[controller.git] /

/*
 * Copyright (c) 2014 Cisco Systems, Inc. and others.  All rights reserved.
 * Copyright (c) 2015 Brocade Communications Systems, Inc. and others.  All rights reserved.
 *
 * This program and the accompanying materials are made available under the
 * terms of the Eclipse Public License v1.0 which accompanies this distribution,
 * and is available at http://www.eclipse.org/legal/epl-v10.html
 */
package org.opendaylight.controller.cluster.raft;

import static com.google.common.base.Verify.verify;
import static java.util.Objects.requireNonNull;

import akka.actor.ActorRef;
import akka.actor.ActorSelection;
import akka.actor.PoisonPill;
import akka.actor.Status;
import akka.persistence.JournalProtocol;
import akka.persistence.SnapshotProtocol;
import com.google.common.annotations.VisibleForTesting;
import edu.umd.cs.findbugs.annotations.SuppressFBWarnings;
import java.util.ArrayList;
import java.util.Collection;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Objects;
import java.util.Optional;
import java.util.concurrent.TimeUnit;
import org.apache.commons.lang3.time.DurationFormatUtils;
import org.eclipse.jdt.annotation.NonNull;
import org.eclipse.jdt.annotation.Nullable;
import org.opendaylight.controller.cluster.DataPersistenceProvider;
import org.opendaylight.controller.cluster.DelegatingPersistentDataProvider;
import org.opendaylight.controller.cluster.NonPersistentDataProvider;
import org.opendaylight.controller.cluster.PersistentDataProvider;
import org.opendaylight.controller.cluster.common.actor.AbstractUntypedPersistentActor;
import org.opendaylight.controller.cluster.mgmt.api.FollowerInfo;
import org.opendaylight.controller.cluster.notifications.LeaderStateChanged;
import org.opendaylight.controller.cluster.notifications.RoleChanged;
import org.opendaylight.controller.cluster.raft.base.messages.ApplyState;
import org.opendaylight.controller.cluster.raft.base.messages.CheckConsensusReached;
import org.opendaylight.controller.cluster.raft.base.messages.InitiateCaptureSnapshot;
import org.opendaylight.controller.cluster.raft.base.messages.LeaderTransitioning;
import org.opendaylight.controller.cluster.raft.base.messages.Replicate;
import org.opendaylight.controller.cluster.raft.base.messages.SwitchBehavior;
import org.opendaylight.controller.cluster.raft.behaviors.AbstractLeader;
import org.opendaylight.controller.cluster.raft.behaviors.AbstractRaftActorBehavior;
import org.opendaylight.controller.cluster.raft.behaviors.Follower;
import org.opendaylight.controller.cluster.raft.behaviors.RaftActorBehavior;
import org.opendaylight.controller.cluster.raft.client.messages.FindLeader;
import org.opendaylight.controller.cluster.raft.client.messages.FindLeaderReply;
import org.opendaylight.controller.cluster.raft.client.messages.GetOnDemandRaftState;
import org.opendaylight.controller.cluster.raft.client.messages.OnDemandRaftState;
import org.opendaylight.controller.cluster.raft.client.messages.Shutdown;
import org.opendaylight.controller.cluster.raft.messages.Payload;
import org.opendaylight.controller.cluster.raft.messages.RequestLeadership;
import org.opendaylight.controller.cluster.raft.persisted.ApplyJournalEntries;
import org.opendaylight.controller.cluster.raft.persisted.NoopPayload;
import org.opendaylight.controller.cluster.raft.persisted.ServerConfigurationPayload;
import org.opendaylight.controller.cluster.raft.persisted.SimpleReplicatedLogEntry;
import org.opendaylight.yangtools.concepts.Identifier;
import org.opendaylight.yangtools.concepts.Immutable;

/**
 * RaftActor encapsulates a state machine that needs to be kept synchronized
 * in a cluster. It implements the RAFT algorithm as described in the paper
 * <a href='https://ramcloud.stanford.edu/wiki/download/attachments/11370504/raft.pdf'>
 * In Search of an Understandable Consensus Algorithm</a>
 *
 * <p>
 * RaftActor has 3 states and each state has a certain behavior associated
 * with it. A Raft actor can behave as,
 * <ul>
 * <li> A Leader </li>
 * <li> A Follower (or) </li>
 * <li> A Candidate </li>
 * </ul>
 *
 * <p>
 * A RaftActor MUST be a Leader in order to accept requests from clients to
 * change the state of it's encapsulated state machine. Once a RaftActor becomes
 * a Leader it is also responsible for ensuring that all followers ultimately
 * have the same log and therefore the same state machine as itself.
 *
 * <p>
 * The current behavior of a RaftActor determines how election for leadership
 * is initiated and how peer RaftActors react to request for votes.
 *
 * <p>
 * Each RaftActor also needs to know the current election term. It uses this
 * information for a couple of things. One is to simply figure out who it
 * voted for in the last election. Another is to figure out if the message
 * it received to update it's state is stale.
 *
 * <p>
 * The RaftActor uses akka-persistence to store it's replicated log.
 * Furthermore through it's behaviors a Raft Actor determines
 * <ul>
 * <li> when a log entry should be persisted </li>
 * <li> when a log entry should be applied to the state machine (and) </li>
 * <li> when a snapshot should be saved </li>
 * </ul>
 */
public abstract class RaftActor extends AbstractUntypedPersistentActor {

    private static final long APPLY_STATE_DELAY_THRESHOLD_IN_NANOS = TimeUnit.MILLISECONDS.toNanos(50L); // 50 millis

