Slice front-end request messages
[controller.git] / opendaylight / md-sal / sal-distributed-datastore / src / main / java / org / opendaylight / controller / cluster / datastore / Shard.java
index c98e11d60a2e063395ed0b428d2eb5baaee77ea2..ebe5498481d9fe3befdabafbb4976f635a7b15b3 100644 (file)
@@ -11,20 +11,50 @@ package org.opendaylight.controller.cluster.datastore;
 import akka.actor.ActorRef;
 import akka.actor.ActorSelection;
 import akka.actor.Cancellable;
+import akka.actor.ExtendedActorSystem;
 import akka.actor.Props;
+import akka.actor.Status;
+import akka.actor.Status.Failure;
+import akka.serialization.JavaSerializer;
 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.base.Ticker;
+import com.google.common.base.Verify;
+import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableMap;
+import com.google.common.collect.Range;
 import java.io.IOException;
+import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collections;
 import java.util.Map;
 import java.util.concurrent.TimeUnit;
 import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
+import org.opendaylight.controller.cluster.access.ABIVersion;
+import org.opendaylight.controller.cluster.access.commands.ConnectClientRequest;
+import org.opendaylight.controller.cluster.access.commands.ConnectClientSuccess;
+import org.opendaylight.controller.cluster.access.commands.LocalHistoryRequest;
+import org.opendaylight.controller.cluster.access.commands.NotLeaderException;
+import org.opendaylight.controller.cluster.access.commands.OutOfSequenceEnvelopeException;
+import org.opendaylight.controller.cluster.access.commands.TransactionRequest;
+import org.opendaylight.controller.cluster.access.concepts.ClientIdentifier;
+import org.opendaylight.controller.cluster.access.concepts.FrontendIdentifier;
+import org.opendaylight.controller.cluster.access.concepts.LocalHistoryIdentifier;
+import org.opendaylight.controller.cluster.access.concepts.Request;
+import org.opendaylight.controller.cluster.access.concepts.RequestEnvelope;
+import org.opendaylight.controller.cluster.access.concepts.RequestException;
+import org.opendaylight.controller.cluster.access.concepts.RequestSuccess;
+import org.opendaylight.controller.cluster.access.concepts.RetiredGenerationException;
+import org.opendaylight.controller.cluster.access.concepts.RuntimeRequestException;
+import org.opendaylight.controller.cluster.access.concepts.SliceableMessage;
 import org.opendaylight.controller.cluster.access.concepts.TransactionIdentifier;
+import org.opendaylight.controller.cluster.access.concepts.UnsupportedRequestException;
 import org.opendaylight.controller.cluster.common.actor.CommonConfig;
+import org.opendaylight.controller.cluster.common.actor.Dispatchers;
+import org.opendaylight.controller.cluster.common.actor.Dispatchers.DispatcherType;
 import org.opendaylight.controller.cluster.common.actor.MessageTracker;
 import org.opendaylight.controller.cluster.common.actor.MessageTracker.Error;
 import org.opendaylight.controller.cluster.common.actor.MeteringBehavior;
@@ -40,26 +70,35 @@ import org.opendaylight.controller.cluster.datastore.messages.CloseTransactionCh
 import org.opendaylight.controller.cluster.datastore.messages.CommitTransaction;
 import org.opendaylight.controller.cluster.datastore.messages.CreateTransaction;
 import org.opendaylight.controller.cluster.datastore.messages.CreateTransactionReply;
-import org.opendaylight.controller.cluster.datastore.messages.DatastoreSnapshot;
-import org.opendaylight.controller.cluster.datastore.messages.DatastoreSnapshot.ShardSnapshot;
 import org.opendaylight.controller.cluster.datastore.messages.ForwardedReadyTransaction;
 import org.opendaylight.controller.cluster.datastore.messages.GetShardDataTree;
+import org.opendaylight.controller.cluster.datastore.messages.MakeLeaderLocal;
+import org.opendaylight.controller.cluster.datastore.messages.OnDemandShardState;
 import org.opendaylight.controller.cluster.datastore.messages.PeerAddressResolved;
+import org.opendaylight.controller.cluster.datastore.messages.PersistAbortTransactionPayload;
 import org.opendaylight.controller.cluster.datastore.messages.ReadyLocalTransaction;
 import org.opendaylight.controller.cluster.datastore.messages.RegisterChangeListener;
 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.utils.Dispatchers;
+import org.opendaylight.controller.cluster.datastore.persisted.AbortTransactionPayload;
+import org.opendaylight.controller.cluster.datastore.persisted.DatastoreSnapshot;
+import org.opendaylight.controller.cluster.datastore.persisted.DatastoreSnapshot.ShardSnapshot;
+import org.opendaylight.controller.cluster.messaging.MessageAssembler;
+import org.opendaylight.controller.cluster.messaging.MessageSlicer;
+import org.opendaylight.controller.cluster.messaging.SliceOptions;
 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.LeadershipTransferFailedException;
 import org.opendaylight.controller.cluster.raft.RaftActor;
 import org.opendaylight.controller.cluster.raft.RaftActorRecoveryCohort;
 import org.opendaylight.controller.cluster.raft.RaftActorSnapshotCohort;
 import org.opendaylight.controller.cluster.raft.RaftState;
 import org.opendaylight.controller.cluster.raft.base.messages.FollowerInitialSyncUpStatus;
+import org.opendaylight.controller.cluster.raft.client.messages.OnDemandRaftState;
 import org.opendaylight.controller.cluster.raft.messages.AppendEntriesReply;
+import org.opendaylight.controller.cluster.raft.messages.RequestLeadership;
 import org.opendaylight.controller.cluster.raft.messages.ServerRemoved;
 import org.opendaylight.controller.cluster.raft.protobuff.client.messages.Payload;
 import org.opendaylight.yangtools.concepts.Identifier;
@@ -67,12 +106,14 @@ import org.opendaylight.yangtools.yang.data.api.schema.tree.DataValidationFailed
 import org.opendaylight.yangtools.yang.data.api.schema.tree.TipProducingDataTree;
 import org.opendaylight.yangtools.yang.data.api.schema.tree.TreeType;
 import org.opendaylight.yangtools.yang.model.api.SchemaContext;
+import org.opendaylight.yangtools.yang.model.api.SchemaContextProvider;
 import scala.concurrent.duration.Duration;
 import scala.concurrent.duration.FiniteDuration;
 
