Address comments in https://git.opendaylight.org/gerrit/#/c/18392/
[controller.git] / opendaylight / md-sal / sal-distributed-datastore / src / main / java / org / opendaylight / controller / cluster / datastore / Shard.java
index a3109b66b1116853ebb6977c99f72527468fea44..71afb5c9b7394215406926306f7555b034231b47 100644 (file)
@@ -10,161 +10,162 @@ package org.opendaylight.controller.cluster.datastore;
 
 import akka.actor.ActorRef;
 import akka.actor.ActorSelection;
-import akka.actor.PoisonPill;
+import akka.actor.Cancellable;
 import akka.actor.Props;
-import akka.event.Logging;
-import akka.event.LoggingAdapter;
 import akka.japi.Creator;
 import akka.persistence.RecoveryFailure;
 import akka.serialization.Serialization;
 import com.google.common.annotations.VisibleForTesting;
 import com.google.common.base.Optional;
 import com.google.common.base.Preconditions;
-import com.google.common.collect.Lists;
-import com.google.common.util.concurrent.CheckedFuture;
 import com.google.common.util.concurrent.FutureCallback;
 import com.google.common.util.concurrent.Futures;
 import com.google.common.util.concurrent.ListenableFuture;
-import com.google.protobuf.ByteString;
-import com.google.protobuf.InvalidProtocolBufferException;
+import java.io.IOException;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.concurrent.TimeUnit;
+import javax.annotation.Nonnull;
 import org.opendaylight.controller.cluster.common.actor.CommonConfig;
 import org.opendaylight.controller.cluster.common.actor.MeteringBehavior;
+import org.opendaylight.controller.cluster.datastore.ShardCommitCoordinator.CohortEntry;
+import org.opendaylight.controller.cluster.datastore.exceptions.NoShardLeaderException;
 import org.opendaylight.controller.cluster.datastore.identifiers.ShardIdentifier;
 import org.opendaylight.controller.cluster.datastore.identifiers.ShardTransactionIdentifier;
 import org.opendaylight.controller.cluster.datastore.jmx.mbeans.shard.ShardMBeanFactory;
 import org.opendaylight.controller.cluster.datastore.jmx.mbeans.shard.ShardStats;
+import org.opendaylight.controller.cluster.datastore.messages.AbortTransaction;
+import org.opendaylight.controller.cluster.datastore.messages.AbortTransactionReply;
+import org.opendaylight.controller.cluster.datastore.messages.ActorInitialized;
+import org.opendaylight.controller.cluster.datastore.messages.BatchedModifications;
+import org.opendaylight.controller.cluster.datastore.messages.CanCommitTransaction;
 import org.opendaylight.controller.cluster.datastore.messages.CloseTransactionChain;
+import org.opendaylight.controller.cluster.datastore.messages.CommitTransaction;
 import org.opendaylight.controller.cluster.datastore.messages.CommitTransactionReply;
 import org.opendaylight.controller.cluster.datastore.messages.CreateTransaction;
 import org.opendaylight.controller.cluster.datastore.messages.CreateTransactionReply;
-import org.opendaylight.controller.cluster.datastore.messages.EnableNotification;
-import org.opendaylight.controller.cluster.datastore.messages.ForwardedCommitTransaction;
+import org.opendaylight.controller.cluster.datastore.messages.ForwardedReadyTransaction;
 import org.opendaylight.controller.cluster.datastore.messages.PeerAddressResolved;
-import org.opendaylight.controller.cluster.datastore.messages.ReadData;
-import org.opendaylight.controller.cluster.datastore.messages.ReadDataReply;
+import org.opendaylight.controller.cluster.datastore.messages.ReadyLocalTransaction;
 import org.opendaylight.controller.cluster.datastore.messages.RegisterChangeListener;
-import org.opendaylight.controller.cluster.datastore.messages.RegisterChangeListenerReply;
+import org.opendaylight.controller.cluster.datastore.messages.RegisterDataTreeChangeListener;
+import org.opendaylight.controller.cluster.datastore.messages.ShardLeaderStateChanged;
 import org.opendaylight.controller.cluster.datastore.messages.UpdateSchemaContext;
 import org.opendaylight.controller.cluster.datastore.modification.Modification;
+import org.opendaylight.controller.cluster.datastore.modification.ModificationPayload;
 import org.opendaylight.controller.cluster.datastore.modification.MutableCompositeModification;
-import org.opendaylight.controller.cluster.datastore.node.NormalizedNodeToNodeCodec;
+import org.opendaylight.controller.cluster.datastore.utils.Dispatchers;
+import org.opendaylight.controller.cluster.datastore.utils.MessageTracker;
+import org.opendaylight.controller.cluster.notifications.LeaderStateChanged;
+import org.opendaylight.controller.cluster.notifications.RegisterRoleChangeListener;
+import org.opendaylight.controller.cluster.notifications.RoleChangeNotifier;
 import org.opendaylight.controller.cluster.raft.RaftActor;
-import org.opendaylight.controller.cluster.raft.ReplicatedLogEntry;
-import org.opendaylight.controller.cluster.raft.base.messages.CaptureSnapshotReply;
+import org.opendaylight.controller.cluster.raft.RaftActorRecoveryCohort;
+import org.opendaylight.controller.cluster.raft.RaftActorSnapshotCohort;
+import org.opendaylight.controller.cluster.raft.base.messages.FollowerInitialSyncUpStatus;
+import org.opendaylight.controller.cluster.raft.messages.AppendEntriesReply;
+import org.opendaylight.controller.cluster.raft.protobuff.client.messages.CompositeModificationByteStringPayload;
 import org.opendaylight.controller.cluster.raft.protobuff.client.messages.CompositeModificationPayload;
-import org.opendaylight.controller.cluster.raft.protobuff.client.messages.Payload;
-import org.opendaylight.controller.md.sal.common.api.data.AsyncDataChangeListener;
-import org.opendaylight.controller.md.sal.common.api.data.ReadFailedException;
-import org.opendaylight.controller.md.sal.dom.store.impl.InMemoryDOMDataStore;
-import org.opendaylight.controller.md.sal.dom.store.impl.InMemoryDOMDataStoreFactory;
-import org.opendaylight.controller.protobuff.messages.common.NormalizedNodeMessages;
-import org.opendaylight.controller.sal.core.spi.data.DOMStoreReadTransaction;
-import org.opendaylight.controller.sal.core.spi.data.DOMStoreThreePhaseCommitCohort;
-import org.opendaylight.controller.sal.core.spi.data.DOMStoreTransactionChain;
-import org.opendaylight.controller.sal.core.spi.data.DOMStoreTransactionFactory;
-import org.opendaylight.controller.sal.core.spi.data.DOMStoreWriteTransaction;
-import org.opendaylight.yangtools.concepts.ListenerRegistration;
-import org.opendaylight.yangtools.yang.data.api.YangInstanceIdentifier;
-import org.opendaylight.yangtools.yang.data.api.schema.NormalizedNode;
+import org.opendaylight.yangtools.yang.data.api.schema.tree.DataTree;
+import org.opendaylight.yangtools.yang.data.api.schema.tree.DataTreeCandidate;
+import org.opendaylight.yangtools.yang.data.api.schema.tree.DataValidationFailedException;
+import org.opendaylight.yangtools.yang.data.api.schema.tree.ModificationType;
 import org.opendaylight.yangtools.yang.model.api.SchemaContext;
-import java.util.ArrayList;
-import java.util.Collection;
-import java.util.HashMap;
-import java.util.List;
-import java.util.Map;
-import java.util.concurrent.ExecutionException;
+import scala.concurrent.duration.Duration;
+import scala.concurrent.duration.FiniteDuration;
 
