opendaylight/md-sal/sal-akka-raft/src/main/java/org/opendaylight/controller/cluster/raft/RaftActor.java

   1 /*
   2  * Copyright (c) 2014 Cisco Systems, Inc. and others.  All rights reserved.
   3  * Copyright (c) 2015 Brocade Communications Systems, Inc. and others.  All rights reserved.
   4  *
   5  * This program and the accompanying materials are made available under the
   6  * terms of the Eclipse Public License v1.0 which accompanies this distribution,
   7  * and is available at http://www.eclipse.org/legal/epl-v10.html
   8  */
   9
  10 package org.opendaylight.controller.cluster.raft;
  11
  12 import akka.actor.ActorRef;
  13 import akka.actor.ActorSelection;
  14 import akka.japi.Procedure;
  15 import com.google.common.annotations.VisibleForTesting;
  16 import com.google.common.base.Objects;
  17 import com.google.common.base.Optional;
  18 import com.google.common.base.Supplier;
  19 import com.google.common.collect.Lists;
  20 import java.io.Serializable;
  21 import java.util.Collection;
  22 import java.util.HashMap;
  23 import java.util.List;
  24 import java.util.Map;
  25 import java.util.concurrent.TimeUnit;
  26 import javax.annotation.Nonnull;
  27 import org.apache.commons.lang3.time.DurationFormatUtils;
  28 import org.opendaylight.controller.cluster.DataPersistenceProvider;
  29 import org.opendaylight.controller.cluster.DelegatingPersistentDataProvider;
  30 import org.opendaylight.controller.cluster.NonPersistentDataProvider;
  31 import org.opendaylight.controller.cluster.PersistentDataProvider;
  32 import org.opendaylight.controller.cluster.common.actor.AbstractUntypedPersistentActor;
  33 import org.opendaylight.controller.cluster.notifications.LeaderStateChanged;
  34 import org.opendaylight.controller.cluster.notifications.RoleChanged;
  35 import org.opendaylight.controller.cluster.raft.base.messages.ApplyJournalEntries;
  36 import org.opendaylight.controller.cluster.raft.base.messages.ApplyState;
  37 import org.opendaylight.controller.cluster.raft.base.messages.InitiateCaptureSnapshot;
  38 import org.opendaylight.controller.cluster.raft.base.messages.Replicate;
  39 import org.opendaylight.controller.cluster.raft.base.messages.SwitchBehavior;
  40 import org.opendaylight.controller.cluster.raft.behaviors.AbstractLeader;
  41 import org.opendaylight.controller.cluster.raft.behaviors.DelegatingRaftActorBehavior;
  42 import org.opendaylight.controller.cluster.raft.behaviors.Follower;
  43 import org.opendaylight.controller.cluster.raft.behaviors.RaftActorBehavior;
  44 import org.opendaylight.controller.cluster.raft.client.messages.FindLeader;
  45 import org.opendaylight.controller.cluster.raft.client.messages.FindLeaderReply;
  46 import org.opendaylight.controller.cluster.raft.client.messages.FollowerInfo;
  47 import org.opendaylight.controller.cluster.raft.client.messages.GetOnDemandRaftState;
  48 import org.opendaylight.controller.cluster.raft.client.messages.OnDemandRaftState;
  49 import org.opendaylight.controller.cluster.raft.protobuff.client.messages.Payload;
  50 import org.slf4j.Logger;
  51 import org.slf4j.LoggerFactory;
  52
  53 /**
  54  * RaftActor encapsulates a state machine that needs to be kept synchronized
  55  * in a cluster. It implements the RAFT algorithm as described in the paper
  56  * <a href='https://ramcloud.stanford.edu/wiki/download/attachments/11370504/raft.pdf'>
  57  * In Search of an Understandable Consensus Algorithm</a>
  58  * <p/>
  59  * RaftActor has 3 states and each state has a certain behavior associated
  60  * with it. A Raft actor can behave as,
  61  * <ul>
  62  * <li> A Leader </li>
  63  * <li> A Follower (or) </li>
  64  * <li> A Candidate </li>
  65  * </ul>
  66  * <p/>
  67  * <p/>
  68  * A RaftActor MUST be a Leader in order to accept requests from clients to
  69  * change the state of it's encapsulated state machine. Once a RaftActor becomes
  70  * a Leader it is also responsible for ensuring that all followers ultimately
  71  * have the same log and therefore the same state machine as itself.