    /**
     * This context should NOT be passed directly to any other actor it is
     * only to be consumed by the RaftActorBehaviors.
     */
    private final RaftActorContextImpl context;

    private final DelegatingPersistentDataProvider delegatingPersistenceProvider;

    private final PersistentDataProvider persistentProvider;

    private final BehaviorStateTracker behaviorStateTracker = new BehaviorStateTracker();

    private RaftActorRecoverySupport raftRecovery;

    private RaftActorSnapshotMessageSupport snapshotSupport;

    private RaftActorServerConfigurationSupport serverConfigurationSupport;

    private boolean shuttingDown;

    @SuppressFBWarnings(value = "MC_OVERRIDABLE_METHOD_CALL_IN_CONSTRUCTOR", justification = "Akka class design")
    protected RaftActor(final String id, final Map<String, String> peerAddresses,
         final Optional<ConfigParams> configParams, final short payloadVersion) {

        persistentProvider = new PersistentDataProvider(this);
        delegatingPersistenceProvider = new RaftActorDelegatingPersistentDataProvider(null, persistentProvider);

        context = new RaftActorContextImpl(getSelf(), getContext(), id,
            new ElectionTermImpl(persistentProvider, id, LOG), -1, -1, peerAddresses,
            configParams.isPresent() ? configParams.get() : new DefaultConfigParamsImpl(),
            delegatingPersistenceProvider, this::handleApplyState, LOG, this::executeInSelf);

        context.setPayloadVersion(payloadVersion);
        context.setReplicatedLog(ReplicatedLogImpl.newInstance(context));
    }

    @Override
    public void preStart() throws Exception {
        LOG.info("Starting recovery for {} with journal batch size {}", persistenceId(),
                context.getConfigParams().getJournalRecoveryLogBatchSize());

        super.preStart();

        snapshotSupport = newRaftActorSnapshotMessageSupport();
        serverConfigurationSupport = new RaftActorServerConfigurationSupport(this);
    }

    @Override
    public void postStop() throws Exception {
        context.close();
        super.postStop();
    }

    @Override
    protected void handleRecover(final Object message) {
        if (raftRecovery == null) {
            raftRecovery = newRaftActorRecoverySupport();
        }

        boolean recoveryComplete = raftRecovery.handleRecoveryMessage(message, persistentProvider);
        if (recoveryComplete) {
            onRecoveryComplete();

            initializeBehavior();

            raftRecovery = null;
        }
    }

    protected RaftActorRecoverySupport newRaftActorRecoverySupport() {
        return new RaftActorRecoverySupport(context, getRaftActorRecoveryCohort());
    }

    @VisibleForTesting
    void initializeBehavior() {
        changeCurrentBehavior(new Follower(context));
    }

    @VisibleForTesting
    @SuppressWarnings("checkstyle:IllegalCatch")
    protected void changeCurrentBehavior(final RaftActorBehavior newBehavior) {
        final RaftActorBehavior currentBehavior = getCurrentBehavior();
        if (currentBehavior != null) {
            try {
                currentBehavior.close();
            } catch (Exception e) {
                LOG.warn("{}: Error closing behavior {}", persistence(), currentBehavior, e);
            }
        }

        final BehaviorState state = behaviorStateTracker.capture(currentBehavior);
        setCurrentBehavior(newBehavior);
        handleBehaviorChange(state, newBehavior);
    }

    /**
     * Method exposed for subclasses to plug-in their logic. This method is invoked by {@link #handleCommand(Object)}
     * for messages which are not handled by this class. Subclasses overriding this class should fall back to this
     * implementation for messages which they do not handle
     *
     * @param message Incoming command message
     */
    protected void handleNonRaftCommand(final Object message) {
        unhandled(message);
    }

    /**
     * Handles a message.
     *
     * @deprecated This method is not final for testing purposes. DO NOT OVERRIDE IT, override
     *             {@link #handleNonRaftCommand(Object)} instead.
     */
    @Deprecated
    @Override
    // FIXME: make this method final once our unit tests do not need to override it
    protected void handleCommand(final Object message) {
        if (serverConfigurationSupport.handleMessage(message, getSender())) {
            return;
        }
        if (snapshotSupport.handleSnapshotMessage(message, getSender())) {
            return;
        }
        if (message instanceof ApplyState) {
            ApplyState applyState = (ApplyState) message;

            if (!hasFollowers()) {
                // for single node, the capture should happen after the apply state
                // as we delete messages from the persistent journal which have made it to the snapshot
                // capturing the snapshot before applying makes the persistent journal and snapshot out of sync
                // and recovery shows data missing
                context.getReplicatedLog().captureSnapshotIfReady(applyState.getReplicatedLogEntry());

                context.getSnapshotManager().trimLog(context.getLastApplied());
            }

            possiblyHandleBehaviorMessage(message);
        } else if (message instanceof ApplyJournalEntries) {
            ApplyJournalEntries applyEntries = (ApplyJournalEntries) message;
            LOG.debug("{}: Persisting ApplyJournalEntries with index={}", persistenceId(), applyEntries.getToIndex());