 /**
  * A Shard represents a portion of the logical data tree.
- * <p/>
+ *
+ * <p>
  * Our Shard uses InMemoryDataTree as it's internal representation and delegates all requests it
  */
 public class Shard extends RaftActor {
@@ -93,9 +134,27 @@ public class Shard extends RaftActor {
         }
     };
 
+    static final Object RESUME_NEXT_PENDING_TRANSACTION = new Object() {
+        @Override
+        public String toString() {
+            return "resumeNextPendingTransaction";
+        }
+    };
+
     // FIXME: shard names should be encapsulated in their own class and this should be exposed as a constant.
     public static final String DEFAULT_NAME = "default";
 
+    private static final Collection<ABIVersion> SUPPORTED_ABIVERSIONS;
+
+    static {
+        final ABIVersion[] values = ABIVersion.values();
+        final ABIVersion[] real = Arrays.copyOfRange(values, 1, values.length - 1);
+        SUPPORTED_ABIVERSIONS = ImmutableList.copyOf(real).reverse();
+    }
+
+    // FIXME: make this a dynamic property based on mailbox size and maximum number of clients
+    private static final int CLIENT_MAX_MESSAGES = 1000;
+
     // The state of this Shard
     private final ShardDataTree store;
 
@@ -128,7 +187,14 @@ public class Shard extends RaftActor {
 
     private final ShardTransactionMessageRetrySupport messageRetrySupport;
 
-    private final FrontendMetadata frontendMetadata = new FrontendMetadata();
+    private final FrontendMetadata frontendMetadata;
+    private Map<FrontendIdentifier, LeaderFrontendState> knownFrontends = ImmutableMap.of();
+    private boolean paused;
+
+    private final MessageSlicer responseMessageSlicer;
+    private final Dispatchers dispatchers;
+
+    private final MessageAssembler requestMessageAssembler;
 
     protected Shard(final AbstractBuilder<?, ?> builder) {
         super(builder.getId().toString(), builder.getPeerAddresses(),
@@ -137,21 +203,23 @@ public class Shard extends RaftActor {
         this.name = builder.getId().toString();
         this.datastoreContext = builder.getDatastoreContext();
         this.restoreFromSnapshot = builder.getRestoreFromSnapshot();
+        this.frontendMetadata = new FrontendMetadata(name);
 
         setPersistence(datastoreContext.isPersistent());
 