  72  * <p/>
  73  * <p/>
  74  * The current behavior of a RaftActor determines how election for leadership
  75  * is initiated and how peer RaftActors react to request for votes.
  76  * <p/>
  77  * <p/>
  78  * Each RaftActor also needs to know the current election term. It uses this
  79  * information for a couple of things. One is to simply figure out who it
  80  * voted for in the last election. Another is to figure out if the message
  81  * it received to update it's state is stale.
  82  * <p/>
  83  * <p/>
  84  * The RaftActor uses akka-persistence to store it's replicated log.
  85  * Furthermore through it's behaviors a Raft Actor determines
  86  * <p/>
  87  * <ul>
  88  * <li> when a log entry should be persisted </li>
  89  * <li> when a log entry should be applied to the state machine (and) </li>
  90  * <li> when a snapshot should be saved </li>
  91  * </ul>
  92  */
  93 public abstract class RaftActor extends AbstractUntypedPersistentActor {
  94
  95     private static final long APPLY_STATE_DELAY_THRESHOLD_IN_NANOS = TimeUnit.MILLISECONDS.toNanos(50L); // 50 millis
  96
  97     protected final Logger LOG = LoggerFactory.getLogger(getClass());
  98
  99     /**
 100      * The current state determines the current behavior of a RaftActor
 101      * A Raft Actor always starts off in the Follower State
 102      */
 103     private final DelegatingRaftActorBehavior currentBehavior = new DelegatingRaftActorBehavior();
 104
 105     /**
 106      * This context should NOT be passed directly to any other actor it is
 107      * only to be consumed by the RaftActorBehaviors
 108      */
 109     private final RaftActorContextImpl context;
 110
 111     private final DelegatingPersistentDataProvider delegatingPersistenceProvider = new DelegatingPersistentDataProvider(null);
 112
 113     private final PersistentDataProvider persistentProvider;
 114
 115     private RaftActorRecoverySupport raftRecovery;
 116
 117     private RaftActorSnapshotMessageSupport snapshotSupport;
 118
 119     private final BehaviorStateHolder reusableBehaviorStateHolder = new BehaviorStateHolder();
 120
 121     private final SwitchBehaviorSupplier reusableSwitchBehaviorSupplier = new SwitchBehaviorSupplier();
 122
 123     private RaftActorServerConfigurationSupport serverConfigurationSupport;
 124
 125     public RaftActor(String id, Map<String, String> peerAddresses,
 126          Optional<ConfigParams> configParams, short payloadVersion) {
 127
 128         persistentProvider = new PersistentDataProvider(this);
 129         context = new RaftActorContextImpl(this.getSelf(),
 130             this.getContext(), id, new ElectionTermImpl(persistentProvider, id, LOG),
 131             -1, -1, peerAddresses,
 132             (configParams.isPresent() ? configParams.get(): new DefaultConfigParamsImpl()),
 133             delegatingPersistenceProvider, LOG);
 134
 135         context.setPayloadVersion(payloadVersion);
 136         context.setReplicatedLog(ReplicatedLogImpl.newInstance(context, currentBehavior));
 137     }
 138
 139     @Override
 140     public void preStart() throws Exception {
 141         LOG.info("Starting recovery for {} with journal batch size {}", persistenceId(),
 142                 context.getConfigParams().getJournalRecoveryLogBatchSize());
 143
 144         super.preStart();
 145
 146         snapshotSupport = newRaftActorSnapshotMessageSupport();
 147         serverConfigurationSupport = new RaftActorServerConfigurationSupport(getRaftActorContext());
 148     }
 149
 150     @Override
 151     public void postStop() {
 152         if(currentBehavior.getDelegate() != null) {
 153             try {
 154                 currentBehavior.close();
 155             } catch (Exception e) {
 156                 LOG.debug("{}: Error closing behavior {}", persistenceId(), currentBehavior.state());
 157             }
 158         }
 159
 160         super.postStop();
 161     }
 162
 163     @Override
 164     public void handleRecover(Object message) {
 165         if(raftRecovery == null) {
 166             raftRecovery = newRaftActorRecoverySupport();
