerSet, event));
 156             } else if (eventScope == DataChangeScope.SUBTREE && listenerScope == DataChangeScope.SUBTREE) {
 157                 taskListBuilder.add(new ChangeListenerNotifyTask(listenerSet, event));
 158             }
 159         }
 160     }
 161
 162     /**
 163      *
 164      * Add notification tasks with merged event
 165      *
 166      * Separate Events by scope and creates merged notification tasks for each
 167      * and every scope which is present.
 168      *
 169      * Adds merged events to task list based on scope requested by client.
 170      *
 171      * @param taskListBuilder
 172      * @param listeners
 173      * @param entries
 174      */
 175     private static void addNotificationTasksAndMergeEvents(
 176             final ImmutableList.Builder<ChangeListenerNotifyTask> taskListBuilder, final ListenerTree.Node listeners,
 177             final Collection<DOMImmutableDataChangeEvent> entries) {
 178
 179         final Builder baseBuilder = builder(DataChangeScope.BASE);
 180         final Builder oneBuilder = builder(DataChangeScope.ONE);
 181         final Builder subtreeBuilder = builder(DataChangeScope.SUBTREE);
 182
 183         boolean baseModified = false;
 184         boolean oneModified = false;
 185         boolean subtreeModified = false;
 186         for (final DOMImmutableDataChangeEvent entry : entries) {
 187             switch (entry.getScope()) {
 188             // Absence of breaks is intentional here. Subtree contains base and
 189             // one, one also contains base
 190             case BASE:
 191                 baseBuilder.merge(entry);
 192                 baseModified = true;
 193             case ONE:
 194                 oneBuilder.merge(entry);
 195                 oneModified = true;
 196             case SUBTREE:
 197                 subtreeBuilder.merge(entry);
 198                 subtreeModified = true;
 199             }
 200         }
 201
 202         if (baseModified) {
 203             addNotificationTaskExclusively(taskListBuilder, listeners, baseBuilder.build());
 204         }
 205         if (oneModified) {
 206             addNotificationTaskExclusively(taskListBuilder, listeners, oneBuilder.build());
 207         }
 208         if (subtreeModified) {
 209             addNotificationTaskExclusively(taskListBuilder, listeners, subtreeBuilder.build());
 210         }
 211     }
 212
 213     private static void addNotificationTaskExclusively(
 214             final ImmutableList.Builder<ChangeListenerNotifyTask> taskListBuilder, final Node listeners,
 215             final DOMImmutableDataChangeEvent event) {
 216         for (DataChangeListenerRegistration<?> listener : listeners.getListeners()) {
 217             if (listener.getScope() == event.getScope()) {
 218                 Set<DataChangeListenerRegistration<?>> listenerSet = Collections
 219                         .<DataChangeListenerRegistration<?>> singleton(listener);
 220                 taskListBuilder.add(new ChangeListenerNotifyTask(listenerSet, event));
 221             }
 222         }
 223     }
 224
 225     /**
 226      * Resolves data change event for supplied node
 227      *
 228      * @param path
 229      *            Path to current node in tree
 230      * @param listeners
 231      *            Collection of Listener registration nodes interested in
 232      *            subtree
 233      * @param modification
 234      *            Modification of current node
 235      * @param before
 236      *            - Original (before) state of current node
 237      * @param after
 238      *            - After state of current node
 239      * @return Data Change Event of this node and all it's children
 240      */
 241     private DOMImmutableDataChangeEvent resolveAnyChangeEvent(final InstanceIdentifier path,
 242             final Collection<ListenerTree.Node> listeners, final DataTreeCandidateNode node) {
 243
 244         if (node.getModificationType() != ModificationType.UNMODIFIED &&
 245                 !node.getDataAfter().isPresent() && !node.getDataBefore().isPresent()) {
 246             LOG.debug("Modification at {} has type {}, but no before- and after-data. Assuming unchanged.",
 247                     path, node.getModificationType());
 248             return NO_CHANGE;
 249         }
 250
 251         // no before and after state is present
 252
 253         switch (node.getModificationType()) {
 254         case SUBTREE_MODIFIED:
 255             return resolveSubtreeChangeEvent(path, listeners, node);
 256         case MERGE:
 257         case WRITE:
 258             Preconditions.checkArgument(node.getDataAfter().isPresent(),
 259                     "Modification at {} has type {} but no after-data", path, node.getModificationType());
 260             if (node.getDataBefore().isPresent()) {
 261                 return resolveReplacedEvent(path, listeners, node.getDataBefore().get(), node.getDataAfter().get());
 262             } else {
 263                 return resolveCreateEvent(path, listeners, node.getDataAfter().get());
 264             }
 265         case DELETE:
 266             Preconditions.checkArgument(node.getDataBefore().isPresent(),
 267                     "Modification at {} has type {} but no before-data", path, node.getModificationType());
 268             return resolveDeleteEvent(path, listeners, node.getDataBefore().get());
 269         case UNMODIFIED:
 270             return NO_CHANGE;
 271         }
 272
 273         throw new IllegalStateException(String.format("Unhandled node state %s at %s", node.getModificationType(), path));
 274     }
 275
 276     private DOMImmutableDataChangeEvent resolveReplacedEvent(final InstanceIdentifier path,
 277             final Collection<Node> listeners, final NormalizedNode<?, ?> beforeData,
 278             final NormalizedNode<?, ?> afterData) {
 279
 280         if (beforeData instanceof NormalizedNodeContainer<?, ?, ?>) {
 281             // Node is container (contains child) and we have interested
 282             // listeners registered for it, that means we need to do
 283             // resolution of changes on children level and can not
 284             // shortcut resolution.