         LOG.info("Shard created : {}, persistent : {}", name, datastoreContext.isPersistent());
 
         ShardDataTreeChangeListenerPublisherActorProxy treeChangeListenerPublisher =
-                new ShardDataTreeChangeListenerPublisherActorProxy(getContext(), name + "-DTCL-publisher");
+                new ShardDataTreeChangeListenerPublisherActorProxy(getContext(), name + "-DTCL-publisher", name);
         ShardDataChangeListenerPublisherActorProxy dataChangeListenerPublisher =
-                new ShardDataChangeListenerPublisherActorProxy(getContext(), name + "-DCL-publisher");
+                new ShardDataChangeListenerPublisherActorProxy(getContext(), name + "-DCL-publisher", name);
         if (builder.getDataTree() != null) {
             store = new ShardDataTree(this, builder.getSchemaContext(), builder.getDataTree(),
-                    treeChangeListenerPublisher, dataChangeListenerPublisher, name);
+                    treeChangeListenerPublisher, dataChangeListenerPublisher, name, frontendMetadata);
         } else {
             store = new ShardDataTree(this, builder.getSchemaContext(), builder.getTreeType(),
-                    treeChangeListenerPublisher, dataChangeListenerPublisher, name);
+                    builder.getDatastoreContext().getStoreRoot(), treeChangeListenerPublisher,
+                    dataChangeListenerPublisher, name, frontendMetadata);
         }
 
         shardMBean = ShardMBeanFactory.getShardStatsMBean(name, datastoreContext.getDataStoreMXBeanType(), this);
@@ -170,14 +238,25 @@ public class Shard extends RaftActor {
         appendEntriesReplyTracker = new MessageTracker(AppendEntriesReply.class,
                 getRaftActorContext().getConfigParams().getIsolatedCheckIntervalInMillis());
 
+        dispatchers = new Dispatchers(context().system().dispatchers());
         transactionActorFactory = new ShardTransactionActorFactory(store, datastoreContext,
-                new Dispatchers(context().system().dispatchers()).getDispatcherPath(
-                        Dispatchers.DispatcherType.Transaction), self(), getContext(), shardMBean);
+            dispatchers.getDispatcherPath(Dispatchers.DispatcherType.Transaction),
+                self(), getContext(), shardMBean, builder.getId().getShardName());
 
         snapshotCohort = ShardSnapshotCohort.create(getContext(), builder.getId().getMemberName(), store, LOG,
             this.name);
 
         messageRetrySupport = new ShardTransactionMessageRetrySupport(this);
+
+        responseMessageSlicer = MessageSlicer.builder().logContext(this.name)
+                .messageSliceSize(datastoreContext.getMaximumMessageSliceSize())
+                .fileBackedStreamFactory(getRaftActorContext().getFileBackedOutputStreamFactory())
+                .expireStateAfterInactivity(2, TimeUnit.MINUTES).build();
+
+        requestMessageAssembler = MessageAssembler.builder().logContext(this.name)
+                .fileBackedStreamFactory(getRaftActorContext().getFileBackedOutputStreamFactory())
+                .assembledMessageCallback((message, sender) -> self().tell(message, sender))
+                .expireStateAfterInactivity(datastoreContext.getRequestTimeout(), TimeUnit.NANOSECONDS).build();
     }
 
     private void setTransactionCommitTimeout() {
@@ -221,14 +300,22 @@ public class Shard extends RaftActor {
 
     @Override
     protected void handleNonRaftCommand(final Object message) {
-        try (final MessageTracker.Context context = appendEntriesReplyTracker.received(message)) {
+        try (MessageTracker.Context context = appendEntriesReplyTracker.received(message)) {
             final Optional<Error> maybeError = context.error();
             if (maybeError.isPresent()) {
                 LOG.trace("{} : AppendEntriesReply failed to arrive at the expected interval {}", persistenceId(),
                     maybeError.get());
             }
 