 167         }
 168
 169         boolean recoveryComplete = raftRecovery.handleRecoveryMessage(message, persistentProvider);
 170         if(recoveryComplete) {
 171             onRecoveryComplete();
 172
 173             initializeBehavior();
 174
 175             raftRecovery = null;
 176         }
 177     }
 178
 179     protected RaftActorRecoverySupport newRaftActorRecoverySupport() {
 180         return new RaftActorRecoverySupport(context, currentBehavior, getRaftActorRecoveryCohort());
 181     }
 182
 183     protected void initializeBehavior(){
 184         changeCurrentBehavior(new Follower(context));
 185     }
 186
 187     protected void changeCurrentBehavior(RaftActorBehavior newBehavior){
 188         reusableBehaviorStateHolder.init(getCurrentBehavior());
 189         setCurrentBehavior(newBehavior);
 190         handleBehaviorChange(reusableBehaviorStateHolder, getCurrentBehavior());
 191     }
 192
 193     @Override
 194     public void handleCommand(final Object message) {
 195         if(serverConfigurationSupport.handleMessage(message, this, getSender())) {
 196             return;
 197         } else if (message instanceof ApplyState){
 198             ApplyState applyState = (ApplyState) message;
 199
 200             long elapsedTime = (System.nanoTime() - applyState.getStartTime());
 201             if(elapsedTime >= APPLY_STATE_DELAY_THRESHOLD_IN_NANOS){
 202                 LOG.warn("ApplyState took more time than expected. Elapsed Time = {} ms ApplyState = {}",
 203                         TimeUnit.NANOSECONDS.toMillis(elapsedTime), applyState);
 204             }
 205
 206             if(LOG.isDebugEnabled()) {
 207                 LOG.debug("{}: Applying state for log index {} data {}",
 208                     persistenceId(), applyState.getReplicatedLogEntry().getIndex(),
 209                     applyState.getReplicatedLogEntry().getData());
 210             }
 211
 212             applyState(applyState.getClientActor(), applyState.getIdentifier(),
 213                 applyState.getReplicatedLogEntry().getData());
 214
 215             if (!hasFollowers()) {
 216                 // for single node, the capture should happen after the apply state
 217                 // as we delete messages from the persistent journal which have made it to the snapshot
 218                 // capturing the snapshot before applying makes the persistent journal and snapshot out of sync
 219                 // and recovery shows data missing
 220                 context.getReplicatedLog().captureSnapshotIfReady(applyState.getReplicatedLogEntry());
 221
 222                 context.getSnapshotManager().trimLog(context.getLastApplied(), currentBehavior);
 223             }
 224
 225         } else if (message instanceof ApplyJournalEntries){
 226             ApplyJournalEntries applyEntries = (ApplyJournalEntries) message;
 227             if(LOG.isDebugEnabled()) {
 228                 LOG.debug("{}: Persisting ApplyLogEntries with index={}", persistenceId(), applyEntries.getToIndex());
 229             }
 230
 231             persistence().persist(applyEntries, NoopProcedure.instance());
 232
 233         } else if (message instanceof FindLeader) {
 234             getSender().tell(
 235                 new FindLeaderReply(getLeaderAddress()),
 236                 getSelf()
 237             );
 238         } else if(message instanceof GetOnDemandRaftState) {
 239             onGetOnDemandRaftStats();
 240         } else if(message instanceof InitiateCaptureSnapshot) {
 241             captureSnapshot();
 242         } else if(message instanceof SwitchBehavior){
 243             switchBehavior(((SwitchBehavior) message));
 244         } else if(!snapshotSupport.handleSnapshotMessage(message)) {
 245             switchBehavior(reusableSwitchBehaviorSupplier.handleMessage(getSender(), message));
 246         }
 247     }
 248
 249     private void switchBehavior(SwitchBehavior message) {
 250         if(!getRaftActorContext().getRaftPolicy().automaticElectionsEnabled()) {
 251             RaftState newState = message.getNewState();