            persistence().persistAsync(applyEntries, NoopProcedure.instance());
        } else if (message instanceof FindLeader) {
            getSender().tell(new FindLeaderReply(getLeaderAddress()), getSelf());
        } else if (message instanceof GetOnDemandRaftState) {
            onGetOnDemandRaftStats();
        } else if (message instanceof InitiateCaptureSnapshot) {
            captureSnapshot();
        } else if (message instanceof SwitchBehavior) {
            switchBehavior((SwitchBehavior) message);
        } else if (message instanceof LeaderTransitioning) {
            onLeaderTransitioning((LeaderTransitioning)message);
        } else if (message instanceof Shutdown) {
            onShutDown();
        } else if (message instanceof Runnable) {
            ((Runnable)message).run();
        } else if (message instanceof NoopPayload) {
            persistData(null, null, (NoopPayload) message, false);
        } else if (message instanceof RequestLeadership) {
            onRequestLeadership((RequestLeadership) message);
        } else if (!possiblyHandleBehaviorMessage(message)) {
            if (message instanceof JournalProtocol.Response
                && delegatingPersistenceProvider.handleJournalResponse((JournalProtocol.Response) message)) {
                LOG.debug("{}: handled a journal response", persistenceId());
            } else if (message instanceof SnapshotProtocol.Response
                && delegatingPersistenceProvider.handleSnapshotResponse((SnapshotProtocol.Response) message)) {
                LOG.debug("{}: handled a snapshot response", persistenceId());
            } else {
                handleNonRaftCommand(message);
            }
        }
    }

    private void onRequestLeadership(final RequestLeadership message) {
        LOG.debug("{}: onRequestLeadership {}", persistenceId(), message);
        if (!isLeader()) {
            // non-leader cannot satisfy leadership request
            LOG.warn("{}: onRequestLeadership {} was sent to non-leader."
                    + " Current behavior: {}. Sending failure response",
                    persistenceId(), message, getCurrentBehavior().state());
            message.getReplyTo().tell(new LeadershipTransferFailedException("Cannot transfer leader to "
                    + message.getRequestedFollowerId()
                    + ". RequestLeadership message was sent to non-leader " + persistenceId()), getSelf());
            return;
        }

        final String requestedFollowerId = message.getRequestedFollowerId();
        final ActorRef replyTo = message.getReplyTo();
        initiateLeadershipTransfer(new RaftActorLeadershipTransferCohort.OnComplete() {
            @Override
            public void onSuccess(final ActorRef raftActorRef) {
                // sanity check
                if (!requestedFollowerId.equals(getLeaderId())) {
                    onFailure(raftActorRef);
                }

                LOG.debug("{}: Leadership transferred successfully to {}", persistenceId(), requestedFollowerId);
                replyTo.tell(new Status.Success(null), getSelf());
            }

            @Override
            public void onFailure(final ActorRef raftActorRef) {
                LOG.debug("{}: LeadershipTransfer request from {} failed", persistenceId(), requestedFollowerId);
                replyTo.tell(new Status.Failure(
                        new LeadershipTransferFailedException(
                                "Failed to transfer leadership to " + requestedFollowerId
                                        + ". Follower is not ready to become leader")),
                        getSelf());
            }
        }, message.getRequestedFollowerId(), RaftActorLeadershipTransferCohort.USE_DEFAULT_LEADER_TIMEOUT);
    }

    private boolean possiblyHandleBehaviorMessage(final Object message) {
        final RaftActorBehavior currentBehavior = getCurrentBehavior();
        final BehaviorState state = behaviorStateTracker.capture(currentBehavior);

        // A behavior indicates that it processed the change by returning a reference to the next behavior
        // to be used. A null return indicates it has not processed the message and we should be passing it to
        // the subclass for handling.
        final RaftActorBehavior nextBehavior = currentBehavior.handleMessage(getSender(), message);
        if (nextBehavior != null) {
            switchBehavior(state, nextBehavior);
            return true;
        }

        return false;
    }

    private void initiateLeadershipTransfer(final RaftActorLeadershipTransferCohort.OnComplete onComplete,
            final @Nullable String followerId, final long newLeaderTimeoutInMillis) {
        LOG.debug("{}: Initiating leader transfer", persistenceId());

        RaftActorLeadershipTransferCohort leadershipTransferInProgress = context.getRaftActorLeadershipTransferCohort();
        if (leadershipTransferInProgress == null) {
            leadershipTransferInProgress = new RaftActorLeadershipTransferCohort(this, followerId);
            leadershipTransferInProgress.setNewLeaderTimeoutInMillis(newLeaderTimeoutInMillis);
            leadershipTransferInProgress.addOnComplete(new RaftActorLeadershipTransferCohort.OnComplete() {
                @Override
                public void onSuccess(final ActorRef raftActorRef) {
                    context.setRaftActorLeadershipTransferCohort(null);
                }

                @Override
                public void onFailure(final ActorRef raftActorRef) {
                    context.setRaftActorLeadershipTransferCohort(null);
                }
            });

            leadershipTransferInProgress.addOnComplete(onComplete);

            context.setRaftActorLeadershipTransferCohort(leadershipTransferInProgress);
            leadershipTransferInProgress.init();