 /**
  * A Shard represents a portion of the logical data tree <br/>
  * <p>
- * Our Shard uses InMemoryDataStore as it's internal representation and delegates all requests it
+ * Our Shard uses InMemoryDataTree as it's internal representation and delegates all requests it
  * </p>
  */
 public class Shard extends RaftActor {
 
-    public static final String DEFAULT_NAME = "default";
-
-    // The state of this Shard
-    private final InMemoryDOMDataStore store;
+    private static final Object TX_COMMIT_TIMEOUT_CHECK_MESSAGE = "txCommitTimeoutCheck";
 
-    private final Map<Object, DOMStoreThreePhaseCommitCohort>
-        modificationToCohort = new HashMap<>();
+    @VisibleForTesting
+    static final Object GET_SHARD_MBEAN_MESSAGE = "getShardMBeanMessage";
 
-    private final LoggingAdapter LOG =
-        Logging.getLogger(getContext().system(), this);
+    @VisibleForTesting
+    static final String DEFAULT_NAME = "default";
 
-    // By default persistent will be true and can be turned off using the system
-    // property shard.persistent
-    private final boolean persistent;
+    // The state of this Shard
+    private final ShardDataTree store;
 
     /// The name of this shard
-    private final ShardIdentifier name;
+    private final String name;
 
     private final ShardStats shardMBean;
 
-    private final List<ActorSelection> dataChangeListeners = new ArrayList<>();
+    private DatastoreContext datastoreContext;
 
-    private final DatastoreContext datastoreContext;
+    private final ShardCommitCoordinator commitCoordinator;
 
-    private SchemaContext schemaContext;
+    private long transactionCommitTimeout;
 
-    private ActorRef createSnapshotTransaction;
+    private Cancellable txCommitTimeoutCheckSchedule;
 
-    /**
-     * Coordinates persistence recovery on startup.
-     */
-    private ShardRecoveryCoordinator recoveryCoordinator;
-    private List<Object> currentLogRecoveryBatch;
+    private final Optional<ActorRef> roleChangeNotifier;
 
-    private final Map<String, DOMStoreTransactionChain> transactionChains = new HashMap<>();
+    private final MessageTracker appendEntriesReplyTracker;
 
-    protected Shard(ShardIdentifier name, Map<ShardIdentifier, String> peerAddresses,
-            DatastoreContext datastoreContext, SchemaContext schemaContext) {
-        super(name.toString(), mapPeerAddresses(peerAddresses),
-                Optional.of(datastoreContext.getShardRaftConfig()));
+    private final ShardTransactionActorFactory transactionActorFactory;
 
-        this.name = name;
-        this.datastoreContext = datastoreContext;
-        this.schemaContext = schemaContext;
+    private final ShardSnapshotCohort snapshotCohort;
 
-        String setting = System.getProperty("shard.persistent");
+    private final DataTreeChangeListenerSupport treeChangeSupport = new DataTreeChangeListenerSupport(this);
+    private final DataChangeListenerSupport changeSupport = new DataChangeListenerSupport(this);
 
-        this.persistent = !"false".equals(setting);
+    protected Shard(final ShardIdentifier name, final Map<String, String> peerAddresses,
+            final DatastoreContext datastoreContext, final SchemaContext schemaContext) {
+        super(name.toString(), new HashMap<>(peerAddresses), Optional.of(datastoreContext.getShardRaftConfig()),
+                DataStoreVersions.CURRENT_VERSION);
 
-        LOG.info("Shard created : {} persistent : {}", name, persistent);
+        this.name = name.toString();
+        this.datastoreContext = datastoreContext;
 
-        store = InMemoryDOMDataStoreFactory.create(name.toString(), null,
-                datastoreContext.getDataStoreProperties());
+        setPersistence(datastoreContext.isPersistent());
 
-        if(schemaContext != null) {
-            store.onGlobalContextUpdated(schemaContext);
-        }
+        LOG.info("Shard created : {}, persistent : {}", name, datastoreContext.isPersistent());
+
+        store = new ShardDataTree(schemaContext);
 
         shardMBean = ShardMBeanFactory.getShardStatsMBean(name.toString(),
                 datastoreContext.getDataStoreMXBeanType());
-        shardMBean.setDataStoreExecutor(store.getDomStoreExecutor());
-        shardMBean.setNotificationManager(store.getDataChangeListenerNotificationManager());
+        shardMBean.setShardActor(getSelf());
 
         if (isMetricsCaptureEnabled()) {
             getContext().become(new MeteringBehavior(this));
         }
-    }
 
-    private static Map<String, String> mapPeerAddresses(
-        Map<ShardIdentifier, String> peerAddresses) {
-        Map<String, String> map = new HashMap<>();
+        commitCoordinator = new ShardCommitCoordinator(store,
+                datastoreContext.getShardCommitQueueExpiryTimeoutInMillis(),
+                datastoreContext.getShardTransactionCommitQueueCapacity(), self(), LOG, this.name);
 
-        for (Map.Entry<ShardIdentifier, String> entry : peerAddresses
-            .entrySet()) {
-            map.put(entry.getKey().toString(), entry.getValue());
-        }
+        setTransactionCommitTimeout();
+
+        // create a notifier actor for each cluster member
+        roleChangeNotifier = createRoleChangeNotifier(name.toString());
+
+        appendEntriesReplyTracker = new MessageTracker(AppendEntriesReply.class,
+                getRaftActorContext().getConfigParams().getIsolatedCheckIntervalInMillis());
+
+        transactionActorFactory = new ShardTransactionActorFactory(store, datastoreContext,
+                new Dispatchers(context().system().dispatchers()).getDispatcherPath(
+                        Dispatchers.DispatcherType.Transaction), self(), getContext(), shardMBean);
 
