opendaylight/md-sal/sal-distributed-datastore/src/main/java/org/opendaylight/controller/cluster/datastore/entityownership/EntityOwnershipShard.java

   1 /*
   2  * Copyright (c) 2015 Brocade Communications 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.cluster.datastore.entityownership;
   9
  10 import static org.opendaylight.controller.cluster.datastore.entityownership.EntityOwnersModel.CANDIDATE_NAME_NODE_ID;
  11 import static org.opendaylight.controller.cluster.datastore.entityownership.EntityOwnersModel.CANDIDATE_NODE_ID;
  12 import static org.opendaylight.controller.cluster.datastore.entityownership.EntityOwnersModel.ENTITY_ID_NODE_ID;
  13 import static org.opendaylight.controller.cluster.datastore.entityownership.EntityOwnersModel.ENTITY_ID_QNAME;
  14 import static org.opendaylight.controller.cluster.datastore.entityownership.EntityOwnersModel.ENTITY_NODE_ID;
  15 import static org.opendaylight.controller.cluster.datastore.entityownership.EntityOwnersModel.ENTITY_OWNERS_PATH;
  16 import static org.opendaylight.controller.cluster.datastore.entityownership.EntityOwnersModel.ENTITY_OWNER_NODE_ID;
  17 import static org.opendaylight.controller.cluster.datastore.entityownership.EntityOwnersModel.ENTITY_OWNER_QNAME;
  18 import static org.opendaylight.controller.cluster.datastore.entityownership.EntityOwnersModel.ENTITY_TYPES_PATH;
  19 import static org.opendaylight.controller.cluster.datastore.entityownership.EntityOwnersModel.ENTITY_TYPE_NODE_ID;
  20 import static org.opendaylight.controller.cluster.datastore.entityownership.EntityOwnersModel.ENTITY_TYPE_QNAME;
  21 import static org.opendaylight.controller.cluster.datastore.entityownership.EntityOwnersModel.candidateMapEntry;
  22 import static org.opendaylight.controller.cluster.datastore.entityownership.EntityOwnersModel.candidateNodeKey;
  23 import static org.opendaylight.controller.cluster.datastore.entityownership.EntityOwnersModel.candidatePath;
  24 import static org.opendaylight.controller.cluster.datastore.entityownership.EntityOwnersModel.createEntity;
  25 import static org.opendaylight.controller.cluster.datastore.entityownership.EntityOwnersModel.entityOwnersWithCandidate;
  26 import akka.actor.ActorRef;
  27 import akka.actor.ActorSelection;
  28 import akka.actor.Cancellable;
  29 import akka.actor.Props;
  30 import akka.pattern.Patterns;
  31 import com.google.common.base.Optional;
  32 import com.google.common.base.Strings;
  33 import java.util.ArrayList;
  34 import java.util.Collection;
  35 import java.util.HashMap;
  36 import java.util.HashSet;
  37 import java.util.Map;
  38 import java.util.Set;
  39 import java.util.concurrent.TimeUnit;
  40 import org.opendaylight.controller.cluster.datastore.DatastoreContext;
  41 import org.opendaylight.controller.cluster.datastore.Shard;
  42 import org.opendaylight.controller.cluster.datastore.entityownership.messages.CandidateAdded;
  43 import org.opendaylight.controller.cluster.datastore.entityownership.messages.CandidateRemoved;
  44 import org.opendaylight.controller.cluster.datastore.entityownership.messages.RegisterCandidateLocal;
  45 import org.opendaylight.controller.cluster.datastore.entityownership.messages.RegisterListenerLocal;
  46 import org.opendaylight.controller.cluster.datastore.entityownership.messages.SelectOwner;
  47 import org.opendaylight.controller.cluster.datastore.entityownership.messages.UnregisterCandidateLocal;
  48 import org.opendaylight.controller.cluster.datastore.entityownership.messages.UnregisterListenerLocal;
  49 import org.opendaylight.controller.cluster.datastore.entityownership.selectionstrategy.EntityOwnerSelectionStrategy;
  50 import org.opendaylight.controller.cluster.datastore.entityownership.selectionstrategy.EntityOwnerSelectionStrategyConfig;
  51 import org.opendaylight.controller.cluster.datastore.identifiers.ShardIdentifier;
  52 import org.opendaylight.controller.cluster.datastore.messages.BatchedModifications;
  53 import org.opendaylight.controller.cluster.datastore.messages.PeerDown;
  54 import org.opendaylight.controller.cluster.datastore.messages.PeerUp;
  55 import org.opendaylight.controller.cluster.datastore.messages.SuccessReply;