        } else {
            LOG.debug("{}: prior leader transfer in progress - adding callback", persistenceId());
            leadershipTransferInProgress.addOnComplete(onComplete);
        }
    }

    private void onShutDown() {
        LOG.debug("{}: onShutDown", persistenceId());

        if (shuttingDown) {
            return;
        }

        shuttingDown = true;

        final RaftActorBehavior currentBehavior = context.getCurrentBehavior();
        switch (currentBehavior.state()) {
            case Leader:
            case PreLeader:
                // Fall-through to more work
                break;
            default:
                // For non-leaders shutdown is a no-op
                self().tell(PoisonPill.getInstance(), self());
                return;
        }

        if (context.hasFollowers()) {
            initiateLeadershipTransfer(new RaftActorLeadershipTransferCohort.OnComplete() {
                @Override
                public void onSuccess(final ActorRef raftActorRef) {
                    LOG.debug("{}: leader transfer succeeded - sending PoisonPill", persistenceId());
                    raftActorRef.tell(PoisonPill.getInstance(), raftActorRef);
                }

                @Override
                public void onFailure(final ActorRef raftActorRef) {
                    LOG.debug("{}: leader transfer failed - sending PoisonPill", persistenceId());
                    raftActorRef.tell(PoisonPill.getInstance(), raftActorRef);
                }
            }, null, TimeUnit.MILLISECONDS.convert(2, TimeUnit.SECONDS));
        } else {
            pauseLeader(new TimedRunnable(context.getConfigParams().getElectionTimeOutInterval(), this) {
                @Override
                protected void doRun() {
                    self().tell(PoisonPill.getInstance(), self());
                }

                @Override
                protected void doCancel() {
                    self().tell(PoisonPill.getInstance(), self());
                }
            });
        }
    }

    private void onLeaderTransitioning(final LeaderTransitioning leaderTransitioning) {
        LOG.debug("{}: onLeaderTransitioning: {}", persistenceId(), leaderTransitioning);
        Optional<ActorRef> roleChangeNotifier = getRoleChangeNotifier();
        if (getRaftState() == RaftState.Follower && roleChangeNotifier.isPresent()
                && leaderTransitioning.getLeaderId().equals(getCurrentBehavior().getLeaderId())) {
            roleChangeNotifier.get().tell(newLeaderStateChanged(getId(), null,
                getCurrentBehavior().getLeaderPayloadVersion()), getSelf());
        }
    }

    private void switchBehavior(final SwitchBehavior message) {
        if (!getRaftActorContext().getRaftPolicy().automaticElectionsEnabled()) {
            RaftState newState = message.getNewState();
            if (newState == RaftState.Leader || newState == RaftState.Follower) {
                getRaftActorContext().getTermInformation().updateAndPersist(message.getNewTerm(), "");
                switchBehavior(behaviorStateTracker.capture(getCurrentBehavior()),
                    AbstractRaftActorBehavior.createBehavior(context, message.getNewState()));
            } else {
                LOG.warn("Switching to behavior : {} - not supported", newState);
            }
        }
    }

    private void switchBehavior(final BehaviorState oldBehaviorState, final RaftActorBehavior nextBehavior) {
        setCurrentBehavior(nextBehavior);
        handleBehaviorChange(oldBehaviorState, nextBehavior);
    }

    @VisibleForTesting
    RaftActorSnapshotMessageSupport newRaftActorSnapshotMessageSupport() {
        return new RaftActorSnapshotMessageSupport(context, getRaftActorSnapshotCohort());
    }

    private void onGetOnDemandRaftStats() {
        // Debugging message to retrieve raft stats.

        Map<String, String> peerAddresses = new HashMap<>();
        Map<String, Boolean> peerVotingStates = new HashMap<>();
        for (PeerInfo info: context.getPeers()) {
            peerVotingStates.put(info.getId(), info.isVoting());
            peerAddresses.put(info.getId(), info.getAddress() != null ? info.getAddress() : "");
        }

        final RaftActorBehavior currentBehavior = context.getCurrentBehavior();
        OnDemandRaftState.AbstractBuilder<?, ?> builder = newOnDemandRaftStateBuilder()
                .commitIndex(context.getCommitIndex())
                .currentTerm(context.getTermInformation().getCurrentTerm())
                .inMemoryJournalDataSize(replicatedLog().dataSize())
                .inMemoryJournalLogSize(replicatedLog().size())
                .isSnapshotCaptureInitiated(context.getSnapshotManager().isCapturing())
                .lastApplied(context.getLastApplied())
                .lastIndex(replicatedLog().lastIndex())
                .lastTerm(replicatedLog().lastTerm())
                .leader(getLeaderId())
                .raftState(currentBehavior.state().toString())
                .replicatedToAllIndex(currentBehavior.getReplicatedToAllIndex())
                .snapshotIndex(replicatedLog().getSnapshotIndex())
                .snapshotTerm(replicatedLog().getSnapshotTerm())
                .votedFor(context.getTermInformation().getVotedFor())
                .isVoting(context.isVotingMember())
                .peerAddresses(peerAddresses)
                .peerVotingStates(peerVotingStates)
                .customRaftPolicyClassName(context.getConfigParams().getCustomRaftPolicyImplementationClass());