 285             LOG.trace("Resolving subtree replace event for {} before {}, after {}",path,beforeData,afterData);
 286             @SuppressWarnings("unchecked")
 287             NormalizedNodeContainer<?, PathArgument, NormalizedNode<PathArgument, ?>> beforeCont = (NormalizedNodeContainer<?, PathArgument, NormalizedNode<PathArgument, ?>>) beforeData;
 288             @SuppressWarnings("unchecked")
 289             NormalizedNodeContainer<?, PathArgument, NormalizedNode<PathArgument, ?>> afterCont = (NormalizedNodeContainer<?, PathArgument, NormalizedNode<PathArgument, ?>>) afterData;
 290             return resolveNodeContainerReplaced(path, listeners, beforeCont, afterCont);
 291         } else if (!beforeData.equals(afterData)) {
 292             // Node is Leaf type (does not contain child nodes)
 293             // so normal equals method is sufficient for determining change.
 294             LOG.trace("Resolving leaf replace event for {} , before {}, after {}",path,beforeData,afterData);
 295             DOMImmutableDataChangeEvent event = builder(DataChangeScope.BASE).setBefore(beforeData).setAfter(afterData)
 296                     .addUpdated(path, beforeData, afterData).build();
 297             addPartialTask(listeners, event);
 298             return event;
 299         } else {
 300             return NO_CHANGE;
 301         }
 302     }
 303
 304     private DOMImmutableDataChangeEvent resolveNodeContainerReplaced(final InstanceIdentifier path,
 305             final Collection<Node> listeners,
 306             final NormalizedNodeContainer<?, PathArgument, NormalizedNode<PathArgument, ?>> beforeCont,
 307                     final NormalizedNodeContainer<?, PathArgument, NormalizedNode<PathArgument, ?>> afterCont) {
 308         final Set<PathArgument> alreadyProcessed = new HashSet<>();
 309         final List<DOMImmutableDataChangeEvent> childChanges = new LinkedList<>();
 310
 311         DataChangeScope potentialScope = DataChangeScope.BASE;
 312         // We look at all children from before and compare it with after state.
 313         for (NormalizedNode<PathArgument, ?> beforeChild : beforeCont.getValue()) {
 314             PathArgument childId = beforeChild.getIdentifier();
 315             alreadyProcessed.add(childId);
 316             InstanceIdentifier childPath = path.node(childId);
 317             Collection<ListenerTree.Node> childListeners = getListenerChildrenWildcarded(listeners, childId);
 318             Optional<NormalizedNode<PathArgument, ?>> afterChild = afterCont.getChild(childId);
 319             DOMImmutableDataChangeEvent childChange = resolveNodeContainerChildUpdated(childPath, childListeners,
 320                     beforeChild, afterChild);
 321             // If change is empty (equals to NO_CHANGE)
 322             if (childChange != NO_CHANGE) {
 323                 childChanges.add(childChange);
 324             }
 325
 326         }
 327
 328         for (NormalizedNode<PathArgument, ?> afterChild : afterCont.getValue()) {
 329             PathArgument childId = afterChild.getIdentifier();
 330             if (!alreadyProcessed.contains(childId)) {
 331                 // We did not processed that child already
 332                 // and it was not present in previous loop, that means it is
 333                 // created.
