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