  56 import org.opendaylight.controller.cluster.datastore.modification.DeleteModification;
  57 import org.opendaylight.controller.cluster.datastore.modification.MergeModification;
  58 import org.opendaylight.controller.cluster.datastore.modification.WriteModification;
  59 import org.opendaylight.controller.md.sal.common.api.clustering.Entity;
  60 import org.opendaylight.yangtools.yang.data.api.YangInstanceIdentifier;
  61 import org.opendaylight.yangtools.yang.data.api.YangInstanceIdentifier.PathArgument;
  62 import org.opendaylight.yangtools.yang.data.api.schema.DataContainerChild;
  63 import org.opendaylight.yangtools.yang.data.api.schema.MapEntryNode;
  64 import org.opendaylight.yangtools.yang.data.api.schema.MapNode;
  65 import org.opendaylight.yangtools.yang.data.api.schema.NormalizedNode;
  66 import org.opendaylight.yangtools.yang.data.api.schema.tree.DataTreeSnapshot;
  67 import org.opendaylight.yangtools.yang.data.impl.schema.ImmutableNodes;
  68 import org.opendaylight.yangtools.yang.model.api.SchemaContext;
  69 import scala.concurrent.Future;
  70 import scala.concurrent.duration.FiniteDuration;
  71
  72 /**
  73  * Special Shard for EntityOwnership.
  74  *
  75  * @author Thomas Pantelis
  76  */
  77 class EntityOwnershipShard extends Shard {
  78     private final String localMemberName;
  79     private final EntityOwnershipShardCommitCoordinator commitCoordinator;
  80     private final EntityOwnershipListenerSupport listenerSupport;
  81     private final Set<String> downPeerMemberNames = new HashSet<>();
  82     private final Map<String, String> peerIdToMemberNames = new HashMap<>();
  83     private EntityOwnerSelectionStrategyConfig strategyConfig;
  84     private Map<YangInstanceIdentifier, Cancellable> entityToScheduledOwnershipTask = new HashMap<>();
  85
  86     private static DatastoreContext noPersistenceDatastoreContext(DatastoreContext datastoreContext) {
  87         return DatastoreContext.newBuilderFrom(datastoreContext).persistent(false).build();
  88     }
  89
  90     protected EntityOwnershipShard(ShardIdentifier name, Map<String, String> peerAddresses,
  91                                    DatastoreContext datastoreContext, SchemaContext schemaContext, String localMemberName,
  92                                    EntityOwnerSelectionStrategyConfig strategyConfig) {
  93         super(name, peerAddresses, noPersistenceDatastoreContext(datastoreContext), schemaContext);
  94         this.localMemberName = localMemberName;
  95         this.commitCoordinator = new EntityOwnershipShardCommitCoordinator(localMemberName, LOG);
  96         this.listenerSupport = new EntityOwnershipListenerSupport(getContext(), persistenceId());
  97         this.strategyConfig = strategyConfig;
  98
  99         for(String peerId: peerAddresses.keySet()) {
 100             ShardIdentifier shardId = ShardIdentifier.builder().fromShardIdString(peerId).build();
 101             peerIdToMemberNames.put(peerId, shardId.getMemberName());
 102         }
 103     }
 104
 105     @Override
 106     protected void onDatastoreContext(DatastoreContext context) {
 107         super.onDatastoreContext(noPersistenceDatastoreContext(context));
 108     }
 109
 110     @Override
 111     protected void onRecoveryComplete() {
 112         super.onRecoveryComplete();
 113
 114         new CandidateListChangeListener(getSelf(), persistenceId()).init(getDataStore());
 115         new EntityOwnerChangeListener(localMemberName, listenerSupport).init(getDataStore());
 116     }
 117
 118     @Override
 119     public void onReceiveCommand(final Object message) throws Exception {
 120         if(message instanceof RegisterCandidateLocal) {
 121             onRegisterCandidateLocal((RegisterCandidateLocal) message);
 122         } else if(message instanceof UnregisterCandidateLocal) {
 123             onUnregisterCandidateLocal((UnregisterCandidateLocal)message);
 124         } else if(message instanceof CandidateAdded){
 125             onCandidateAdded((CandidateAdded) message);
 126         } else if(message instanceof CandidateRemoved){