 334                 Collection<ListenerTree.Node> childListeners = getListenerChildrenWildcarded(listeners, childId);
 335                 InstanceIdentifier childPath = path.node(childId);
 336                 childChanges.add(resolveSameEventRecursivelly(childPath , childListeners, afterChild,
 337                         DOMImmutableDataChangeEvent.getCreateEventFactory()));
 338             }
 339         }
 340         if (childChanges.isEmpty()) {
 341             return NO_CHANGE;
 342         }
 343
 344         Builder eventBuilder = builder(potentialScope) //
 345                 .setBefore(beforeCont) //
 346                 .setAfter(afterCont)
 347                 .addUpdated(path, beforeCont, afterCont);
 348         for (DOMImmutableDataChangeEvent childChange : childChanges) {
 349             eventBuilder.merge(childChange);
 350         }
 351
 352         DOMImmutableDataChangeEvent replaceEvent = eventBuilder.build();
 353         addPartialTask(listeners, replaceEvent);
 354         return replaceEvent;
 355     }
 356
 357     private DOMImmutableDataChangeEvent resolveNodeContainerChildUpdated(final InstanceIdentifier path,
 358             final Collection<Node> listeners, final NormalizedNode<PathArgument, ?> before,
 359             final Optional<NormalizedNode<PathArgument, ?>> after) {
 360
 361         if (after.isPresent()) {
 362             // REPLACE or SUBTREE Modified
 363             return resolveReplacedEvent(path, listeners, before, after.get());
 364
 365         } else {
 366             // AFTER state is not present - child was deleted.
 367             return resolveSameEventRecursivelly(path, listeners, before,
 368                     DOMImmutableDataChangeEvent.getRemoveEventFactory());
 369         }
 370     }
 371
 372     /**
 373      * Resolves create events deep down the interest listener tree.
 374      *
 375      *
 376      * @param path
 377      * @param listeners
 378      * @param afterState
 379      * @return
 380      */
 381     private DOMImmutableDataChangeEvent resolveCreateEvent(final InstanceIdentifier path,
 382             final Collection<ListenerTree.Node> listeners, final NormalizedNode<?, ?> afterState) {
 383         @SuppressWarnings({ "unchecked", "rawtypes" })
 384         final NormalizedNode<PathArgument, ?> node = (NormalizedNode) afterState;
 385         return resolveSameEventRecursivelly(path, listeners, node, DOMImmutableDataChangeEvent.getCreateEventFactory());
 386     }
 387
 388     private DOMImmutableDataChangeEvent resolveDeleteEvent(final InstanceIdentifier path,
 389             final Collection<ListenerTree.Node> listeners, final NormalizedNode<?, ?> beforeState) {
 390
 391         @SuppressWarnings({ "unchecked", "rawtypes" })
 392         final NormalizedNode<PathArgument, ?> node = (NormalizedNode) beforeState;
 393         return resolveSameEventRecursivelly(path, listeners, node, DOMImmutableDataChangeEvent.getRemoveEventFactory());
 394     }
 395
 396     private DOMImmutableDataChangeEvent resolveSameEventRecursivelly(final InstanceIdentifier path,
 397             final Collection<Node> listeners, final NormalizedNode<PathArgument, ?> node,
 398             final SimpleEventFactory eventFactory) {
 399         final DOMImmutableDataChangeEvent event = eventFactory.create(path, node);
 400         DOMImmutableDataChangeEvent propagateEvent = event;
 401         // We have listeners for this node or it's children, so we will try
 402         // to do additional processing
 403         if (node instanceof NormalizedNodeContainer<?, ?, ?>) {
 404             LOG.trace("Resolving subtree recursive event for {}, type {}", path, eventFactory);
 405
 406             Builder eventBuilder = builder(DataChangeScope.BASE);
 407             eventBuilder.merge(event);
 408             eventBuilder.setBefore(event.getOriginalSubtree());
 409             eventBuilder.setAfter(event.getUpdatedSubtree());
 410
 411             // Node has children, so we will try to resolve it's children
 412             // changes.