-        return map;
+        snapshotCohort = new ShardSnapshotCohort(transactionActorFactory, store, LOG, this.name);
+    }
+
+    private void setTransactionCommitTimeout() {
+        transactionCommitTimeout = TimeUnit.MILLISECONDS.convert(
+                datastoreContext.getShardTransactionCommitTimeoutInSeconds(), TimeUnit.SECONDS) / 2;
     }
 
     public static Props props(final ShardIdentifier name,
-        final Map<ShardIdentifier, String> peerAddresses,
-        DatastoreContext datastoreContext, SchemaContext schemaContext) {
+        final Map<String, String> peerAddresses,
+        final DatastoreContext datastoreContext, final SchemaContext schemaContext) {
         Preconditions.checkNotNull(name, "name should not be null");
         Preconditions.checkNotNull(peerAddresses, "peerAddresses should not be null");
         Preconditions.checkNotNull(datastoreContext, "dataStoreContext should not be null");
@@ -173,479 +174,542 @@ public class Shard extends RaftActor {
         return Props.create(new ShardCreator(name, peerAddresses, datastoreContext, schemaContext));
     }
 
-    @Override public void onReceiveRecover(Object message) {
+    private Optional<ActorRef> createRoleChangeNotifier(String shardId) {
+        ActorRef shardRoleChangeNotifier = this.getContext().actorOf(
+            RoleChangeNotifier.getProps(shardId), shardId + "-notifier");
+        return Optional.of(shardRoleChangeNotifier);
+    }
+
+    @Override
+    public void postStop() {
+        LOG.info("Stopping Shard {}", persistenceId());
+
+        super.postStop();
+
+        if(txCommitTimeoutCheckSchedule != null) {
+            txCommitTimeoutCheckSchedule.cancel();
+        }
+
+        shardMBean.unregisterMBean();
+    }
+
+    @Override
+    public void onReceiveRecover(final Object message) throws Exception {
         if(LOG.isDebugEnabled()) {
-            LOG.debug("onReceiveRecover: Received message {} from {}",
-                message.getClass().toString(),
-                getSender());
+            LOG.debug("{}: onReceiveRecover: Received message {} from {}", persistenceId(),
+                message.getClass().toString(), getSender());
         }
 
         if (message instanceof RecoveryFailure){
-            LOG.error(((RecoveryFailure) message).cause(), "Recovery failed because of this cause");
+            LOG.error("{}: Recovery failed because of this cause",
+                    persistenceId(), ((RecoveryFailure) message).cause());
+
+            // Even though recovery failed, we still need to finish our recovery, eg send the
+            // ActorInitialized message and start the txCommitTimeoutCheckSchedule.
+            onRecoveryComplete();
         } else {
             super.onReceiveRecover(message);
+            if(LOG.isTraceEnabled()) {
+                appendEntriesReplyTracker.begin();
+            }
         }
     }
 
-    @Override public void onReceiveCommand(Object message) {
-        if(LOG.isDebugEnabled()) {
-            LOG.debug("onReceiveCommand: Received message {} from {}",
-                message.getClass().toString(),
-                getSender());
+    @Override
+    public void onReceiveCommand(final Object message) throws Exception {
+
+        MessageTracker.Context context = appendEntriesReplyTracker.received(message);
+
+        if(context.error().isPresent()){
+            LOG.trace("{} : AppendEntriesReply failed to arrive at the expected interval {}", persistenceId(),
+                    context.error());
         }
 
-        if(message.getClass().equals(ReadDataReply.SERIALIZABLE_CLASS)) {
-            // This must be for install snapshot. Don't want to open this up and trigger
-            // deSerialization
-            self()
-                .tell(new CaptureSnapshotReply(ReadDataReply.getNormalizedNodeByteString(message)),
-                    self());
-
-            createSnapshotTransaction = null;
-            // Send a PoisonPill instead of sending close transaction because we do not really need
-            // a response
-            getSender().tell(PoisonPill.getInstance(), self());
-
-        } else if (message.getClass().equals(CloseTransactionChain.SERIALIZABLE_CLASS)){
-            closeTransactionChain(CloseTransactionChain.fromSerializable(message));
-        } else if (message instanceof RegisterChangeListener) {
-            registerChangeListener((RegisterChangeListener) message);
-        } else if (message instanceof UpdateSchemaContext) {
-            updateSchemaContext((UpdateSchemaContext) message);
-        } else if (message instanceof ForwardedCommitTransaction) {
-            handleForwardedCommit((ForwardedCommitTransaction) message);
-        } else if (message.getClass()
-            .equals(CreateTransaction.SERIALIZABLE_CLASS)) {
-            if (isLeader()) {
-                createTransaction(CreateTransaction.fromSerializable(message));
-            } else if (getLeader() != null) {
-                getLeader().forward(message, getContext());
+        try {
+            if (CreateTransaction.SERIALIZABLE_CLASS.isInstance(message)) {
+                handleCreateTransaction(message);
+            } else if (BatchedModifications.class.isInstance(message)) {
+                handleBatchedModifications((BatchedModifications)message);
+            } else if (message instanceof ForwardedReadyTransaction) {
+                commitCoordinator.handleForwardedReadyTransaction((ForwardedReadyTransaction) message,
+                        getSender(), this);
+            } else if (message instanceof ReadyLocalTransaction) {
+                handleReadyLocalTransaction((ReadyLocalTransaction)message);
+            } else if (CanCommitTransaction.SERIALIZABLE_CLASS.isInstance(message)) {
+                handleCanCommitTransaction(CanCommitTransaction.fromSerializable(message));
+            } else if (CommitTransaction.SERIALIZABLE_CLASS.isInstance(message)) {
+                handleCommitTransaction(CommitTransaction.fromSerializable(message));
+            } else if (AbortTransaction.SERIALIZABLE_CLASS.isInstance(message)) {
+                handleAbortTransaction(AbortTransaction.fromSerializable(message));
+            } else if (CloseTransactionChain.SERIALIZABLE_CLASS.isInstance(message)) {
+                closeTransactionChain(CloseTransactionChain.fromSerializable(message));
+            } else if (message instanceof RegisterChangeListener) {
+                changeSupport.onMessage((RegisterChangeListener) message, isLeader());
+            } else if (message instanceof RegisterDataTreeChangeListener) {
+                treeChangeSupport.onMessage((RegisterDataTreeChangeListener) message, isLeader());
+            } else if (message instanceof UpdateSchemaContext) {
+                updateSchemaContext((UpdateSchemaContext) message);
+            } else if (message instanceof PeerAddressResolved) {
+                PeerAddressResolved resolved = (PeerAddressResolved) message;
+                setPeerAddress(resolved.getPeerId().toString(),
+                        resolved.getPeerAddress());
+            } else if (message.equals(TX_COMMIT_TIMEOUT_CHECK_MESSAGE)) {
+                handleTransactionCommitTimeoutCheck();
+            } else if(message instanceof DatastoreContext) {
+                onDatastoreContext((DatastoreContext)message);
+            } else if(message instanceof RegisterRoleChangeListener){
+                roleChangeNotifier.get().forward(message, context());
+            } else if (message instanceof FollowerInitialSyncUpStatus) {
+                shardMBean.setFollowerInitialSyncStatus(((FollowerInitialSyncUpStatus) message).isInitialSyncDone());
+                context().parent().tell(message, self());
+            } else if(GET_SHARD_MBEAN_MESSAGE.equals(message)){
+                sender().tell(getShardMBean(), self());
             } else {
-                getSender().tell(new akka.actor.Status.Failure(new IllegalStateException(
-                    "Could not find leader so transaction cannot be created")), getSelf());
+                super.onReceiveCommand(message);
             }
-        } else if (message instanceof PeerAddressResolved) {
-            PeerAddressResolved resolved = (PeerAddressResolved) message;
-            setPeerAddress(resolved.getPeerId().toString(),
-                resolved.getPeerAddress());
-        } else {
-            super.onReceiveCommand(message);
+        } finally {
+            context.done();
         }
     }
 