        ReplicatedLogEntry lastLogEntry = replicatedLog().last();
        if (lastLogEntry != null) {
            builder.lastLogIndex(lastLogEntry.getIndex());
            builder.lastLogTerm(lastLogEntry.getTerm());
        }

        if (getCurrentBehavior() instanceof AbstractLeader) {
            AbstractLeader leader = (AbstractLeader)getCurrentBehavior();
            Collection<String> followerIds = leader.getFollowerIds();
            List<FollowerInfo> followerInfoList = new ArrayList<>(followerIds.size());
            for (String id: followerIds) {
                final FollowerLogInformation info = leader.getFollower(id);
                followerInfoList.add(new FollowerInfo(id, info.getNextIndex(), info.getMatchIndex(),
                        info.isFollowerActive(), DurationFormatUtils.formatDurationHMS(
                            TimeUnit.NANOSECONDS.toMillis(info.nanosSinceLastActivity())),
                        context.getPeerInfo(info.getId()).isVoting()));
            }

            builder.followerInfoList(followerInfoList);
        }

        sender().tell(builder.build(), self());

    }

    protected OnDemandRaftState.AbstractBuilder<?, ?> newOnDemandRaftStateBuilder() {
        return OnDemandRaftState.builder();
    }

    private void handleBehaviorChange(final BehaviorState oldBehaviorState, final RaftActorBehavior currentBehavior) {
        RaftActorBehavior oldBehavior = oldBehaviorState.getBehavior();

        if (oldBehavior != currentBehavior) {
            onStateChanged();
        }

        String lastLeaderId = oldBehavior == null ? null : oldBehaviorState.getLastLeaderId();
        String lastValidLeaderId = oldBehavior == null ? null : oldBehaviorState.getLastValidLeaderId();
        String oldBehaviorStateName = oldBehavior == null ? null : oldBehavior.state().name();

        // it can happen that the state has not changed but the leader has changed.
        Optional<ActorRef> roleChangeNotifier = getRoleChangeNotifier();
        if (!Objects.equals(lastLeaderId, currentBehavior.getLeaderId())
                || oldBehaviorState.getLeaderPayloadVersion() != currentBehavior.getLeaderPayloadVersion()) {
            if (roleChangeNotifier.isPresent()) {
                roleChangeNotifier.get().tell(newLeaderStateChanged(getId(), currentBehavior.getLeaderId(),
                        currentBehavior.getLeaderPayloadVersion()), getSelf());
            }

            onLeaderChanged(lastValidLeaderId, currentBehavior.getLeaderId());

            RaftActorLeadershipTransferCohort leadershipTransferInProgress =
                    context.getRaftActorLeadershipTransferCohort();
            if (leadershipTransferInProgress != null) {
                leadershipTransferInProgress.onNewLeader(currentBehavior.getLeaderId());
            }

            serverConfigurationSupport.onNewLeader(currentBehavior.getLeaderId());
        }

        if (roleChangeNotifier.isPresent()
                && (oldBehavior == null || oldBehavior.state() != currentBehavior.state())) {
            roleChangeNotifier.get().tell(new RoleChanged(getId(), oldBehaviorStateName ,
                    currentBehavior.state().name()), getSelf());
        }
    }

    private void handleApplyState(final ApplyState applyState) {
        long startTime = System.nanoTime();

        Payload payload = applyState.getReplicatedLogEntry().getData();
        if (LOG.isDebugEnabled()) {
            LOG.debug("{}: Applying state for log index {} data {}",
                persistenceId(), applyState.getReplicatedLogEntry().getIndex(), payload);
        }

        if (!(payload instanceof NoopPayload) && !(payload instanceof ServerConfigurationPayload)) {
            applyState(applyState.getClientActor(), applyState.getIdentifier(), payload);
        }

        long elapsedTime = System.nanoTime() - startTime;
        if (elapsedTime >= APPLY_STATE_DELAY_THRESHOLD_IN_NANOS) {
            LOG.debug("ApplyState took more time than expected. Elapsed Time = {} ms ApplyState = {}",
                    TimeUnit.NANOSECONDS.toMillis(elapsedTime), applyState);
        }

        // Send the ApplyState message back to self to handle further processing asynchronously.
        self().tell(applyState, self());
    }

    protected LeaderStateChanged newLeaderStateChanged(final String memberId, final String leaderId,
            final short leaderPayloadVersion) {
        return new LeaderStateChanged(memberId, leaderId, leaderPayloadVersion);
    }

    @Override
    public long snapshotSequenceNr() {
        // When we do a snapshot capture, we also capture and save the sequence-number of the persistent journal,
        // so that we can delete the persistent journal based on the saved sequence-number
        // However , when akka replays the journal during recovery, it replays it from the sequence number when the
        // snapshot was saved and not the number we saved. We would want to override it , by asking akka to use the
        // last-sequence number known to us.
        return context.getSnapshotManager().getLastSequenceNumber();
    }