 127             onCandidateRemoved((CandidateRemoved) message);
 128         } else if(message instanceof PeerDown) {
 129             onPeerDown((PeerDown) message);
 130         } else if(message instanceof PeerUp) {
 131             onPeerUp((PeerUp) message);
 132         } else if(message instanceof RegisterListenerLocal) {
 133             onRegisterListenerLocal((RegisterListenerLocal)message);
 134         } else if(message instanceof UnregisterListenerLocal) {
 135             onUnregisterListenerLocal((UnregisterListenerLocal) message);
 136         } else if(message instanceof SelectOwner) {
 137             onSelectOwner((SelectOwner) message);
 138         } else if(!commitCoordinator.handleMessage(message, this)) {
 139             super.onReceiveCommand(message);
 140         }
 141     }
 142
 143     private void onSelectOwner(SelectOwner selectOwner) {
 144         String currentOwner = getCurrentOwner(selectOwner.getEntityPath());
 145         if(Strings.isNullOrEmpty(currentOwner)) {
 146             writeNewOwner(selectOwner.getEntityPath(), newOwner(selectOwner.getAllCandidates(),
 147                     selectOwner.getOwnerSelectionStrategy()));
 148
 149             Cancellable cancellable = entityToScheduledOwnershipTask.get(selectOwner.getEntityPath());
 150             if(cancellable != null){
 151                 if(!cancellable.isCancelled()){
 152                     cancellable.cancel();
 153                 }
 154                 entityToScheduledOwnershipTask.remove(selectOwner.getEntityPath());
 155             }
 156         }
 157     }
 158
 159     private void onRegisterCandidateLocal(RegisterCandidateLocal registerCandidate) {
 160         LOG.debug("{}: onRegisterCandidateLocal: {}", persistenceId(), registerCandidate);
 161
 162         listenerSupport.setHasCandidateForEntity(registerCandidate.getEntity());
 163
 164         NormalizedNode<?, ?> entityOwners = entityOwnersWithCandidate(registerCandidate.getEntity().getType(),
 165                 registerCandidate.getEntity().getId(), localMemberName);
 166         commitCoordinator.commitModification(new MergeModification(ENTITY_OWNERS_PATH, entityOwners), this);
 167
 168         getSender().tell(SuccessReply.INSTANCE, getSelf());
 169     }
 170
 171     private void onUnregisterCandidateLocal(UnregisterCandidateLocal unregisterCandidate) {
 172         LOG.debug("{}: onUnregisterCandidateLocal: {}", persistenceId(), unregisterCandidate);
 173
 174         Entity entity = unregisterCandidate.getEntity();
 175         listenerSupport.unsetHasCandidateForEntity(entity);
 176
 177         YangInstanceIdentifier candidatePath = candidatePath(entity.getType(), entity.getId(), localMemberName);
 178         commitCoordinator.commitModification(new DeleteModification(candidatePath), this);
 179
 180         getSender().tell(SuccessReply.INSTANCE, getSelf());
 181     }
 182
 183     private void onRegisterListenerLocal(final RegisterListenerLocal registerListener) {
 184         LOG.debug("{}: onRegisterListenerLocal: {}", persistenceId(), registerListener);
 185
 186         listenerSupport.addEntityOwnershipListener(registerListener.getEntityType(), registerListener.getListener());
 187
 188         getSender().tell(SuccessReply.INSTANCE, getSelf());
 189
 190         searchForEntitiesOwnedBy(localMemberName, new EntityWalker() {
 191             @Override
 192             public void onEntity(MapEntryNode entityTypeNode, MapEntryNode entityNode) {
 193                 Optional<DataContainerChild<? extends PathArgument, ?>> possibleType =
 194                         entityTypeNode.getChild(ENTITY_TYPE_NODE_ID);
 195                 String entityType = possibleType.isPresent() ? possibleType.get().getValue().toString() : null;
 196                 if (registerListener.getEntityType().equals(entityType)) {
 197                     Entity entity = new Entity(entityType,
 198                             (YangInstanceIdentifier) entityNode.getChild(ENTITY_ID_NODE_ID).get().getValue());
 199                     listenerSupport.notifyEntityOwnershipListener(entity, false, true, true, registerListener.getListener());
 200                 }