 252             if( newState == RaftState.Leader || newState == RaftState.Follower) {
 253                 switchBehavior(reusableSwitchBehaviorSupplier.handleMessage(getSender(), message));
 254                 getRaftActorContext().getTermInformation().updateAndPersist(message.getNewTerm(), "");
 255             } else {
 256                 LOG.warn("Switching to behavior : {} - not supported", newState);
 257             }
 258         }
 259     }
 260
 261     private void switchBehavior(Supplier<RaftActorBehavior> supplier){
 262         reusableBehaviorStateHolder.init(getCurrentBehavior());
 263
 264         setCurrentBehavior(supplier.get());
 265
 266         handleBehaviorChange(reusableBehaviorStateHolder, getCurrentBehavior());
 267     }
 268
 269     protected RaftActorSnapshotMessageSupport newRaftActorSnapshotMessageSupport() {
 270         return new RaftActorSnapshotMessageSupport(context, currentBehavior,
 271                 getRaftActorSnapshotCohort());
 272     }
 273
 274     private void onGetOnDemandRaftStats() {
 275         // Debugging message to retrieve raft stats.
 276
 277         Map<String, String> peerAddresses = new HashMap<>();
 278         for(String peerId: context.getPeerIds()) {
 279             peerAddresses.put(peerId, context.getPeerAddress(peerId));
 280         }
 281
 282         OnDemandRaftState.Builder builder = OnDemandRaftState.builder()
 283                 .commitIndex(context.getCommitIndex())
 284                 .currentTerm(context.getTermInformation().getCurrentTerm())
 285                 .inMemoryJournalDataSize(replicatedLog().dataSize())
 286                 .inMemoryJournalLogSize(replicatedLog().size())
 287                 .isSnapshotCaptureInitiated(context.getSnapshotManager().isCapturing())
 288                 .lastApplied(context.getLastApplied())
 289                 .lastIndex(replicatedLog().lastIndex())
 290                 .lastTerm(replicatedLog().lastTerm())
 291                 .leader(getLeaderId())
 292                 .raftState(currentBehavior.state().toString())
 293                 .replicatedToAllIndex(currentBehavior.getReplicatedToAllIndex())
 294                 .snapshotIndex(replicatedLog().getSnapshotIndex())
 295                 .snapshotTerm(replicatedLog().getSnapshotTerm())
 296                 .votedFor(context.getTermInformation().getVotedFor())
 297                 .peerAddresses(peerAddresses);
 298
 299         ReplicatedLogEntry lastLogEntry = getLastLogEntry();
 300         if (lastLogEntry != null) {
 301             builder.lastLogIndex(lastLogEntry.getIndex());
 302             builder.lastLogTerm(lastLogEntry.getTerm());
 303         }
 304
 305         if(getCurrentBehavior() instanceof AbstractLeader) {
 306             AbstractLeader leader = (AbstractLeader)getCurrentBehavior();
 307             Collection<String> followerIds = leader.getFollowerIds();
 308             List<FollowerInfo> followerInfoList = Lists.newArrayListWithCapacity(followerIds.size());
 309             for(String id: followerIds) {
 310                 final FollowerLogInformation info = leader.getFollower(id);
 311                 followerInfoList.add(new FollowerInfo(id, info.getNextIndex(), info.getMatchIndex(),
 312                         info.isFollowerActive(), DurationFormatUtils.formatDurationHMS(info.timeSinceLastActivity())));
 313             }
 314
 315             builder.followerInfoList(followerInfoList);
 316         }
 317
 318         sender().tell(builder.build(), self());
 319
 320     }
 321
 322     private void handleBehaviorChange(BehaviorStateHolder oldBehaviorState, RaftActorBehavior currentBehavior) {
 323         RaftActorBehavior oldBehavior = oldBehaviorState.getBehavior();
 324
 325         if (oldBehavior != currentBehavior){
 326             onStateChanged();
 327         }
 328
 329         String lastValidLeaderId = oldBehavior == null ? null : oldBehaviorState.getLastValidLeaderId();
 330         String oldBehaviorStateName = oldBehavior == null ? null : oldBehavior.state().name();
 331
 332         // it can happen that the state has not changed but the leader has changed.