    /**
     * Persists the given Payload in the journal and replicates to any followers. After successful completion,
     * {@link #applyState(ActorRef, Identifier, Object)} is notified.
     *
     * @param clientActor optional ActorRef that is provided via the applyState callback
     * @param identifier the payload identifier
     * @param data the payload data to persist
     * @param batchHint if true, an attempt is made to delay immediate replication and batch the payload with
     *        subsequent payloads for efficiency. Otherwise the payload is immediately replicated.
     */
    protected final void persistData(final ActorRef clientActor, final Identifier identifier, final Payload data,
            final boolean batchHint) {
        ReplicatedLogEntry replicatedLogEntry = new SimpleReplicatedLogEntry(
            context.getReplicatedLog().lastIndex() + 1,
            context.getTermInformation().getCurrentTerm(), data);
        replicatedLogEntry.setPersistencePending(true);

        LOG.debug("{}: Persist data {}", persistenceId(), replicatedLogEntry);

        final RaftActorContext raftContext = getRaftActorContext();

        boolean wasAppended = replicatedLog().appendAndPersist(replicatedLogEntry, persistedLogEntry -> {
            // Clear the persistence pending flag in the log entry.
            persistedLogEntry.setPersistencePending(false);

            if (!hasFollowers()) {
                // Increment the Commit Index and the Last Applied values
                raftContext.setCommitIndex(persistedLogEntry.getIndex());
                raftContext.setLastApplied(persistedLogEntry.getIndex());

                // Apply the state immediately.
                handleApplyState(new ApplyState(clientActor, identifier, persistedLogEntry));

                // Send a ApplyJournalEntries message so that we write the fact that we applied
                // the state to durable storage
                self().tell(new ApplyJournalEntries(persistedLogEntry.getIndex()), self());

            } else {
                context.getReplicatedLog().captureSnapshotIfReady(replicatedLogEntry);

                // Local persistence is complete so send the CheckConsensusReached message to the behavior (which
                // normally should still be the leader) to check if consensus has now been reached in conjunction with
                // follower replication.
                getCurrentBehavior().handleMessage(getSelf(), CheckConsensusReached.INSTANCE);
            }
        }, true);

        if (wasAppended && hasFollowers()) {
            // Send log entry for replication.
            getCurrentBehavior().handleMessage(getSelf(), new Replicate(clientActor, identifier, replicatedLogEntry,
                    !batchHint));
        }
    }

    private ReplicatedLog replicatedLog() {
        return context.getReplicatedLog();
    }

    protected String getId() {
        return context.getId();
    }

    @VisibleForTesting
    void setCurrentBehavior(final RaftActorBehavior behavior) {
        context.setCurrentBehavior(behavior);
    }

    protected RaftActorBehavior getCurrentBehavior() {
        return context.getCurrentBehavior();
    }

    /**
     * Derived actors can call the isLeader method to check if the current
     * RaftActor is the Leader or not.
     *
     * @return true it this RaftActor is a Leader false otherwise
     */
    protected boolean isLeader() {
        return context.getId().equals(getCurrentBehavior().getLeaderId());
    }

    protected final boolean isLeaderActive() {
        return getRaftState() != RaftState.IsolatedLeader && getRaftState() != RaftState.PreLeader
                && !shuttingDown && !isLeadershipTransferInProgress();
    }

    protected boolean isLeadershipTransferInProgress() {
        RaftActorLeadershipTransferCohort leadershipTransferInProgress = context.getRaftActorLeadershipTransferCohort();
        return leadershipTransferInProgress != null && leadershipTransferInProgress.isTransferring();
    }

    /**
     * Derived actor can call getLeader if they need a reference to the Leader.
     * This would be useful for example in forwarding a request to an actor
     * which is the leader
     *
     * @return A reference to the leader if known, null otherwise
     */
    public ActorSelection getLeader() {
        String leaderAddress = getLeaderAddress();

        if (leaderAddress == null) {
            return null;
        }

        return context.actorSelection(leaderAddress);
    }

    /**
     * Returns the id of the current leader.
     *
     * @return the current leader's id
     */
    protected final String getLeaderId() {
        return getCurrentBehavior().getLeaderId();
    }

    @VisibleForTesting
    protected final RaftState getRaftState() {
        return getCurrentBehavior().state();
    }

    protected Long getCurrentTerm() {
        return context.getTermInformation().getCurrentTerm();
    }

    protected RaftActorContext getRaftActorContext() {
        return context;
    }

    protected void updateConfigParams(final ConfigParams configParams) {

        // obtain the RaftPolicy for oldConfigParams and the updated one.
        String oldRaftPolicy = context.getConfigParams().getCustomRaftPolicyImplementationClass();
        String newRaftPolicy = configParams.getCustomRaftPolicyImplementationClass();

        LOG.debug("{}: RaftPolicy used with prev.config {}, RaftPolicy used with newConfig {}", persistenceId(),
            oldRaftPolicy, newRaftPolicy);
        context.setConfigParams(configParams);
        if (!Objects.equals(oldRaftPolicy, newRaftPolicy)) {
            // The RaftPolicy was modified. If the current behavior is Follower then re-initialize to Follower
            // but transfer the previous leaderId so it doesn't immediately try to schedule an election. This
            // avoids potential disruption. Otherwise, switch to Follower normally.
            RaftActorBehavior behavior = getCurrentBehavior();
            if (behavior != null && behavior.state() == RaftState.Follower) {
                String previousLeaderId = behavior.getLeaderId();
                short previousLeaderPayloadVersion = behavior.getLeaderPayloadVersion();