 201             }
 202         });
 203     }
 204
 205     private void onUnregisterListenerLocal(UnregisterListenerLocal unregisterListener) {
 206         LOG.debug("{}: onUnregisterListenerLocal: {}", persistenceId(), unregisterListener);
 207
 208         listenerSupport.removeEntityOwnershipListener(unregisterListener.getEntityType(), unregisterListener.getListener());
 209
 210         getSender().tell(SuccessReply.INSTANCE, getSelf());
 211     }
 212
 213     void tryCommitModifications(final BatchedModifications modifications) {
 214         if(isLeader()) {
 215             LOG.debug("{}: Committing BatchedModifications {} locally", persistenceId(), modifications.getTransactionID());
 216
 217             // Note that it's possible the commit won't get consensus and will timeout and not be applied
 218             // to the state. However we don't need to retry it in that case b/c it will be committed to
 219             // the journal first and, once a majority of followers come back on line and it is replicated,
 220             // it will be applied at that point.
 221             handleBatchedModificationsLocal(modifications, self());
 222         } else {
 223             final ActorSelection leader = getLeader();
 224             if (leader != null) {
 225                 if(LOG.isDebugEnabled()) {
 226                     LOG.debug("{}: Sending BatchedModifications {} to leader {}", persistenceId(),
 227                             modifications.getTransactionID(), leader);
 228                 }
 229
 230                 Future<Object> future = Patterns.ask(leader, modifications, TimeUnit.SECONDS.toMillis(
 231                         getDatastoreContext().getShardTransactionCommitTimeoutInSeconds()));
 232
 233                 Patterns.pipe(future, getContext().dispatcher()).pipeTo(getSelf(), ActorRef.noSender());
 234             }
 235         }
 236     }
 237
 238     boolean hasLeader() {
 239         return getLeader() != null && !isIsolatedLeader();
 240     }
 241
 242     @Override
 243     protected void onStateChanged() {
 244         super.onStateChanged();
 245
 246         commitCoordinator.onStateChanged(this, isLeader());
 247     }
 248
 249     @Override
 250     protected void onLeaderChanged(String oldLeader, String newLeader) {
 251         super.onLeaderChanged(oldLeader, newLeader);
 252
 253         LOG.debug("{}: onLeaderChanged: oldLeader: {}, newLeader: {}, isLeader: {}", persistenceId(), oldLeader,
 254                 newLeader, isLeader());
 255
 256         if(isLeader()) {
 257             // We were just elected leader. If the old leader is down, select new owners for the entities
 258             // owned by the down leader.
 259
 260             String oldLeaderMemberName = peerIdToMemberNames.get(oldLeader);
 261
 262             LOG.debug("{}: oldLeaderMemberName: {}", persistenceId(), oldLeaderMemberName);
 263
 264             if(downPeerMemberNames.contains(oldLeaderMemberName)) {
 265                 selectNewOwnerForEntitiesOwnedBy(oldLeaderMemberName);
 266             }
 267         }
 268     }
 269
 270     private void onCandidateRemoved(CandidateRemoved message) {
 271         LOG.debug("{}: onCandidateRemoved: {}", persistenceId(), message);
 272
 273         if(isLeader()) {
 274             String currentOwner = getCurrentOwner(message.getEntityPath());
 275             if(message.getRemovedCandidate().equals(currentOwner)){
 276                 writeNewOwner(message.getEntityPath(),
 277                         newOwner(message.getRemainingCandidates(), getEntityOwnerElectionStrategy(message.getEntityPath())));
 278             }
 279         } else {
 280             // We're not the leader. If the removed candidate is our local member then check if we actually
 281             // have a local candidate registered. If we do then we must have been partitioned from the leader
 282             // and the leader removed our candidate since the leader can't tell the difference between a
 283             // temporary network partition and a node's process actually restarted. So, in that case, re-add
 284             // our candidate.