 333         Optional<ActorRef> roleChangeNotifier = getRoleChangeNotifier();
 334         if(!Objects.equal(lastValidLeaderId, currentBehavior.getLeaderId()) ||
 335            oldBehaviorState.getLeaderPayloadVersion() != currentBehavior.getLeaderPayloadVersion()) {
 336             if(roleChangeNotifier.isPresent()) {
 337                 roleChangeNotifier.get().tell(newLeaderStateChanged(getId(), currentBehavior.getLeaderId(),
 338                         currentBehavior.getLeaderPayloadVersion()), getSelf());
 339             }
 340
 341             onLeaderChanged(lastValidLeaderId, currentBehavior.getLeaderId());
 342         }
 343
 344         if (roleChangeNotifier.isPresent() &&
 345                 (oldBehavior == null || (oldBehavior.state() != currentBehavior.state()))) {
 346             roleChangeNotifier.get().tell(new RoleChanged(getId(), oldBehaviorStateName ,
 347                     currentBehavior.state().name()), getSelf());
 348         }
 349     }
 350
 351     protected LeaderStateChanged newLeaderStateChanged(String memberId, String leaderId, short leaderPayloadVersion) {
 352         return new LeaderStateChanged(memberId, leaderId, leaderPayloadVersion);
 353     }
 354
 355     @Override
 356     public long snapshotSequenceNr() {
 357         // When we do a snapshot capture, we also capture and save the sequence-number of the persistent journal,
 358         // so that we can delete the persistent journal based on the saved sequence-number
 359         // However , when akka replays the journal during recovery, it replays it from the sequence number when the snapshot
 360         // was saved and not the number we saved.
 361         // We would want to override it , by asking akka to use the last-sequence number known to us.
 362         return context.getSnapshotManager().getLastSequenceNumber();
 363     }
 364
 365     /**
 366      * When a derived RaftActor needs to persist something it must call
 367      * persistData.
 368      *
 369      * @param clientActor
 370      * @param identifier
 371      * @param data
 372      */
 373     protected void persistData(final ActorRef clientActor, final String identifier,
 374         final Payload data) {
 375
 376         ReplicatedLogEntry replicatedLogEntry = new ReplicatedLogImplEntry(
 377             context.getReplicatedLog().lastIndex() + 1,
 378             context.getTermInformation().getCurrentTerm(), data);
 379
 380         if(LOG.isDebugEnabled()) {
 381             LOG.debug("{}: Persist data {}", persistenceId(), replicatedLogEntry);
 382         }
 383
 384         final RaftActorContext raftContext = getRaftActorContext();
 385
 386         replicatedLog().appendAndPersist(replicatedLogEntry, new Procedure<ReplicatedLogEntry>() {
 387             @Override
 388             public void apply(ReplicatedLogEntry replicatedLogEntry) {
 389                 if (!hasFollowers()){
 390                     // Increment the Commit Index and the Last Applied values
 391                     raftContext.setCommitIndex(replicatedLogEntry.getIndex());
 392                     raftContext.setLastApplied(replicatedLogEntry.getIndex());
 393
 394                     // Apply the state immediately.