-            if (CreateTransaction.isSerializedType(message)) {
+            store.resetTransactionBatch();
+
+            if (message instanceof RequestEnvelope) {
+                handleRequestEnvelope((RequestEnvelope)message);
+            } else if (requestMessageAssembler.isHandledMessage(message)) {
+                handleRequestAssemblerMessage(message);
+            } else if (message instanceof ConnectClientRequest) {
+                handleConnectClient((ConnectClientRequest)message);
+            } else if (CreateTransaction.isSerializedType(message)) {
                 handleCreateTransaction(message);
             } else if (message instanceof BatchedModifications) {
                 handleBatchedModifications((BatchedModifications)message);
@@ -254,8 +341,7 @@ public class Shard extends RaftActor {
                 PeerAddressResolved resolved = (PeerAddressResolved) message;
                 setPeerAddress(resolved.getPeerId(), resolved.getPeerAddress());
             } else if (TX_COMMIT_TIMEOUT_CHECK_MESSAGE.equals(message)) {
-                store.checkForExpiredTransactions(transactionCommitTimeout);
-                commitCoordinator.checkForExpiredTransactions(transactionCommitTimeout, this);
+                commitTimeoutCheck();
             } else if (message instanceof DatastoreContext) {
                 onDatastoreContext((DatastoreContext)message);
             } else if (message instanceof RegisterRoleChangeListener) {
@@ -274,12 +360,213 @@ public class Shard extends RaftActor {
             } else if (message instanceof DataTreeCohortActorRegistry.CohortRegistryCommand) {
                 store.processCohortRegistryCommand(getSender(),
                         (DataTreeCohortActorRegistry.CohortRegistryCommand) message);
-            } else {
+            } else if (message instanceof PersistAbortTransactionPayload) {
+                final TransactionIdentifier txId = ((PersistAbortTransactionPayload) message).getTransactionId();
+                persistPayload(txId, AbortTransactionPayload.create(txId), true);
+            } else if (message instanceof MakeLeaderLocal) {
+                onMakeLeaderLocal();
+            } else if (RESUME_NEXT_PENDING_TRANSACTION.equals(message)) {
+                store.resumeNextPendingTransaction();
+            } else if (!responseMessageSlicer.handleMessage(message)) {
                 super.handleNonRaftCommand(message);
             }
         }
     }
 