-    private void closeTransactionChain(CloseTransactionChain closeTransactionChain) {
-        DOMStoreTransactionChain chain =
-            transactionChains.remove(closeTransactionChain.getTransactionChainId());
+    @Override
+    protected Optional<ActorRef> getRoleChangeNotifier() {
+        return roleChangeNotifier;
+    }
 
-        if(chain != null) {
-            chain.close();
-        }
+    @Override
+    protected LeaderStateChanged newLeaderStateChanged(String memberId, String leaderId) {
+        return new ShardLeaderStateChanged(memberId, leaderId,
+                isLeader() ? Optional.<DataTree>of(store.getDataTree()) : Optional.<DataTree>absent());
     }
 
-    private ActorRef createTypedTransactionActor(
-        int transactionType,
-        ShardTransactionIdentifier transactionId,
-        String transactionChainId ) {
+    private void onDatastoreContext(DatastoreContext context) {
+        datastoreContext = context;
 
-        DOMStoreTransactionFactory factory = store;
+        commitCoordinator.setQueueCapacity(datastoreContext.getShardTransactionCommitQueueCapacity());
 
-        if(!transactionChainId.isEmpty()) {
-            factory = transactionChains.get(transactionChainId);
-            if(factory == null){
-                DOMStoreTransactionChain transactionChain = store.createTransactionChain();
-                transactionChains.put(transactionChainId, transactionChain);
-                factory = transactionChain;
-            }
-        }
+        setTransactionCommitTimeout();
 
-        if(this.schemaContext == null){
-            throw new NullPointerException("schemaContext should not be null");
+        if(datastoreContext.isPersistent() && !persistence().isRecoveryApplicable()) {
+            setPersistence(true);
+        } else if(!datastoreContext.isPersistent() && persistence().isRecoveryApplicable()) {
+            setPersistence(false);
         }
 
-        if (transactionType
-            == TransactionProxy.TransactionType.READ_ONLY.ordinal()) {
-
-            shardMBean.incrementReadOnlyTransactionCount();
-
-            return getContext().actorOf(
-                ShardTransaction.props(factory.newReadOnlyTransaction(), getSelf(),
-                        schemaContext,datastoreContext, shardMBean), transactionId.toString());
-
-        } else if (transactionType
-            == TransactionProxy.TransactionType.READ_WRITE.ordinal()) {
+        updateConfigParams(datastoreContext.getShardRaftConfig());
+    }
 
-            shardMBean.incrementReadWriteTransactionCount();
+    private void handleTransactionCommitTimeoutCheck() {
+        CohortEntry cohortEntry = commitCoordinator.getCurrentCohortEntry();
+        if(cohortEntry != null) {
+            if(cohortEntry.isExpired(transactionCommitTimeout)) {
+                LOG.warn("{}: Current transaction {} has timed out after {} ms - aborting",
+                        persistenceId(), cohortEntry.getTransactionID(), transactionCommitTimeout);
 
-            return getContext().actorOf(
-                ShardTransaction.props(factory.newReadWriteTransaction(), getSelf(),
-                        schemaContext, datastoreContext, shardMBean), transactionId.toString());
+                doAbortTransaction(cohortEntry.getTransactionID(), null);
+            }
+        }
 
+        commitCoordinator.cleanupExpiredCohortEntries();
+    }
 
-        } else if (transactionType
-            == TransactionProxy.TransactionType.WRITE_ONLY.ordinal()) {
+    private static boolean isEmptyCommit(final DataTreeCandidate candidate) {
+        return ModificationType.UNMODIFIED.equals(candidate.getRootNode().getModificationType());
+    }
 
-            shardMBean.incrementWriteOnlyTransactionCount();
+    void continueCommit(final CohortEntry cohortEntry) throws Exception {
+        final DataTreeCandidate candidate = cohortEntry.getCohort().getCandidate();
 
-            return getContext().actorOf(
-                ShardTransaction.props(factory.newWriteOnlyTransaction(), getSelf(),
-                        schemaContext, datastoreContext, shardMBean), transactionId.toString());
+        // If we do not have any followers and we are not using persistence
+        // or if cohortEntry has no modifications
+        // we can apply modification to the state immediately
+        if ((!hasFollowers() && !persistence().isRecoveryApplicable()) || isEmptyCommit(candidate)) {
+            applyModificationToState(cohortEntry.getReplySender(), cohortEntry.getTransactionID(), candidate);
         } else {
-            throw new IllegalArgumentException(
-                "Shard="+name + ":CreateTransaction message has unidentified transaction type="
-                    + transactionType);
+            Shard.this.persistData(cohortEntry.getReplySender(), cohortEntry.getTransactionID(),
+                DataTreeCandidatePayload.create(candidate));
         }
     }
 
-    private void createTransaction(CreateTransaction createTransaction) {
-        createTransaction(createTransaction.getTransactionType(),
-            createTransaction.getTransactionId(), createTransaction.getTransactionChainId());
+    private void handleCommitTransaction(final CommitTransaction commit) {
+        if(!commitCoordinator.handleCommit(commit.getTransactionID(), getSender(), this)) {
+            shardMBean.incrementFailedTransactionsCount();
+        }
     }
 
-    private ActorRef createTransaction(int transactionType, String remoteTransactionId, String transactionChainId) {
+    private void finishCommit(@Nonnull final ActorRef sender, @Nonnull final String transactionID, @Nonnull final CohortEntry cohortEntry) {
+        LOG.debug("{}: Finishing commit for transaction {}", persistenceId(), cohortEntry.getTransactionID());
 
-        ShardTransactionIdentifier transactionId =
-            ShardTransactionIdentifier.builder()
-                .remoteTransactionId(remoteTransactionId)
-                .build();
-        if(LOG.isDebugEnabled()) {
-            LOG.debug("Creating transaction : {} ", transactionId);
-        }
-        ActorRef transactionActor =
-            createTypedTransactionActor(transactionType, transactionId, transactionChainId);
-
-        getSender()
-            .tell(new CreateTransactionReply(
-                    Serialization.serializedActorPath(transactionActor),
-                    remoteTransactionId).toSerializable(),
-                getSelf());
+        try {
+            // We block on the future here so we don't have to worry about possibly accessing our
+            // state on a different thread outside of our dispatcher. Also, the data store
+            // currently uses a same thread executor anyway.
+            cohortEntry.getCohort().commit().get();
 