 285             if(localMemberName.equals(message.getRemovedCandidate()) &&
 286                     listenerSupport.hasCandidateForEntity(createEntity(message.getEntityPath()))) {
 287                 LOG.debug("Local candidate member was removed but a local candidate is registered for {}" +
 288                     " - adding back local candidate", message.getEntityPath());
 289
 290                 commitCoordinator.commitModification(new MergeModification(
 291                         candidatePath(message.getEntityPath(), localMemberName),
 292                         candidateMapEntry(localMemberName)), this);
 293             }
 294         }
 295     }
 296
 297     private EntityOwnerSelectionStrategy getEntityOwnerElectionStrategy(YangInstanceIdentifier entityPath) {
 298         final String entityType = EntityOwnersModel.entityTypeFromEntityPath(entityPath);
 299         return strategyConfig.createStrategy(entityType);
 300     }
 301
 302     private void onCandidateAdded(CandidateAdded message) {
 303         if(!isLeader()){
 304             return;
 305         }
 306
 307         LOG.debug("{}: onCandidateAdded: {}", persistenceId(), message);
 308
 309         // Since a node's candidate member is only added by the node itself, we can assume the node is up so
 310         // remove it from the downPeerMemberNames.
 311         downPeerMemberNames.remove(message.getNewCandidate());
 312
 313         String currentOwner = getCurrentOwner(message.getEntityPath());
 314         EntityOwnerSelectionStrategy strategy = getEntityOwnerElectionStrategy(message.getEntityPath());
 315         if(Strings.isNullOrEmpty(currentOwner)){
 316             if(strategy.getSelectionDelayInMillis() == 0L) {
 317                 writeNewOwner(message.getEntityPath(), newOwner(message.getAllCandidates(), strategy));
 318             } else {
 319                 scheduleOwnerSelection(message.getEntityPath(), message.getAllCandidates(), strategy);
 320             }
 321         }
 322     }
 323
 324     private void onPeerDown(PeerDown peerDown) {
 325         LOG.info("{}: onPeerDown: {}", persistenceId(), peerDown);
 326
 327         String downMemberName = peerDown.getMemberName();
 328         if(downPeerMemberNames.add(downMemberName) && isLeader()) {
 329             // Remove the down peer as a candidate from all entities.
 330             removeCandidateFromEntities(downMemberName);
 331         }
 332     }
 333
 334     private void onPeerUp(PeerUp peerUp) {
 335         LOG.debug("{}: onPeerUp: {}", persistenceId(), peerUp);
 336
 337         peerIdToMemberNames.put(peerUp.getPeerId(), peerUp.getMemberName());
 338         downPeerMemberNames.remove(peerUp.getMemberName());
 339     }
 340
 341     private void selectNewOwnerForEntitiesOwnedBy(String owner) {
 342         final BatchedModifications modifications = commitCoordinator.newBatchedModifications();
 343         searchForEntitiesOwnedBy(owner, new EntityWalker() {
 344             @Override
 345             public void onEntity(MapEntryNode entityTypeNode, MapEntryNode entityNode) {
 346
 347                 YangInstanceIdentifier entityPath = YangInstanceIdentifier.builder(ENTITY_TYPES_PATH).
 348                         node(entityTypeNode.getIdentifier()).node(ENTITY_NODE_ID).node(entityNode.getIdentifier()).