+    private void handleRequestAssemblerMessage(Object message) {
+        dispatchers.getDispatcher(DispatcherType.Serialization).execute(() -> {
+            JavaSerializer.currentSystem().value_$eq((ExtendedActorSystem) context().system());
+            requestMessageAssembler.handleMessage(message, self());
+        });
+    }
+
+    @SuppressWarnings("checkstyle:IllegalCatch")
+    private void handleRequestEnvelope(final RequestEnvelope envelope) {
+        final long now = ticker().read();
+        try {
+            final RequestSuccess<?, ?> success = handleRequest(envelope, now);
+            if (success != null) {
+                final long executionTimeNanos = ticker().read() - now;
+                if (success instanceof SliceableMessage) {
+                    dispatchers.getDispatcher(DispatcherType.Serialization).execute(() ->
+                        responseMessageSlicer.slice(SliceOptions.builder().identifier(success.getTarget())
+                            .message(envelope.newSuccessEnvelope(success, executionTimeNanos))
+                            .sendTo(envelope.getMessage().getReplyTo()).replyTo(self())
+                            .onFailureCallback(t -> {
+                                LOG.warn("Error slicing response {}", success, t);
+                            }).build()));
+                } else {
+                    envelope.sendSuccess(success, executionTimeNanos);
+                }
+            }
+        } catch (RequestException e) {
+            LOG.debug("{}: request {} failed", persistenceId(), envelope, e);
+            envelope.sendFailure(e, ticker().read() - now);
+        } catch (Exception e) {
+            LOG.debug("{}: request {} caused failure", persistenceId(), envelope, e);
+            envelope.sendFailure(new RuntimeRequestException("Request failed to process", e),
+                ticker().read() - now);
+        }
+    }
+
+    private void commitTimeoutCheck() {
+        store.checkForExpiredTransactions(transactionCommitTimeout, this::updateAccess);
+        commitCoordinator.checkForExpiredTransactions(transactionCommitTimeout, this);
+        requestMessageAssembler.checkExpiredAssembledMessageState();
+    }
+
+    private Optional<Long> updateAccess(final SimpleShardDataTreeCohort cohort) {
+        final FrontendIdentifier frontend = cohort.getIdentifier().getHistoryId().getClientId().getFrontendId();
+        final LeaderFrontendState state = knownFrontends.get(frontend);
+        if (state == null) {
+            // Not tell-based protocol, do nothing
+            return Optional.absent();
+        }
+
+        if (isIsolatedLeader()) {
+            // We are isolated and no new request can come through until we emerge from it. We are still updating
+            // liveness of frontend when we see it attempting to communicate. Use the last access timer.
+            return Optional.of(state.getLastSeenTicks());
+        }
+
+        // If this frontend has freshly connected, give it some time to catch up before killing its transactions.
+        return Optional.of(state.getLastConnectTicks());
+    }
+
+    private void onMakeLeaderLocal() {
+        LOG.debug("{}: onMakeLeaderLocal received", persistenceId());
+        if (isLeader()) {
+            getSender().tell(new Status.Success(null), getSelf());
+            return;
+        }
+
+        final ActorSelection leader = getLeader();
+
+        if (leader == null) {
+            // Leader is not present. The cluster is most likely trying to
+            // elect a leader and we should let that run its normal course
+
+            // TODO we can wait for the election to complete and retry the
+            // request. We can also let the caller retry by sending a flag
+            // in the response indicating the request is "reTryable".
+            getSender().tell(new Failure(
+                    new LeadershipTransferFailedException("We cannot initiate leadership transfer to local node. "
+                            + "Currently there is no leader for " + persistenceId())),
+                    getSelf());
+            return;
+        }
+
+        leader.tell(new RequestLeadership(getId(), getSender()), getSelf());
+    }
+
+    // Acquire our frontend tracking handle and verify generation matches
+    @Nullable
+    private LeaderFrontendState findFrontend(final ClientIdentifier clientId) throws RequestException {
+        final LeaderFrontendState existing = knownFrontends.get(clientId.getFrontendId());
+        if (existing != null) {
+            final int cmp = Long.compareUnsigned(existing.getIdentifier().getGeneration(), clientId.getGeneration());
+            if (cmp == 0) {
+                existing.touch();
+                return existing;
+            }
+            if (cmp > 0) {
+                LOG.debug("{}: rejecting request from outdated client {}", persistenceId(), clientId);