                LOG.debug("{}: Re-initializing to Follower with previous leaderId {}", persistenceId(),
                        previousLeaderId);

                changeCurrentBehavior(new Follower(context, previousLeaderId, previousLeaderPayloadVersion));
            } else {
                initializeBehavior();
            }
        }
    }

    public final DataPersistenceProvider persistence() {
        return delegatingPersistenceProvider.getDelegate();
    }

    public void setPersistence(final DataPersistenceProvider provider) {
        delegatingPersistenceProvider.setDelegate(provider);
    }

    protected void setPersistence(final boolean persistent) {
        DataPersistenceProvider currentPersistence = persistence();
        if (persistent && (currentPersistence == null || !currentPersistence.isRecoveryApplicable())) {
            setPersistence(new PersistentDataProvider(this));

            if (getCurrentBehavior() != null) {
                LOG.info("{}: Persistence has been enabled - capturing snapshot", persistenceId());
                captureSnapshot();
            }
        } else if (!persistent && (currentPersistence == null || currentPersistence.isRecoveryApplicable())) {
            setPersistence(new NonPersistentDataProvider(this) {
                /*
                 * The way snapshotting works is,
                 * <ol>
                 * <li> RaftActor calls createSnapshot on the Shard
                 * <li> Shard sends a CaptureSnapshotReply and RaftActor then calls saveSnapshot
                 * <li> When saveSnapshot is invoked on the akka-persistence API it uses the SnapshotStore to save
                 * the snapshot. The SnapshotStore sends SaveSnapshotSuccess or SaveSnapshotFailure. When the
                 * RaftActor gets SaveSnapshot success it commits the snapshot to the in-memory journal. This
                 * commitSnapshot is mimicking what is done in SaveSnapshotSuccess.
                 * </ol>
                 */
                @Override
                public void saveSnapshot(final Object object) {
                    // Make saving Snapshot successful
                    // Committing the snapshot here would end up calling commit in the creating state which would
                    // be a state violation. That's why now we send a message to commit the snapshot.
                    self().tell(RaftActorSnapshotMessageSupport.COMMIT_SNAPSHOT, self());
                }
            });
        }
    }

    /**
     * setPeerAddress sets the address of a known peer at a later time.
     *
     * <p>
     * This is to account for situations where a we know that a peer
     * exists but we do not know an address up-front. This may also be used in
     * situations where a known peer starts off in a different location and we
     * need to change it's address
     *
     * <p>
     * Note that if the peerId does not match the list of peers passed to
     * this actor during construction an IllegalStateException will be thrown.
     */
    protected void setPeerAddress(final String peerId, final String peerAddress) {
        context.setPeerAddress(peerId, peerAddress);
    }

    /**
     * The applyState method will be called by the RaftActor when some data
     * needs to be applied to the actor's state.
     *
     * @param clientActor A reference to the client who sent this message. This
     *                    is the same reference that was passed to persistData
     *                    by the derived actor. clientActor may be null when
     *                    the RaftActor is behaving as a follower or during
     *                    recovery.
     * @param identifier  The identifier of the persisted data. This is also
     *                    the same identifier that was passed to persistData by
     *                    the derived actor. identifier may be null when
     *                    the RaftActor is behaving as a follower or during
     *                    recovery
     * @param data        A piece of data that was persisted by the persistData call.
     *                    This should NEVER be null.
     */
    protected abstract void applyState(ActorRef clientActor, Identifier identifier, Object data);

    /**
     * Returns the RaftActorRecoveryCohort to participate in persistence recovery.
     */
    protected abstract @NonNull RaftActorRecoveryCohort getRaftActorRecoveryCohort();

    /**
     * This method is called when recovery is complete.
     */
    protected abstract void onRecoveryComplete();

    /**
     * Returns the RaftActorSnapshotCohort to participate in snapshot captures.
     */
    protected abstract @NonNull RaftActorSnapshotCohort getRaftActorSnapshotCohort();

    /**
     * This method will be called by the RaftActor when the state of the
     * RaftActor changes. The derived actor can then use methods like
     * isLeader or getLeader to do something useful
     */
    protected abstract void onStateChanged();

    /**
     * Notifier Actor for this RaftActor to notify when a role change happens.
     *
     * @return ActorRef - ActorRef of the notifier or Optional.absent if none.
     */
    protected abstract Optional<ActorRef> getRoleChangeNotifier();

    /**
     * This method is called on the leader when a voting change operation completes.
     */
    protected void onVotingStateChangeComplete() {
    }

    /**
     * This method is called prior to operations such as leadership transfer and actor shutdown when the leader
     * must pause or stop its duties. This method allows derived classes to gracefully pause or finish current
     * work prior to performing the operation. On completion of any work, the run method must be called on the
     * given Runnable to proceed with the given operation. <b>Important:</b> the run method must be called on
     * this actor's thread dispatcher as as it modifies internal state.
     *
     * <p>
     * The default implementation immediately runs the operation.
     *
     * @param operation the operation to run
     */
    protected void pauseLeader(final Runnable operation) {
        operation.run();
    }