-        return transactionActor;
-    }
+            sender.tell(CommitTransactionReply.INSTANCE.toSerializable(), getSelf());
 
-    private void syncCommitTransaction(DOMStoreWriteTransaction transaction)
-        throws ExecutionException, InterruptedException {
-        DOMStoreThreePhaseCommitCohort commitCohort = transaction.ready();
-        commitCohort.preCommit().get();
-        commitCohort.commit().get();
-    }
+            shardMBean.incrementCommittedTransactionCount();
+            shardMBean.setLastCommittedTransactionTime(System.currentTimeMillis());
 
+        } catch (Exception e) {
+            sender.tell(new akka.actor.Status.Failure(e), getSelf());
 
-    private void commit(final ActorRef sender, Object serialized) {
-        Modification modification = MutableCompositeModification
-            .fromSerializable(serialized, schemaContext);
-        DOMStoreThreePhaseCommitCohort cohort =
-            modificationToCohort.remove(serialized);
-        if (cohort == null) {
-            // If there's no cached cohort then we must be applying replicated state.
-            commitWithNewTransaction(serialized);
-            return;
+            LOG.error("{}, An exception occurred while committing transaction {}", persistenceId(),
+                    transactionID, e);
+            shardMBean.incrementFailedTransactionsCount();
+        } finally {
+            commitCoordinator.currentTransactionComplete(transactionID, true);
         }
+    }
 
-        if(sender == null) {
-            LOG.error("Commit failed. Sender cannot be null");
-            return;
-        }
+    private void finishCommit(@Nonnull final ActorRef sender, final @Nonnull String transactionID) {
+        // With persistence enabled, this method is called via applyState by the leader strategy
+        // after the commit has been replicated to a majority of the followers.
 
-        ListenableFuture<Void> future = cohort.commit();
+        CohortEntry cohortEntry = commitCoordinator.getCohortEntryIfCurrent(transactionID);
+        if (cohortEntry == null) {
+            // The transaction is no longer the current commit. This can happen if the transaction
+            // was aborted prior, most likely due to timeout in the front-end. We need to finish
+            // committing the transaction though since it was successfully persisted and replicated
+            // however we can't use the original cohort b/c it was already preCommitted and may
+            // conflict with the current commit or may have been aborted so we commit with a new
+            // transaction.
+            cohortEntry = commitCoordinator.getAndRemoveCohortEntry(transactionID);
+            if(cohortEntry != null) {
+                try {
+                    store.applyForeignCandidate(transactionID, cohortEntry.getCohort().getCandidate());
+                } catch (DataValidationFailedException e) {
+                    shardMBean.incrementFailedTransactionsCount();
+                    LOG.error("{}: Failed to re-apply transaction {}", persistenceId(), transactionID, e);
+                }
 
-        Futures.addCallback(future, new FutureCallback<Void>() {
-            @Override
-            public void onSuccess(Void v) {
-                sender.tell(new CommitTransactionReply().toSerializable(), getSelf());
-                shardMBean.incrementCommittedTransactionCount();
-                shardMBean.setLastCommittedTransactionTime(System.currentTimeMillis());
+                sender.tell(CommitTransactionReply.INSTANCE.toSerializable(), getSelf());
+            } else {
+                // This really shouldn't happen - it likely means that persistence or replication
+                // took so long to complete such that the cohort entry was expired from the cache.
+                IllegalStateException ex = new IllegalStateException(
+                        String.format("%s: Could not finish committing transaction %s - no CohortEntry found",
+                                persistenceId(), transactionID));
+                LOG.error(ex.getMessage());
+                sender.tell(new akka.actor.Status.Failure(ex), getSelf());
             }
-
-            @Override
-            public void onFailure(Throwable t) {
-                LOG.error(t, "An exception happened during commit");
-                shardMBean.incrementFailedTransactionsCount();
-                sender.tell(new akka.actor.Status.Failure(t), getSelf());
+        } else {
+            finishCommit(sender, transactionID, cohortEntry);
+        }
+    }
+
+    private void handleCanCommitTransaction(final CanCommitTransaction canCommit) {
+        LOG.debug("{}: Can committing transaction {}", persistenceId(), canCommit.getTransactionID());
+        commitCoordinator.handleCanCommit(canCommit.getTransactionID(), getSender(), this);
+    }
+
+    private void noLeaderError(Object message) {
+        // TODO: rather than throwing an immediate exception, we could schedule a timer to try again to make
+        // it more resilient in case we're in the process of electing a new leader.
+        getSender().tell(new akka.actor.Status.Failure(new NoShardLeaderException(String.format(
+            "Could not find the leader for shard %s. This typically happens" +
+            " when the system is coming up or recovering and a leader is being elected. Try again" +
+            " later.", persistenceId()))), getSelf());
+    }
+
+    private void handleBatchedModifications(BatchedModifications batched) {
+        // This message is sent to prepare the modifications transaction directly on the Shard as an
+        // optimization to avoid the extra overhead of a separate ShardTransaction actor. On the last
+        // BatchedModifications message, the caller sets the ready flag in the message indicating
+        // modifications are complete. The reply contains the cohort actor path (this actor) for the caller
+        // to initiate the 3-phase commit. This also avoids the overhead of sending an additional
+        // ReadyTransaction message.
+
+        // If we're not the leader then forward to the leader. This is a safety measure - we shouldn't
+        // normally get here if we're not the leader as the front-end (TransactionProxy) should determine
+        // the primary/leader shard. However with timing and caching on the front-end, there's a small
+        // window where it could have a stale leader during leadership transitions.
+        //
+        if(isLeader()) {
+            try {
+                commitCoordinator.handleBatchedModifications(batched, getSender(), this);
+            } catch (Exception e) {
+                LOG.error("{}: Error handling BatchedModifications for Tx {}", persistenceId(),
+                        batched.getTransactionID(), e);
+                getSender().tell(new akka.actor.Status.Failure(e), getSelf());
             }
-        });
-
+        } else {
+            ActorSelection leader = getLeader();
+            if(leader != null) {
+                // TODO: what if this is not the first batch and leadership changed in between batched messages?
+                // We could check if the commitCoordinator already has a cached entry and forward all the previous
+                // batched modifications.
+                LOG.debug("{}: Forwarding BatchedModifications to leader {}", persistenceId(), leader);
+                leader.forward(batched, getContext());
+            } else {
+                noLeaderError(batched);
+            }
+        }
     }
 