+                throw new RetiredGenerationException(existing.getIdentifier().getGeneration());
+            }
+
+            LOG.info("{}: retiring state {}, outdated by request from client {}", persistenceId(), existing, clientId);
+            existing.retire();
+            knownFrontends.remove(clientId.getFrontendId());
+        } else {
+            LOG.debug("{}: client {} is not yet known", persistenceId(), clientId);
+        }
+
+        return null;
+    }
+
+    private LeaderFrontendState getFrontend(final ClientIdentifier clientId) throws RequestException {
+        final LeaderFrontendState ret = findFrontend(clientId);
+        if (ret != null) {
+            return ret;
+        }
+
+        // TODO: a dedicated exception would be better, but this is technically true, too
+        throw new OutOfSequenceEnvelopeException(0);
+    }
+
+    private static @Nonnull ABIVersion selectVersion(final ConnectClientRequest message) {
+        final Range<ABIVersion> clientRange = Range.closed(message.getMinVersion(), message.getMaxVersion());
+        for (ABIVersion v : SUPPORTED_ABIVERSIONS) {
+            if (clientRange.contains(v)) {
+                return v;
+            }
+        }
+
+        throw new IllegalArgumentException(String.format(
+            "No common version between backend versions %s and client versions %s", SUPPORTED_ABIVERSIONS,
+            clientRange));
+    }
+
+    @SuppressWarnings("checkstyle:IllegalCatch")
+    private void handleConnectClient(final ConnectClientRequest message) {
+        try {
+            final ClientIdentifier clientId = message.getTarget();
+            final LeaderFrontendState existing = findFrontend(clientId);
+            if (existing != null) {
+                existing.touch();
+            }
+
+            if (!isLeader() || !isLeaderActive()) {
+                LOG.info("{}: not currently leader, rejecting request {}. isLeader: {}, isLeaderActive: {},"
+                                + "isLeadershipTransferInProgress: {}.",
+                        persistenceId(), message, isLeader(), isLeaderActive(), isLeadershipTransferInProgress());
+                throw new NotLeaderException(getSelf());
+            }
+
+            final ABIVersion selectedVersion = selectVersion(message);
+            final LeaderFrontendState frontend;
+            if (existing == null) {
+                frontend = new LeaderFrontendState(persistenceId(), clientId, store);
+                knownFrontends.put(clientId.getFrontendId(), frontend);
+                LOG.debug("{}: created state {} for client {}", persistenceId(), frontend, clientId);
+            } else {
+                frontend = existing;
+            }
+
+            frontend.reconnect();
+            message.getReplyTo().tell(new ConnectClientSuccess(message.getTarget(), message.getSequence(), getSelf(),
+                ImmutableList.of(), store.getDataTree(), CLIENT_MAX_MESSAGES).toVersion(selectedVersion),
+                ActorRef.noSender());
+        } catch (RequestException | RuntimeException e) {
+            message.getReplyTo().tell(new Failure(e), ActorRef.noSender());
+        }
+    }
+
+    private @Nullable RequestSuccess<?, ?> handleRequest(final RequestEnvelope envelope, final long now)
+            throws RequestException {
+        // We are not the leader, hence we want to fail-fast.
+        if (!isLeader() || paused || !isLeaderActive()) {
+            LOG.debug("{}: not currently active leader, rejecting request {}. isLeader: {}, isLeaderActive: {},"
+                            + "isLeadershipTransferInProgress: {}, paused: {}",
+                    persistenceId(), envelope, isLeader(), isLeaderActive(), isLeadershipTransferInProgress(), paused);
+            throw new NotLeaderException(getSelf());
+        }
+
+        final Request<?, ?> request = envelope.getMessage();
+        if (request instanceof TransactionRequest) {
+            final TransactionRequest<?> txReq = (TransactionRequest<?>)request;
+            final ClientIdentifier clientId = txReq.getTarget().getHistoryId().getClientId();
+            return getFrontend(clientId).handleTransactionRequest(txReq, envelope, now);
+        } else if (request instanceof LocalHistoryRequest) {
+            final LocalHistoryRequest<?> lhReq = (LocalHistoryRequest<?>)request;
+            final ClientIdentifier clientId = lhReq.getTarget().getClientId();
+            return getFrontend(clientId).handleLocalHistoryRequest(lhReq, envelope, now);
+        } else {
+            LOG.warn("{}: rejecting unsupported request {}", persistenceId(), request);
+            throw new UnsupportedRequestException(request);
+        }
+    }
+
     private boolean hasLeader() {
         return getLeaderId() != null;
     }
@@ -314,16 +601,15 @@ public class Shard extends RaftActor {
         updateConfigParams(datastoreContext.getShardRaftConfig());
     }
 