 395                     self().tell(new ApplyState(clientActor, identifier, replicatedLogEntry), self());
 396
 397                     // Send a ApplyJournalEntries message so that we write the fact that we applied
 398                     // the state to durable storage
 399                     self().tell(new ApplyJournalEntries(replicatedLogEntry.getIndex()), self());
 400
 401                 } else if (clientActor != null) {
 402                     context.getReplicatedLog().captureSnapshotIfReady(replicatedLogEntry);
 403
 404                     // Send message for replication
 405                     currentBehavior.handleMessage(getSelf(),
 406                             new Replicate(clientActor, identifier, replicatedLogEntry));
 407                 }
 408             }
 409         });
 410     }
 411
 412     private ReplicatedLog replicatedLog() {
 413         return context.getReplicatedLog();
 414     }
 415
 416     protected String getId() {
 417         return context.getId();
 418     }
 419
 420     @VisibleForTesting
 421     void setCurrentBehavior(RaftActorBehavior behavior) {
 422         currentBehavior.setDelegate(behavior);
 423     }
 424
 425     protected RaftActorBehavior getCurrentBehavior() {
 426         return currentBehavior.getDelegate();
 427     }
 428
 429     /**
 430      * Derived actors can call the isLeader method to check if the current
 431      * RaftActor is the Leader or not
 432      *
 433      * @return true it this RaftActor is a Leader false otherwise
 434      */
 435     protected boolean isLeader() {
 436         return context.getId().equals(currentBehavior.getLeaderId());
 437     }
 438
 439     /**
 440      * Derived actor can call getLeader if they need a reference to the Leader.
 441      * This would be useful for example in forwarding a request to an actor
 442      * which is the leader
 443      *
 444      * @return A reference to the leader if known, null otherwise
 445      */
 446     protected ActorSelection getLeader(){
 447         String leaderAddress = getLeaderAddress();
 448
 449         if(leaderAddress == null){
 450             return null;
 451         }
 452
 453         return context.actorSelection(leaderAddress);
 454     }
 455
 456     /**
 457      *
 458      * @return the current leader's id
 459      */
 460     protected String getLeaderId(){
 461         return currentBehavior.getLeaderId();
 462     }
 463
 464     protected RaftState getRaftState() {
 465         return currentBehavior.state();
 466     }
 467
 468     protected ReplicatedLogEntry getLastLogEntry() {
 469         return replicatedLog().last();
 470     }
 471
 472     protected Long getCurrentTerm(){
 473         return context.getTermInformation().getCurrentTerm();
 474     }
 475
 476     protected Long getCommitIndex(){
 477         return context.getCommitIndex();
 478     }
 479
 480     protected Long getLastApplied(){
 481         return context.getLastApplied();
 482     }
 483
 484     protected RaftActorContext getRaftActorContext() {
 485         return context;
 486     }
 487
 488     protected void updateConfigParams(ConfigParams configParams) {
 489         context.setConfigParams(configParams);
 490     }
 491
 492     public final DataPersistenceProvider persistence() {
 493         return delegatingPersistenceProvider.getDelegate();
 494     }
 495
 496     public void setPersistence(DataPersistenceProvider provider) {
 497         delegatingPersistenceProvider.setDelegate(provider);
 498     }
 499
 500     protected void setPersistence(boolean persistent) {
 501         if(persistent) {
 502             setPersistence(new PersistentDataProvider(this));
 503         } else {
 504             setPersistence(new NonPersistentDataProvider() {
 505                 /**
 506                  * The way snapshotting works is,
 507                  * <ol>
 508                  * <li> RaftActor calls createSnapshot on the Shard
 509                  * <li> Shard sends a CaptureSnapshotReply and RaftActor then calls saveSnapshot
 510                  * <li> When saveSnapshot is invoked on the akka-persistence API it uses the SnapshotStore to save
 511                  * the snapshot. The SnapshotStore sends SaveSnapshotSuccess or SaveSnapshotFailure. When the
 512                  * RaftActor gets SaveSnapshot success it commits the snapshot to the in-memory journal. This
 513                  * commitSnapshot is mimicking what is done in SaveSnapshotSuccess.