 413             @SuppressWarnings("unchecked")
 414             NormalizedNodeContainer<?, PathArgument, NormalizedNode<PathArgument, ?>> container = (NormalizedNodeContainer<?, PathArgument, NormalizedNode<PathArgument, ?>>) node;
 415             for (NormalizedNode<PathArgument, ?> child : container.getValue()) {
 416                 PathArgument childId = child.getIdentifier();
 417                 LOG.trace("Resolving event for child {}", childId);
 418                 Collection<Node> childListeners = getListenerChildrenWildcarded(listeners, childId);
 419                 eventBuilder.merge(resolveSameEventRecursivelly(path.node(childId), childListeners, child, eventFactory));
 420             }
 421             propagateEvent = eventBuilder.build();
 422         }
 423         if (!listeners.isEmpty()) {
 424             addPartialTask(listeners, propagateEvent);
 425         }
 426         return propagateEvent;
 427     }
 428
 429     private DOMImmutableDataChangeEvent resolveSubtreeChangeEvent(final InstanceIdentifier path,
 430             final Collection<ListenerTree.Node> listeners, final DataTreeCandidateNode modification) {
 431
 432         Preconditions.checkArgument(modification.getDataBefore().isPresent(), "Subtree change with before-data not present at path %s", path);
 433         Preconditions.checkArgument(modification.getDataAfter().isPresent(), "Subtree change with after-data not present at path %s", path);
 434
 435         Builder one = builder(DataChangeScope.ONE).
 436                 setBefore(modification.getDataBefore().get()).
 437                 setAfter(modification.getDataAfter().get());
 438         Builder subtree = builder(DataChangeScope.SUBTREE).
 439                 setBefore(modification.getDataBefore().get()).
 440                 setAfter(modification.getDataAfter().get());
 441
 442         for (DataTreeCandidateNode childMod : modification.getChildNodes()) {
 443             PathArgument childId = childMod.getIdentifier();
 444             InstanceIdentifier childPath = path.node(childId);
 445             Collection<ListenerTree.Node> childListeners = getListenerChildrenWildcarded(listeners, childId);
 446
 447             switch (childMod.getModificationType()) {
 448             case WRITE:
 449             case MERGE:
 450             case DELETE:
 451                 one.merge(resolveAnyChangeEvent(childPath, childListeners, childMod));
 452                 break;
 453             case SUBTREE_MODIFIED:
 454                 subtree.merge(resolveSubtreeChangeEvent(childPath, childListeners, childMod));
 455                 break;
 456             case UNMODIFIED:
 457                 // no-op
 458                 break;
 459             }
 460         }
 461         DOMImmutableDataChangeEvent oneChangeEvent = one.build();
 462         subtree.merge(oneChangeEvent);
 463         DOMImmutableDataChangeEvent subtreeEvent = subtree.build();
 464         if (!listeners.isEmpty()) {
 465             addPartialTask(listeners, oneChangeEvent);
 466             addPartialTask(listeners, subtreeEvent);
 467         }
 468         return subtreeEvent;
 469     }
 470
 471     private DOMImmutableDataChangeEvent addPartialTask(final Collection<ListenerTree.Node> listeners,
 472             final DOMImmutableDataChangeEvent event) {
 473         for (ListenerTree.Node listenerNode : listeners) {
 474             if (!listenerNode.getListeners().isEmpty()) {
 475                 LOG.trace("Adding event {} for listeners {}",event,listenerNode);
 476                 events.put(listenerNode, event);
 477             }
 478         }
 479         return event;
 480     }
 481
 482     private static Collection<ListenerTree.Node> getListenerChildrenWildcarded(final Collection<ListenerTree.Node> parentNodes,
 483             final PathArgument child) {
 484         if (parentNodes.isEmpty()) {
 485             return Collections.emptyList();
 486         }
 487         com.google.common.collect.ImmutableList.Builder<ListenerTree.Node> result = ImmutableList.builder();
 488         if (child instanceof NodeWithValue || child instanceof NodeIdentifierWithPredicates) {
 489             NodeIdentifier wildcardedIdentifier = new NodeIdentifier(child.getNodeType());
 490             addChildrenNodesToBuilder(result, parentNodes, wildcardedIdentifier);
 491         }
 492         addChildrenNodesToBuilder(result, parentNodes, child);
 493         return result.build();
 494     }
 495
 496     private static void addChildrenNodesToBuilder(final ImmutableList.Builder<ListenerTree.Node> result,
 497             final Collection<ListenerTree.Node> parentNodes, final PathArgument childIdentifier) {
 498         for (ListenerTree.Node node : parentNodes) {
 499             Optional<ListenerTree.Node> child = node.getChild(childIdentifier);
 500             if (child.isPresent()) {
 501                 result.add(child.get());
 502             }
 503         }
 504     }
 505
 506     public static ResolveDataChangeEventsTask create(final DataTreeCandidate candidate, final ListenerTree listenerTree) {
 507         return new ResolveDataChangeEventsTask(candidate, listenerTree);
 508     }
 509 }