-    boolean canSkipPayload() {
-        // If we do not have any followers and we are not using persistence we can apply modification to the state
-        // immediately
-        return !hasFollowers() && !persistence().isRecoveryApplicable();
-    }
-
     // applyState() will be invoked once consensus is reached on the payload
-    void persistPayload(final TransactionIdentifier transactionId, final Payload payload) {
-        // We are faking the sender
-        persistData(self(), transactionId, payload);
+    void persistPayload(final Identifier id, final Payload payload, final boolean batchHint) {
+        boolean canSkipPayload = !hasFollowers() && !persistence().isRecoveryApplicable();
+        if (canSkipPayload) {
+            applyState(self(), id, payload);
+        } else {
+            // We are faking the sender
+            persistData(self(), id, payload, batchHint);
+        }
     }
 
     private void handleCommitTransaction(final CommitTransaction commit) {
@@ -365,7 +651,7 @@ public class Shard extends RaftActor {
         } catch (Exception e) {
             LOG.error("{}: Error handling BatchedModifications for Tx {}", persistenceId(),
                     batched.getTransactionId(), e);
-            sender.tell(new akka.actor.Status.Failure(e), getSelf());
+            sender.tell(new Failure(e), getSelf());
         }
     }
 
@@ -411,7 +697,7 @@ public class Shard extends RaftActor {
 
     private boolean failIfIsolatedLeader(final ActorRef sender) {
         if (isIsolatedLeader()) {
-            sender.tell(new akka.actor.Status.Failure(new NoShardLeaderException(String.format(
+            sender.tell(new Failure(new NoShardLeaderException(String.format(
                     "Shard %s was the leader but has lost contact with all of its followers. Either all"
                     + " other follower nodes are down or this node is isolated by a network partition.",
                     persistenceId()))), getSelf());
@@ -436,7 +722,7 @@ public class Shard extends RaftActor {
             } catch (Exception e) {
                 LOG.error("{}: Error handling ReadyLocalTransaction for Tx {}", persistenceId(),
                         message.getTransactionId(), e);
-                getSender().tell(new akka.actor.Status.Failure(e), getSelf());
+                getSender().tell(new Failure(e), getSelf());
             }
         } else {
             ActorSelection leader = getLeader();
@@ -477,7 +763,7 @@ public class Shard extends RaftActor {
         doAbortTransaction(abort.getTransactionId(), getSender());
     }
 
-    void doAbortTransaction(final TransactionIdentifier transactionID, final ActorRef sender) {
+    void doAbortTransaction(final Identifier transactionID, final ActorRef sender) {
         commitCoordinator.handleAbort(transactionID, sender, this);
     }
 
@@ -487,13 +773,15 @@ public class Shard extends RaftActor {
         } else if (getLeader() != null) {
             getLeader().forward(message, getContext());
         } else {
-            getSender().tell(new akka.actor.Status.Failure(new NoShardLeaderException(
+            getSender().tell(new Failure(new NoShardLeaderException(
                     "Could not create a shard transaction", persistenceId())), getSelf());
         }
     }
 
     private void closeTransactionChain(final CloseTransactionChain closeTransactionChain) {
-        store.closeTransactionChain(closeTransactionChain.getIdentifier());
+        final LocalHistoryIdentifier id = closeTransactionChain.getIdentifier();
+        store.closeTransactionChain(id, null);
+        store.purgeTransactionChain(id, null);
     }
 
     @SuppressWarnings("checkstyle:IllegalCatch")
@@ -510,7 +798,7 @@ public class Shard extends RaftActor {
             getSender().tell(new CreateTransactionReply(Serialization.serializedActorPath(transactionActor),
                     createTransaction.getTransactionId(), createTransaction.getVersion()).toSerializable(), getSelf());
         } catch (Exception e) {
-            getSender().tell(new akka.actor.Status.Failure(e), getSelf());
+            getSender().tell(new Failure(e), getSelf());
         }
     }
 
@@ -593,7 +881,8 @@ public class Shard extends RaftActor {
                     persistenceId(), getId());
             }
 
-            store.closeAllTransactionChains();
+            paused = false;
+            store.purgeLeaderState();
         }
 
         if (hasLeader && !isIsolatedLeader()) {
@@ -604,9 +893,21 @@ public class Shard extends RaftActor {
     @Override
     protected void onLeaderChanged(final String oldLeader, final String newLeader) {
         shardMBean.incrementLeadershipChangeCount();
+        paused = false;
+
+        if (!isLeader()) {
+            if (!knownFrontends.isEmpty()) {
+                LOG.debug("{}: removing frontend state for {}", persistenceId(), knownFrontends.keySet());
+                knownFrontends = ImmutableMap.of();
+            }
+
+            requestMessageAssembler.close();
+
+            if (!hasLeader()) {
+                // No leader anywhere, nothing else to do
+                return;
+            }
 