 514                  * </ol>
 515                  */
 516                 @Override
 517                 public void saveSnapshot(Object o) {
 518                     // Make saving Snapshot successful
 519                     // Committing the snapshot here would end up calling commit in the creating state which would
 520                     // be a state violation. That's why now we send a message to commit the snapshot.
 521                     self().tell(RaftActorSnapshotMessageSupport.COMMIT_SNAPSHOT, self());
 522                 }
 523             });
 524         }
 525     }
 526
 527     /**
 528      * setPeerAddress sets the address of a known peer at a later time.
 529      * <p>
 530      * This is to account for situations where a we know that a peer
 531      * exists but we do not know an address up-front. This may also be used in
 532      * situations where a known peer starts off in a different location and we
 533      * need to change it's address
 534      * <p>
 535      * Note that if the peerId does not match the list of peers passed to
 536      * this actor during construction an IllegalStateException will be thrown.
 537      *
 538      * @param peerId
 539      * @param peerAddress
 540      */
 541     protected void setPeerAddress(String peerId, String peerAddress){
 542         context.setPeerAddress(peerId, peerAddress);
 543     }
 544
 545     /**
 546      * The applyState method will be called by the RaftActor when some data
 547      * needs to be applied to the actor's state
 548      *
 549      * @param clientActor A reference to the client who sent this message. This
 550      *                    is the same reference that was passed to persistData
 551      *                    by the derived actor. clientActor may be null when
 552      *                    the RaftActor is behaving as a follower or during
 553      *                    recovery.
 554      * @param identifier  The identifier of the persisted data. This is also
 555      *                    the same identifier that was passed to persistData by
 556      *                    the derived actor. identifier may be null when
 557      *                    the RaftActor is behaving as a follower or during
 558      *                    recovery
 559      * @param data        A piece of data that was persisted by the persistData call.
 560      *                    This should NEVER be null.
 561      */
 562     protected abstract void applyState(ActorRef clientActor, String identifier,
 563         Object data);
 564
 565     /**
 566      * Returns the RaftActorRecoveryCohort to participate in persistence recovery.
 567      */
 568     @Nonnull
 569     protected abstract RaftActorRecoveryCohort getRaftActorRecoveryCohort();
 570
 571     /**
 572      * This method is called when recovery is complete.
 573      */
 574     protected abstract void onRecoveryComplete();
 575
 576     /**
 577      * Returns the RaftActorSnapshotCohort to participate in persistence recovery.
 578      */
 579     @Nonnull
 580     protected abstract RaftActorSnapshotCohort getRaftActorSnapshotCohort();
 581
 582     /**
 583      * This method will be called by the RaftActor when the state of the
 584      * RaftActor changes. The derived actor can then use methods like
 585      * isLeader or getLeader to do something useful
 586      */
 587     protected abstract void onStateChanged();
 588
 589     /**
 590      * Notifier Actor for this RaftActor to notify when a role change happens
 591      * @return ActorRef - ActorRef of the notifier or Optional.absent if none.