 349                         node(ENTITY_OWNER_NODE_ID).build();
 350
 351                 Object newOwner = newOwner(getCandidateNames(entityNode), getEntityOwnerElectionStrategy(entityPath));
 352
 353                 LOG.debug("{}: Found entity {}, writing new owner {}", persistenceId(), entityPath, newOwner);
 354
 355                 modifications.addModification(new WriteModification(entityPath,
 356                         ImmutableNodes.leafNode(ENTITY_OWNER_NODE_ID, newOwner)));
 357             }
 358         });
 359
 360         commitCoordinator.commitModifications(modifications, this);
 361     }
 362
 363     private void removeCandidateFromEntities(final String owner) {
 364         final BatchedModifications modifications = commitCoordinator.newBatchedModifications();
 365         searchForEntities(new EntityWalker() {
 366             @Override
 367             public void onEntity(MapEntryNode entityTypeNode, MapEntryNode entityNode) {
 368                 if (hasCandidate(entityNode, owner)) {
 369                     YangInstanceIdentifier entityId =
 370                             (YangInstanceIdentifier) entityNode.getIdentifier().getKeyValues().get(ENTITY_ID_QNAME);
 371                     YangInstanceIdentifier candidatePath = candidatePath(
 372                             entityTypeNode.getIdentifier().getKeyValues().get(ENTITY_TYPE_QNAME).toString(),
 373                             entityId, owner);
 374
 375                     LOG.info("{}: Found entity {}, removing candidate {}, path {}", persistenceId(), entityId,
 376                             owner, candidatePath);
 377
 378                     modifications.addModification(new DeleteModification(candidatePath));
 379                 }
 380             }
 381         });
 382
 383         commitCoordinator.commitModifications(modifications, this);
 384     }
 385
 386     private boolean hasCandidate(MapEntryNode entity, String candidateName) {
 387         return ((MapNode)entity.getChild(CANDIDATE_NODE_ID).get()).getChild(candidateNodeKey(candidateName)).isPresent();
 388     }
 389
 390     private void searchForEntitiesOwnedBy(final String owner, final EntityWalker walker) {
 391         DataTreeSnapshot snapshot = getDataStore().getDataTree().takeSnapshot();
 392         Optional<NormalizedNode<?, ?>> possibleEntityTypes = snapshot.readNode(ENTITY_TYPES_PATH);
 393         if(!possibleEntityTypes.isPresent()) {
 394             return;
 395         }
 396
 397         LOG.debug("{}: Searching for entities owned by {}", persistenceId(), owner);
 398
 399         searchForEntities(new EntityWalker() {
 400             @Override
 401             public void onEntity(MapEntryNode entityTypeNode, MapEntryNode entityNode) {
 402                 Optional<DataContainerChild<? extends PathArgument, ?>> possibleOwner =
 403                         entityNode.getChild(ENTITY_OWNER_NODE_ID);
 404                 if(possibleOwner.isPresent() && owner.equals(possibleOwner.get().getValue().toString())) {
 405                     walker.onEntity(entityTypeNode, entityNode);
 406                 }
 407             }
 408         });
 409     }
 410
 411     private void searchForEntities(EntityWalker walker) {
 412         DataTreeSnapshot snapshot = getDataStore().getDataTree().takeSnapshot();
 413         Optional<NormalizedNode<?, ?>> possibleEntityTypes = snapshot.readNode(ENTITY_TYPES_PATH);
 414         if(!possibleEntityTypes.isPresent()) {
 415             return;
 416         }
 417
 418         for(MapEntryNode entityType:  ((MapNode) possibleEntityTypes.get()).getValue()) {
 419             Optional<DataContainerChild<? extends PathArgument, ?>> possibleEntities =
 420                     entityType.getChild(ENTITY_NODE_ID);
 421             if(!possibleEntities.isPresent()) {
 422                 continue; // shouldn't happen but handle anyway
 423             }
 424
 425             for(MapEntryNode entity:  ((MapNode) possibleEntities.get()).getValue()) {
 426                 walker.onEntity(entityType, entity);
 427             }
 428         }
 429     }
 430
 431     private Collection<String> getCandidateNames(MapEntryNode entity) {
 432         Collection<MapEntryNode> candidates = ((MapNode) entity.getChild(CANDIDATE_NODE_ID).get()).getValue();
 433         Collection<String> candidateNames = new ArrayList<>(candidates.size());
 434         for(MapEntryNode candidate: candidates) {
 435             candidateNames.add(candidate.getChild(CANDIDATE_NAME_NODE_ID).get().getValue().toString());
 436         }
 437
 438         return candidateNames;
 439     }
 440
 441     private void writeNewOwner(YangInstanceIdentifier entityPath, String newOwner) {
 442         LOG.debug("{}: Writing new owner {} for entity {}", persistenceId(), newOwner, entityPath);
 443
 444         commitCoordinator.commitModification(new WriteModification(entityPath.node(ENTITY_OWNER_QNAME),
 445                 ImmutableNodes.leafNode(ENTITY_OWNER_NODE_ID, newOwner)), this);
 446     }
 447
 448     /**
 449      * Schedule a new owner selection job. Cancelling any outstanding job if it has not been cancelled.