-    private void commitWithNewTransaction(Object modification) {
-        DOMStoreWriteTransaction tx = store.newWriteOnlyTransaction();
-        MutableCompositeModification.fromSerializable(modification, schemaContext).apply(tx);
-        try {
-            syncCommitTransaction(tx);
-            shardMBean.incrementCommittedTransactionCount();
-        } catch (InterruptedException | ExecutionException e) {
-            shardMBean.incrementFailedTransactionsCount();
-            LOG.error(e, "Failed to commit");
+    private void handleReadyLocalTransaction(final ReadyLocalTransaction message) {
+        if (isLeader()) {
+            try {
+                commitCoordinator.handleReadyLocalTransaction(message, getSender(), this);
+            } catch (Exception e) {
+                LOG.error("{}: Error handling LocalModifications for Tx {}", persistenceId(),
+                        message.getTransactionID(), e);
+                getSender().tell(new akka.actor.Status.Failure(e), getSelf());
+            }
+        } else {
+            ActorSelection leader = getLeader();
+            if (leader != null) {
+                LOG.debug("{}: Forwarding LocalModifications to leader {}", persistenceId(), leader);
+                leader.forward(message, getContext());
+            } else {
+                noLeaderError(message);
+            }
         }
     }
 
-    private void handleForwardedCommit(ForwardedCommitTransaction message) {
-        Object serializedModification =
-            message.getModification().toSerializable();
+    private void handleAbortTransaction(final AbortTransaction abort) {
+        doAbortTransaction(abort.getTransactionID(), getSender());
+    }
+
+    void doAbortTransaction(final String transactionID, final ActorRef sender) {
+        final CohortEntry cohortEntry = commitCoordinator.getCohortEntryIfCurrent(transactionID);
+        if(cohortEntry != null) {
+            LOG.debug("{}: Aborting transaction {}", persistenceId(), transactionID);
+
+            // We don't remove the cached cohort entry here (ie pass false) in case the Tx was
+            // aborted during replication in which case we may still commit locally if replication
+            // succeeds.
+            commitCoordinator.currentTransactionComplete(transactionID, false);
+
+            final ListenableFuture<Void> future = cohortEntry.getCohort().abort();
+            final ActorRef self = getSelf();
+
+            Futures.addCallback(future, new FutureCallback<Void>() {
+                @Override
+                public void onSuccess(final Void v) {
+                    shardMBean.incrementAbortTransactionsCount();
+
+                    if(sender != null) {
+                        sender.tell(AbortTransactionReply.INSTANCE.toSerializable(), self);
+                    }
+                }
+
+                @Override
+                public void onFailure(final Throwable t) {
+                    LOG.error("{}: An exception happened during abort", persistenceId(), t);
 
-        modificationToCohort
-            .put(serializedModification, message.getCohort());
+                    if(sender != null) {
+                        sender.tell(new akka.actor.Status.Failure(t), self);
+                    }
+                }
+            });
+        }
+    }
 
-        if (persistent) {
-            this.persistData(getSender(), "identifier",
-                new CompositeModificationPayload(serializedModification));
+    private void handleCreateTransaction(final Object message) {
+        if (isLeader()) {
+            createTransaction(CreateTransaction.fromSerializable(message));
+        } else if (getLeader() != null) {
+            getLeader().forward(message, getContext());
         } else {
-            this.commit(getSender(), serializedModification);
+            getSender().tell(new akka.actor.Status.Failure(new NoShardLeaderException(String.format(
+                "Could not find leader for shard %s so transaction cannot be created. This typically happens" +
+                " when the system is coming up or recovering and a leader is being elected. Try again" +
+                " later.", persistenceId()))), getSelf());
         }
     }
 
-    private void updateSchemaContext(UpdateSchemaContext message) {
-        this.schemaContext = message.getSchemaContext();
-        updateSchemaContext(message.getSchemaContext());
-        store.onGlobalContextUpdated(message.getSchemaContext());
+    private void closeTransactionChain(final CloseTransactionChain closeTransactionChain) {
+        store.closeTransactionChain(closeTransactionChain.getTransactionChainId());
     }
 
-    @VisibleForTesting void updateSchemaContext(SchemaContext schemaContext) {
-        store.onGlobalContextUpdated(schemaContext);
+    private ActorRef createTypedTransactionActor(int transactionType,
+            ShardTransactionIdentifier transactionId, String transactionChainId,
+            short clientVersion ) {
+
+        return transactionActorFactory.newShardTransaction(TransactionType.fromInt(transactionType),
+                transactionId, transactionChainId, clientVersion);
     }
 
-    private void registerChangeListener(
-        RegisterChangeListener registerChangeListener) {
+    private void createTransaction(CreateTransaction createTransaction) {
+        try {
+            ActorRef transactionActor = createTransaction(createTransaction.getTransactionType(),
+                createTransaction.getTransactionId(), createTransaction.getTransactionChainId(),
+                createTransaction.getVersion());
 
-        if(LOG.isDebugEnabled()) {
-            LOG.debug("registerDataChangeListener for {}", registerChangeListener
-                .getPath());
+            getSender().tell(new CreateTransactionReply(Serialization.serializedActorPath(transactionActor),
+                    createTransaction.getTransactionId()).toSerializable(), getSelf());
+        } catch (Exception e) {
+            getSender().tell(new akka.actor.Status.Failure(e), getSelf());
         }
+    }
 
-
-        ActorSelection dataChangeListenerPath = getContext()
-            .system().actorSelection(
-                registerChangeListener.getDataChangeListenerPath());
+    private ActorRef createTransaction(int transactionType, String remoteTransactionId,
+            String transactionChainId, short clientVersion) {
 
 
-        // Notify the listener if notifications should be enabled or not
-        // If this shard is the leader then it will enable notifications else
-        // it will not
-        dataChangeListenerPath
-            .tell(new EnableNotification(isLeader()), getSelf());
+        ShardTransactionIdentifier transactionId = new ShardTransactionIdentifier(remoteTransactionId);
 
-        // Now store a reference to the data change listener so it can be notified
-        // at a later point if notifications should be enabled or disabled
-        dataChangeListeners.add(dataChangeListenerPath);
+        if(LOG.isDebugEnabled()) {
+            LOG.debug("{}: Creating transaction : {} ", persistenceId(), transactionId);
+        }
 
-        AsyncDataChangeListener<YangInstanceIdentifier, NormalizedNode<?, ?>>
-            listener = new DataChangeListenerProxy(schemaContext, dataChangeListenerPath);
+        ActorRef transactionActor = createTypedTransactionActor(transactionType, transactionId,
+                transactionChainId, clientVersion);
 
-        ListenerRegistration<AsyncDataChangeListener<YangInstanceIdentifier, NormalizedNode<?, ?>>>
-            registration = store.registerChangeListener(registerChangeListener.getPath(),
-                listener, registerChangeListener.getScope());
-        ActorRef listenerRegistration =
-            getContext().actorOf(
-                DataChangeListenerRegistration.props(registration));
+        return transactionActor;
+    }
 