-        boolean hasLeader = hasLeader();
-        if (hasLeader && !isLeader()) {
             // Another leader was elected. If we were the previous leader and had pending transactions, convert
             // them to transaction messages and send to the new leader.
             ActorSelection leader = getLeader();
@@ -625,9 +926,13 @@ public class Shard extends RaftActor {
                 commitCoordinator.abortPendingTransactions("The transacton was aborted due to inflight leadership "
                         + "change and the leader address isn't available.", this);
             }
+        } else {
+            // We have become the leader, we need to reconstruct frontend state
+            knownFrontends = Verify.verifyNotNull(frontendMetadata.toLeaderState(this));
+            LOG.debug("{}: became leader with frontend state for {}", persistenceId(), knownFrontends.keySet());
         }
 
-        if (hasLeader && !isIsolatedLeader()) {
+        if (!isIsolatedLeader()) {
             messageRetrySupport.retryMessages();
         }
     }
@@ -645,9 +950,33 @@ public class Shard extends RaftActor {
     @Override
     protected void pauseLeader(final Runnable operation) {
         LOG.debug("{}: In pauseLeader, operation: {}", persistenceId(), operation);
+        paused = true;
+
+        // Tell-based protocol can replay transaction state, so it is safe to blow it up when we are paused.
+        knownFrontends.values().forEach(LeaderFrontendState::retire);
+        knownFrontends = ImmutableMap.of();
+
         store.setRunOnPendingTransactionsComplete(operation);
     }
 
+    @Override
+    protected void unpauseLeader() {
+        LOG.debug("{}: In unpauseLeader", persistenceId());
+        paused = false;
+
+        store.setRunOnPendingTransactionsComplete(null);
+
+        // Restore tell-based protocol state as if we were becoming the leader
+        knownFrontends = Verify.verifyNotNull(frontendMetadata.toLeaderState(this));
+    }
+
+    @Override
+    protected OnDemandRaftState.AbstractBuilder<?> newOnDemandRaftStateBuilder() {
+        return OnDemandShardState.newBuilder().treeChangeListenerActors(treeChangeSupport.getListenerActors())
+                .dataChangeListenerActors(changeSupport.getListenerActors())
+                .commitCohortActors(store.getCohortActors());
+    }
+
     @Override
     public String persistenceId() {
         return this.name;
@@ -681,7 +1010,7 @@ public class Shard extends RaftActor {
         private ShardIdentifier id;
         private Map<String, String> peerAddresses = Collections.emptyMap();
         private DatastoreContext datastoreContext;
-        private SchemaContext schemaContext;
+        private SchemaContextProvider schemaContextProvider;
         private DatastoreSnapshot.ShardSnapshot restoreFromSnapshot;
         private TipProducingDataTree dataTree;
         private volatile boolean sealed;
@@ -717,9 +1046,9 @@ public class Shard extends RaftActor {
             return self();
         }
 
-        public T schemaContext(final SchemaContext newSchemaContext) {
+        public T schemaContextProvider(final SchemaContextProvider schemaContextProvider) {
             checkSealed();
-            this.schemaContext = newSchemaContext;
+            this.schemaContextProvider = Preconditions.checkNotNull(schemaContextProvider);
             return self();
         }
 
@@ -748,7 +1077,7 @@ public class Shard extends RaftActor {
         }
 
         public SchemaContext getSchemaContext() {
-            return schemaContext;
+            return Verify.verifyNotNull(schemaContextProvider.getSchemaContext());
         }
 
         public DatastoreSnapshot.ShardSnapshot getRestoreFromSnapshot() {
@@ -775,7 +1104,7 @@ public class Shard extends RaftActor {
             Preconditions.checkNotNull(id, "id should not be null");
             Preconditions.checkNotNull(peerAddresses, "peerAddresses should not be null");
             Preconditions.checkNotNull(datastoreContext, "dataStoreContext should not be null");
-            Preconditions.checkNotNull(schemaContext, "schemaContext should not be null");
+            Preconditions.checkNotNull(schemaContextProvider, "schemaContextProvider should not be null");
         }
 
         public Props props() {
@@ -794,4 +1123,8 @@ public class Shard extends RaftActor {
     Ticker ticker() {
         return Ticker.systemTicker();
     }
+
+    void scheduleNextPendingTransaction() {
+        self().tell(RESUME_NEXT_PENDING_TRANSACTION, ActorRef.noSender());
+    }
 }