    /**
     * This method is invoked when the actions hooked to the leader becoming paused failed to execute and the leader
     * should resume normal operations.
     *
     * <p>
     * Note this method can be invoked even before the operation supplied to {@link #pauseLeader(Runnable)} is invoked.
     */
    protected void unpauseLeader() {

    }

    protected void onLeaderChanged(final String oldLeader, final String newLeader) {
    }

    private String getLeaderAddress() {
        if (isLeader()) {
            return getSelf().path().toString();
        }
        String leaderId = getLeaderId();
        if (leaderId == null) {
            return null;
        }
        String peerAddress = context.getPeerAddress(leaderId);
        LOG.debug("{}: getLeaderAddress leaderId = {} peerAddress = {}", persistenceId(), leaderId, peerAddress);

        return peerAddress;
    }

    protected boolean hasFollowers() {
        return getRaftActorContext().hasFollowers();
    }

    private void captureSnapshot() {
        SnapshotManager snapshotManager = context.getSnapshotManager();

        if (!snapshotManager.isCapturing()) {
            final long idx = getCurrentBehavior().getReplicatedToAllIndex();
            LOG.debug("Take a snapshot of current state. lastReplicatedLog is {} and replicatedToAllIndex is {}",
                replicatedLog().last(), idx);

            snapshotManager.captureWithForcedTrim(replicatedLog().last(), idx);
        }
    }

    /**
     * Switch this member to non-voting status. This is a no-op for all behaviors except when we are the leader,
     * in which case we need to step down.
     */
    void becomeNonVoting() {
        if (isLeader()) {
            initiateLeadershipTransfer(new RaftActorLeadershipTransferCohort.OnComplete() {
                @Override
                public void onSuccess(final ActorRef raftActorRef) {
                    LOG.debug("{}: leader transfer succeeded after change to non-voting", persistenceId());
                    ensureFollowerState();
                }

                @Override
                public void onFailure(final ActorRef raftActorRef) {
                    LOG.debug("{}: leader transfer failed after change to non-voting", persistenceId());
                    ensureFollowerState();
                }

                private void ensureFollowerState() {
                    // Whether or not leadership transfer succeeded, we have to step down as leader and
                    // switch to Follower so ensure that.
                    if (getRaftState() != RaftState.Follower) {
                        initializeBehavior();
                    }
                }
            }, null, RaftActorLeadershipTransferCohort.USE_DEFAULT_LEADER_TIMEOUT);
        }
    }

    /**
     * A point-in-time capture of {@link RaftActorBehavior} state critical for transitioning between behaviors.
     */
    private abstract static class BehaviorState implements Immutable {
        @Nullable abstract RaftActorBehavior getBehavior();

        @Nullable abstract String getLastValidLeaderId();

        @Nullable abstract String getLastLeaderId();

        abstract short getLeaderPayloadVersion();
    }

    /**
     * A {@link BehaviorState} corresponding to non-null {@link RaftActorBehavior} state.
     */
    private static final class SimpleBehaviorState extends BehaviorState {
        private final RaftActorBehavior behavior;
        private final String lastValidLeaderId;
        private final String lastLeaderId;
        private final short leaderPayloadVersion;

        SimpleBehaviorState(final String lastValidLeaderId, final String lastLeaderId,
                final RaftActorBehavior behavior) {
            this.lastValidLeaderId = lastValidLeaderId;
            this.lastLeaderId = lastLeaderId;
            this.behavior = requireNonNull(behavior);
            leaderPayloadVersion = behavior.getLeaderPayloadVersion();
        }

        @Override
        RaftActorBehavior getBehavior() {
            return behavior;
        }

        @Override
        String getLastValidLeaderId() {
            return lastValidLeaderId;
        }

        @Override
        short getLeaderPayloadVersion() {
            return leaderPayloadVersion;
        }

        @Override
        String getLastLeaderId() {
            return lastLeaderId;
        }
    }

    /**
     * Class tracking behavior-related information, which we need to keep around and pass across behavior switches.
     * An instance is created for each RaftActor. It has two functions:
     * - it keeps track of the last leader ID we have encountered since we have been created
     * - it creates state capture needed to transition from one behavior to the next
     */
    private static final class BehaviorStateTracker {
        /**
         * A {@link BehaviorState} corresponding to null {@link RaftActorBehavior} state. Since null behavior is only
         * allowed before we receive the first message, we know the leader ID to be null.
         */
        private static final BehaviorState NULL_BEHAVIOR_STATE = new BehaviorState() {
            @Override
            RaftActorBehavior getBehavior() {
                return null;
            }

            @Override
            String getLastValidLeaderId() {
                return null;
            }

            @Override
            short getLeaderPayloadVersion() {
                return -1;
            }

            @Override
            String getLastLeaderId() {
                return null;
            }
        };

        private String lastValidLeaderId;
        private String lastLeaderId;

        BehaviorState capture(final RaftActorBehavior behavior) {
            if (behavior == null) {
                verify(lastValidLeaderId == null, "Null behavior with non-null last leader");
                return NULL_BEHAVIOR_STATE;
            }

            lastLeaderId = behavior.getLeaderId();
            if (lastLeaderId != null) {
                lastValidLeaderId = lastLeaderId;
            }

            return new SimpleBehaviorState(lastValidLeaderId, lastLeaderId, behavior);
        }
    }
}