-        if(LOG.isDebugEnabled()) {
-            LOG.debug(
-                "registerDataChangeListener sending reply, listenerRegistrationPath = {} "
-                , listenerRegistration.path().toString());
+    private void commitWithNewTransaction(final Modification modification) {
+        ReadWriteShardDataTreeTransaction tx = store.newReadWriteTransaction(modification.toString(), null);
+        modification.apply(tx.getSnapshot());
+        try {
+            snapshotCohort.syncCommitTransaction(tx);
+            shardMBean.incrementCommittedTransactionCount();
+            shardMBean.setLastCommittedTransactionTime(System.currentTimeMillis());
+        } catch (Exception e) {
+            shardMBean.incrementFailedTransactionsCount();
+            LOG.error("{}: Failed to commit", persistenceId(), e);
         }
-
-        getSender()
-            .tell(new RegisterChangeListenerReply(listenerRegistration.path()),
-                getSelf());
     }
 
-    private boolean isMetricsCaptureEnabled(){
-        CommonConfig config = new CommonConfig(getContext().system().settings().config());
-        return config.isMetricCaptureEnabled();
+    private void updateSchemaContext(final UpdateSchemaContext message) {
+        updateSchemaContext(message.getSchemaContext());
     }
 
-    @Override
-    protected
-    void startLogRecoveryBatch(int maxBatchSize) {
-        currentLogRecoveryBatch = Lists.newArrayListWithCapacity(maxBatchSize);
+    @VisibleForTesting
+    void updateSchemaContext(final SchemaContext schemaContext) {
+        store.updateSchemaContext(schemaContext);
+    }
 
-        if(LOG.isDebugEnabled()) {
-            LOG.debug("{} : starting log recovery batch with max size {}", persistenceId(), maxBatchSize);
-        }
+    private boolean isMetricsCaptureEnabled() {
+        CommonConfig config = new CommonConfig(getContext().system().settings().config());
+        return config.isMetricCaptureEnabled();
     }
 
     @Override
-    protected void appendRecoveredLogEntry(Payload data) {
-        if (data instanceof CompositeModificationPayload) {
-            currentLogRecoveryBatch.add(((CompositeModificationPayload) data).getModification());
-        } else {
-            LOG.error("Unknown state received {} during recovery", data);
-        }
+    protected RaftActorSnapshotCohort getRaftActorSnapshotCohort() {
+        return snapshotCohort;
     }
 
     @Override
-    protected void applyRecoverySnapshot(ByteString snapshot) {
-        if(recoveryCoordinator == null) {
-            recoveryCoordinator = new ShardRecoveryCoordinator(persistenceId(), schemaContext);
-        }
-
-        recoveryCoordinator.submit(snapshot, store.newWriteOnlyTransaction());
-
-        if(LOG.isDebugEnabled()) {
-            LOG.debug("{} : submitted recovery sbapshot", persistenceId());
-        }
+    @Nonnull
+    protected RaftActorRecoveryCohort getRaftActorRecoveryCohort() {
+        return new ShardRecoveryCoordinator(store, persistenceId(), LOG);
     }
 
     @Override
-    protected void applyCurrentLogRecoveryBatch() {
-        if(recoveryCoordinator == null) {
-            recoveryCoordinator = new ShardRecoveryCoordinator(persistenceId(), schemaContext);
-        }
-
-        recoveryCoordinator.submit(currentLogRecoveryBatch, store.newWriteOnlyTransaction());
+    protected void onRecoveryComplete() {
+        store.recoveryDone();
+        //notify shard manager
+        getContext().parent().tell(new ActorInitialized(), getSelf());
 
-        if(LOG.isDebugEnabled()) {
-            LOG.debug("{} : submitted log recovery batch with size {}", persistenceId(),
-                    currentLogRecoveryBatch.size());
+        // Being paranoid here - this method should only be called once but just in case...
+        if(txCommitTimeoutCheckSchedule == null) {
+            // Schedule a message to be periodically sent to check if the current in-progress
+            // transaction should be expired and aborted.
+            FiniteDuration period = Duration.create(transactionCommitTimeout / 3, TimeUnit.MILLISECONDS);
+            txCommitTimeoutCheckSchedule = getContext().system().scheduler().schedule(
+                    period, period, getSelf(),
+                    TX_COMMIT_TIMEOUT_CHECK_MESSAGE, getContext().dispatcher(), ActorRef.noSender());
         }
     }
 
     @Override
-    protected void onRecoveryComplete() {
-        if(recoveryCoordinator != null) {
-            Collection<DOMStoreWriteTransaction> txList = recoveryCoordinator.getTransactions();
-
-            if(LOG.isDebugEnabled()) {
-                LOG.debug("{} : recovery complete - committing {} Tx's", persistenceId(), txList.size());
-            }
-
-            for(DOMStoreWriteTransaction tx: txList) {
+    protected void applyState(final ActorRef clientActor, final String identifier, final Object data) {
+        if (data instanceof DataTreeCandidatePayload) {
+            if (clientActor == null) {
+                // No clientActor indicates a replica coming from the leader
                 try {
-                    syncCommitTransaction(tx);
-                    shardMBean.incrementCommittedTransactionCount();
-                } catch (InterruptedException | ExecutionException e) {
-                    shardMBean.incrementFailedTransactionsCount();
-                    LOG.error(e, "Failed to commit");
+                    store.applyForeignCandidate(identifier, ((DataTreeCandidatePayload)data).getCandidate());
+                } catch (DataValidationFailedException | IOException e) {
+                    LOG.error("{}: Error applying replica {}", persistenceId(), identifier, e);
                 }
+            } else {
+                // Replication consensus reached, proceed to commit
+                finishCommit(clientActor, identifier);
             }
-        }
-
-        recoveryCoordinator = null;
-        currentLogRecoveryBatch = null;
-        updateJournalStats();
-    }
-
-    @Override
-    protected void applyState(ActorRef clientActor, String identifier, Object data) {
-
-        if (data instanceof CompositeModificationPayload) {
+        } else if (data instanceof ModificationPayload) {
+            try {
+                applyModificationToState(clientActor, identifier, ((ModificationPayload) data).getModification());
+            } catch (ClassNotFoundException | IOException e) {
+                LOG.error("{}: Error extracting ModificationPayload", persistenceId(), e);
+            }
+        } else if (data instanceof CompositeModificationPayload) {
             Object modification = ((CompositeModificationPayload) data).getModification();
 
-            if (modification != null) {
-                commit(clientActor, modification);
-            } else {
-                LOG.error(
-                    "modification is null - this is very unexpected, clientActor = {}, identifier = {}",
-                    identifier, clientActor != null ? clientActor.path().toString() : null);
-            }
+            applyModificationToState(clientActor, identifier, modification);
+        } else if(data instanceof CompositeModificationByteStringPayload ){
+            Object modification = ((CompositeModificationByteStringPayload) data).getModification();
 