 592      */
 593     protected abstract Optional<ActorRef> getRoleChangeNotifier();
 594
 595     protected void onLeaderChanged(String oldLeader, String newLeader){};
 596
 597     private String getLeaderAddress(){
 598         if(isLeader()){
 599             return getSelf().path().toString();
 600         }
 601         String leaderId = currentBehavior.getLeaderId();
 602         if (leaderId == null) {
 603             return null;
 604         }
 605         String peerAddress = context.getPeerAddress(leaderId);
 606         if(LOG.isDebugEnabled()) {
 607             LOG.debug("{}: getLeaderAddress leaderId = {} peerAddress = {}",
 608                     persistenceId(), leaderId, peerAddress);
 609         }
 610
 611         return peerAddress;
 612     }
 613
 614     protected boolean hasFollowers(){
 615         return getRaftActorContext().hasFollowers();
 616     }
 617
 618     private void captureSnapshot() {
 619         SnapshotManager snapshotManager = context.getSnapshotManager();
 620
 621         if(!snapshotManager.isCapturing()) {
 622             LOG.debug("Take a snapshot of current state. lastReplicatedLog is {} and replicatedToAllIndex is {}",
 623                 replicatedLog().last(), currentBehavior.getReplicatedToAllIndex());
 624
 625             snapshotManager.capture(replicatedLog().last(), currentBehavior.getReplicatedToAllIndex());
 626         }
 627     }
 628
 629     /**
 630      * @deprecated Deprecated in favor of {@link org.opendaylight.controller.cluster.raft.base.messages.DeleteEntries}
 631      *             whose type for fromIndex is long instead of int. This class was kept for backwards
 632      *             compatibility with Helium.
 633      */
 634     // Suppressing this warning as we can't set serialVersionUID to maintain backwards compatibility.
 635     @SuppressWarnings("serial")
 636     @Deprecated
 637     static class DeleteEntries implements Serializable {
 638         private final int fromIndex;
 639
 640         public DeleteEntries(int fromIndex) {
 641             this.fromIndex = fromIndex;
 642         }
 643
 644         public int getFromIndex() {
 645             return fromIndex;
 646         }
 647     }
 648
 649     /**
 650      * @deprecated Deprecated in favor of non-inner class {@link org.opendaylight.controller.cluster.raft.base.messages.UpdateElectionTerm}
 651      *             which has serialVersionUID set. This class was kept for backwards compatibility with Helium.
 652      */
 653     // Suppressing this warning as we can't set serialVersionUID to maintain backwards compatibility.
 654     @SuppressWarnings("serial")
 655     @Deprecated
 656     static class UpdateElectionTerm implements Serializable {
 657         private final long currentTerm;
 658         private final String votedFor;
 659
 660         public UpdateElectionTerm(long currentTerm, String votedFor) {
 661             this.currentTerm = currentTerm;
 662             this.votedFor = votedFor;
 663         }
 664
 665         public long getCurrentTerm() {
 666             return currentTerm;
 667         }
 668
 669         public String getVotedFor() {
 670             return votedFor;
 671         }
 672     }
 673
 674     private static class BehaviorStateHolder {
 675         private RaftActorBehavior behavior;
 676         private String lastValidLeaderId;
 677         private short leaderPayloadVersion;
 678
 679         void init(RaftActorBehavior behavior) {
 680             this.behavior = behavior;
 681             this.leaderPayloadVersion = behavior != null ? behavior.getLeaderPayloadVersion() : -1;
 682
 683             String behaviorLeaderId = behavior != null ? behavior.getLeaderId() : null;
 684             if(behaviorLeaderId != null) {
 685                 this.lastValidLeaderId = behaviorLeaderId;
 686             }
 687         }
 688
 689         RaftActorBehavior getBehavior() {
 690             return behavior;
 691         }
 692
 693         String getLastValidLeaderId() {
 694             return lastValidLeaderId;
 695         }
 696
 697         short getLeaderPayloadVersion() {
 698             return leaderPayloadVersion;
 699         }
 700     }
 701
 702     private class SwitchBehaviorSupplier implements Supplier<RaftActorBehavior> {
 703         private Object message;
 704         private ActorRef sender;
 705
 706         public SwitchBehaviorSupplier handleMessage(ActorRef sender, Object message){
 707             this.sender = sender;
 708             this.message = message;
 709             return this;
 710         }
 711
 712         @Override
 713         public RaftActorBehavior get() {
 714             if(this.message instanceof SwitchBehavior){
 715                 return ((SwitchBehavior) message).getNewState().createBehavior(getRaftActorContext());
 716             }
 717             return currentBehavior.handleMessage(sender, message);
 718         }
 719     }
 720 }