 450      *
 451      * @param entityPath
 452      * @param allCandidates
 453      */
 454     public void scheduleOwnerSelection(YangInstanceIdentifier entityPath, Collection<String> allCandidates,
 455                                        EntityOwnerSelectionStrategy strategy){
 456         Cancellable lastScheduledTask = entityToScheduledOwnershipTask.get(entityPath);
 457         if(lastScheduledTask != null && !lastScheduledTask.isCancelled()){
 458             lastScheduledTask.cancel();
 459         }
 460         lastScheduledTask = context().system().scheduler().scheduleOnce(
 461                 FiniteDuration.apply(strategy.getSelectionDelayInMillis(), TimeUnit.MILLISECONDS)
 462                 , self(), new SelectOwner(entityPath, allCandidates, strategy)
 463                 , context().system().dispatcher(), self());
 464
 465         entityToScheduledOwnershipTask.put(entityPath, lastScheduledTask);
 466     }
 467
 468     private String newOwner(Collection<String> candidates, EntityOwnerSelectionStrategy ownerSelectionStrategy) {
 469         Collection<String> viableCandidates = getViableCandidates(candidates);
 470         if(viableCandidates.size() == 0){
 471             return "";
 472         }
 473         return ownerSelectionStrategy.newOwner(viableCandidates);
 474     }
 475
 476     private Collection<String> getViableCandidates(Collection<String> candidates) {
 477         Collection<String> viableCandidates = new ArrayList<>();
 478
 479         for (String candidate : candidates) {
 480             if (!downPeerMemberNames.contains(candidate)) {
 481                 viableCandidates.add(candidate);
 482             }
 483         }
 484         return viableCandidates;
 485     }
 486
 487     private String getCurrentOwner(YangInstanceIdentifier entityId) {
 488         DataTreeSnapshot snapshot = getDataStore().getDataTree().takeSnapshot();
 489         Optional<NormalizedNode<?, ?>> optionalEntityOwner = snapshot.readNode(entityId.node(ENTITY_OWNER_QNAME));
 490         if(optionalEntityOwner.isPresent()){
 491             return optionalEntityOwner.get().getValue().toString();
 492         }
 493         return null;
 494     }
 495
 496     public static Props props(final ShardIdentifier name, final Map<String, String> peerAddresses,
 497                               final DatastoreContext datastoreContext, final SchemaContext schemaContext,
 498                               final String localMemberName, final EntityOwnerSelectionStrategyConfig strategyConfig) {
 499         return Props.create(new Creator(name, peerAddresses, datastoreContext, schemaContext, localMemberName, strategyConfig));
 500     }
 501
 502     private static class Creator extends AbstractShardCreator {
 503         private static final long serialVersionUID = 1L;
 504
 505         private final String localMemberName;
 506         private final EntityOwnerSelectionStrategyConfig strategyConfig;
 507
 508         Creator(final ShardIdentifier name, final Map<String, String> peerAddresses,
 509                 final DatastoreContext datastoreContext, final SchemaContext schemaContext,
 510                 final String localMemberName, EntityOwnerSelectionStrategyConfig strategyConfig) {
 511             super(name, peerAddresses, datastoreContext, schemaContext);
 512             this.localMemberName = localMemberName;
 513             this.strategyConfig = strategyConfig;
 514         }
 515
 516         @Override
 517         public Shard create() throws Exception {
 518             return new EntityOwnershipShard(name, peerAddresses, datastoreContext, schemaContext, localMemberName, strategyConfig);
 519         }
 520     }
 521
 522     private static interface EntityWalker {
 523         void onEntity(MapEntryNode entityTypeNode, MapEntryNode entityNode);
 524     }
 525 }