+            applyModificationToState(clientActor, identifier, modification);
         } else {
-            LOG.error("Unknown state received {} Class loader = {} CompositeNodeMod.ClassLoader = {}",
-                    data, data.getClass().getClassLoader(),
+            LOG.error("{}: Unknown state received {} Class loader = {} CompositeNodeMod.ClassLoader = {}",
+                    persistenceId(), data, data.getClass().getClassLoader(),
                     CompositeModificationPayload.class.getClassLoader());
         }
-
-        updateJournalStats();
-
     }
 
-    private void updateJournalStats() {
-        ReplicatedLogEntry lastLogEntry = getLastLogEntry();
-
-        if (lastLogEntry != null) {
-            shardMBean.setLastLogIndex(lastLogEntry.getIndex());
-            shardMBean.setLastLogTerm(lastLogEntry.getTerm());
+    private void applyModificationToState(ActorRef clientActor, String identifier, Object modification) {
+        if(modification == null) {
+            LOG.error(
+                    "{}: modification is null - this is very unexpected, clientActor = {}, identifier = {}",
+                    persistenceId(), identifier, clientActor != null ? clientActor.path().toString() : null);
+        } else if(clientActor == null) {
+            // There's no clientActor to which to send a commit reply so we must be applying
+            // replicated state from the leader.
+            commitWithNewTransaction(MutableCompositeModification.fromSerializable(modification));
+        } else {
+            // This must be the OK to commit after replication consensus.
+            finishCommit(clientActor, identifier);
         }
-
-        shardMBean.setCommitIndex(getCommitIndex());
-        shardMBean.setLastApplied(getLastApplied());
     }
 
     @Override
-    protected void createSnapshot() {
-        if (createSnapshotTransaction == null) {
-
-            // Create a transaction. We are really going to treat the transaction as a worker
-            // so that this actor does not get block building the snapshot
-            createSnapshotTransaction = createTransaction(
-                TransactionProxy.TransactionType.READ_ONLY.ordinal(),
-                "createSnapshot", "");
+    protected void onStateChanged() {
+        boolean isLeader = isLeader();
+        changeSupport.onLeadershipChange(isLeader);
+        treeChangeSupport.onLeadershipChange(isLeader);
 
-            createSnapshotTransaction.tell(
-                new ReadData(YangInstanceIdentifier.builder().build()).toSerializable(), self());
+        // If this actor is no longer the leader close all the transaction chains
+        if (!isLeader) {
+            if(LOG.isDebugEnabled()) {
+                LOG.debug(
+                    "{}: onStateChanged: Closing all transaction chains because shard {} is no longer the leader",
+                    persistenceId(), getId());
+            }
 
+            store.closeAllTransactionChains();
         }
     }
 
-    @VisibleForTesting
     @Override
-    protected void applySnapshot(ByteString snapshot) {
-        // Since this will be done only on Recovery or when this actor is a Follower
-        // we can safely commit everything in here. We not need to worry about event notifications
-        // as they would have already been disabled on the follower
-
-        LOG.info("Applying snapshot");
-        try {
-            DOMStoreWriteTransaction transaction = store.newWriteOnlyTransaction();
-            NormalizedNodeMessages.Node serializedNode = NormalizedNodeMessages.Node.parseFrom(snapshot);
-            NormalizedNode<?, ?> node = new NormalizedNodeToNodeCodec(schemaContext)
-                .decode(serializedNode);
-
-            // delete everything first
-            transaction.delete(YangInstanceIdentifier.builder().build());
-
-            // Add everything from the remote node back
-            transaction.write(YangInstanceIdentifier.builder().build(), node);
-            syncCommitTransaction(transaction);
-        } catch (InvalidProtocolBufferException | InterruptedException | ExecutionException e) {
-            LOG.error(e, "An exception occurred when applying snapshot");
-        } finally {
-            LOG.info("Done applying snapshot");
-        }
+    protected void onLeaderChanged(String oldLeader, String newLeader) {
+        shardMBean.incrementLeadershipChangeCount();
     }
 
-    @Override protected void onStateChanged() {
-        for (ActorSelection dataChangeListener : dataChangeListeners) {
-            dataChangeListener
-                .tell(new EnableNotification(isLeader()), getSelf());
-        }
-
-        shardMBean.setRaftState(getRaftState().name());
-        shardMBean.setCurrentTerm(getCurrentTerm());
-
-        // If this actor is no longer the leader close all the transaction chains
-        if(!isLeader()){
-            for(Map.Entry<String, DOMStoreTransactionChain> entry : transactionChains.entrySet()){
-                if(LOG.isDebugEnabled()) {
-                    LOG.debug(
-                        "onStateChanged: Closing transaction chain {} because shard {} is no longer the leader",
-                        entry.getKey(), getId());
-                }
-                entry.getValue().close();
-            }
-
-            transactionChains.clear();
-        }
+    @Override
+    public String persistenceId() {
+        return this.name;
     }
 
-    @Override protected void onLeaderChanged(String oldLeader, String newLeader) {
-        shardMBean.setLeader(newLeader);
+    @VisibleForTesting
+    ShardCommitCoordinator getCommitCoordinator() {
+        return commitCoordinator;
     }
 
-    @Override public String persistenceId() {
-        return this.name.toString();
-    }
 
     private static class ShardCreator implements Creator<Shard> {
 
         private static final long serialVersionUID = 1L;
 
         final ShardIdentifier name;
-        final Map<ShardIdentifier, String> peerAddresses;
+        final Map<String, String> peerAddresses;
         final DatastoreContext datastoreContext;
         final SchemaContext schemaContext;
 
-        ShardCreator(ShardIdentifier name, Map<ShardIdentifier, String> peerAddresses,
-                DatastoreContext datastoreContext, SchemaContext schemaContext) {
+        ShardCreator(final ShardIdentifier name, final Map<String, String> peerAddresses,
+                final DatastoreContext datastoreContext, final SchemaContext schemaContext) {
             this.name = name;
             this.peerAddresses = peerAddresses;
             this.datastoreContext = datastoreContext;
@@ -659,29 +723,8 @@ public class Shard extends RaftActor {
     }
 
     @VisibleForTesting
-    NormalizedNode<?,?> readStore(YangInstanceIdentifier id)
-            throws ExecutionException, InterruptedException {
-        DOMStoreReadTransaction transaction = store.newReadOnlyTransaction();
-
-        CheckedFuture<Optional<NormalizedNode<?, ?>>, ReadFailedException> future =
-            transaction.read(id);
-
-        Optional<NormalizedNode<?, ?>> optional = future.get();
-        NormalizedNode<?, ?> node = optional.isPresent()? optional.get() : null;
-
-        transaction.close();
-
-        return node;
-    }
-
-    @VisibleForTesting
-    void writeToStore(YangInstanceIdentifier id, NormalizedNode<?,?> node)
-        throws ExecutionException, InterruptedException {
-        DOMStoreWriteTransaction transaction = store.newWriteOnlyTransaction();
-
-        transaction.write(id, node);
-
-        syncCommitTransaction(transaction);
+    public ShardDataTree getDataStore() {
+        return store;
     }
 
     @VisibleForTesting