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

   1 /*
   2  * Copyright (c) 2014 Cisco Systems, Inc. and others.  All rights reserved.
   3  *
   4  * This program and the accompanying materials are made available under the
   5  * terms of the Eclipse Public License v1.0 which accompanies this distribution,
   6  * and is available at http://www.eclipse.org/legal/epl-v10.html
   7  */
   8
   9 package org.opendaylight.controller.cluster.raft.behaviors;
  10
  11 import akka.actor.ActorRef;
  12 import akka.actor.ActorSelection;
  13 import akka.actor.Address;
  14 import akka.cluster.Cluster;
  15 import akka.cluster.ClusterEvent.CurrentClusterState;
  16 import akka.cluster.Member;
  17 import akka.cluster.MemberStatus;
  18 import akka.japi.Procedure;
  19 import com.google.common.annotations.VisibleForTesting;
  20 import com.google.common.base.Stopwatch;
  21 import java.util.ArrayList;
  22 import java.util.Optional;
  23 import java.util.Set;
  24 import java.util.concurrent.TimeUnit;
  25 import javax.annotation.Nullable;
  26 import org.opendaylight.controller.cluster.raft.RaftActorContext;
  27 import org.opendaylight.controller.cluster.raft.RaftState;
  28 import org.opendaylight.controller.cluster.raft.ReplicatedLogEntry;
  29 import org.opendaylight.controller.cluster.raft.Snapshot;
  30 import org.opendaylight.controller.cluster.raft.base.messages.ApplySnapshot;
  31 import org.opendaylight.controller.cluster.raft.base.messages.ElectionTimeout;
  32 import org.opendaylight.controller.cluster.raft.base.messages.TimeoutNow;
  33 import org.opendaylight.controller.cluster.raft.messages.AppendEntries;
  34 import org.opendaylight.controller.cluster.raft.messages.AppendEntriesReply;
  35 import org.opendaylight.controller.cluster.raft.messages.InstallSnapshot;
  36 import org.opendaylight.controller.cluster.raft.messages.InstallSnapshotReply;
  37 import org.opendaylight.controller.cluster.raft.messages.RaftRPC;
  38 import org.opendaylight.controller.cluster.raft.messages.RequestVote;
  39 import org.opendaylight.controller.cluster.raft.messages.RequestVoteReply;
  40 import org.opendaylight.controller.cluster.raft.persisted.ServerConfigurationPayload;
  41
  42 /**
  43  * The behavior of a RaftActor in the Follower raft state.
  44  * <p/>
  45  * <ul>
  46  * <li> Respond to RPCs from candidates and leaders
  47  * <li> If election timeout elapses without receiving AppendEntries
  48  * RPC from current leader or granting vote to candidate:
  49  * convert to candidate
  50  * </ul>
  51  */
  52 public class Follower extends AbstractRaftActorBehavior {
  53     private static final int SYNC_THRESHOLD = 10;
  54
  55     private static final long MAX_ELECTION_TIMEOUT_FACTOR = 18;
  56
  57     private final SyncStatusTracker initialSyncStatusTracker;
  58
  59     private final Procedure<ReplicatedLogEntry> appendAndPersistCallback =
  60         logEntry -> context.getReplicatedLog().captureSnapshotIfReady(logEntry);
  61
  62     private final Stopwatch lastLeaderMessageTimer = Stopwatch.createStarted();
  63     private SnapshotTracker snapshotTracker = null;
  64     private String leaderId;
  65     private short leaderPayloadVersion;
  66
  67     public Follower(RaftActorContext context) {
  68         this(context, null, (short)-1);
  69     }
  70
  71     public Follower(RaftActorContext context, String initialLeaderId, short initialLeaderPayloadVersion) {
  72         super(context, RaftState.Follower);
  73         this.leaderId = initialLeaderId;
  74         this.leaderPayloadVersion = initialLeaderPayloadVersion;
  75
  76         initialSyncStatusTracker = new SyncStatusTracker(context.getActor(), getId(), SYNC_THRESHOLD);
  77
  78         if (context.getPeerIds().isEmpty() && getLeaderId() == null) {
  79             actor().tell(TimeoutNow.INSTANCE, actor());
  80         } else {
  81             scheduleElection(electionDuration());
  82         }
  83     }
  84
  85     @Override
  86     public final String getLeaderId() {
  87         return leaderId;
  88     }
  89
  90     @VisibleForTesting
  91     protected final void setLeaderId(@Nullable final String leaderId) {
  92         this.leaderId = leaderId;
  93     }
  94
  95     @Override
  96     public short getLeaderPayloadVersion() {
  97         return leaderPayloadVersion;
  98     }
  99
 100     @VisibleForTesting
 101     protected final void setLeaderPayloadVersion(short leaderPayloadVersion) {
 102         this.leaderPayloadVersion = leaderPayloadVersion;
 103     }
 104
 105     private void restartLastLeaderMessageTimer() {
 106         if (lastLeaderMessageTimer.isRunning()) {
 107             lastLeaderMessageTimer.reset();
 108         }
 109
 110         lastLeaderMessageTimer.start();
 111     }
 112
 113     private boolean isLogEntryPresent(long index) {
 114         if (context.getReplicatedLog().isInSnapshot(index)) {
 115             return true;
 116         }
 117
 118         ReplicatedLogEntry entry = context.getReplicatedLog().get(index);
 119         return entry != null;
 120
 121     }
 122
 123     private void updateInitialSyncStatus(long currentLeaderCommit, String newLeaderId) {
 124         initialSyncStatusTracker.update(newLeaderId, currentLeaderCommit, context.getCommitIndex());
 125     }
 126
 127     @Override
 128     protected RaftActorBehavior handleAppendEntries(ActorRef sender, AppendEntries appendEntries) {
 129
 130         int numLogEntries = appendEntries.getEntries() != null ? appendEntries.getEntries().size() : 0;
 131         if (log.isTraceEnabled()) {
 132             log.trace("{}: handleAppendEntries: {}", logName(), appendEntries);
 133         } else if (log.isDebugEnabled() && numLogEntries > 0) {
 134             log.debug("{}: handleAppendEntries: {}", logName(), appendEntries);
 135         }
 136
 137         // TODO : Refactor this method into a bunch of smaller methods
 138         // to make it easier to read. Before refactoring ensure tests
 139         // cover the code properly
 140
 141         if (snapshotTracker != null && !snapshotTracker.getLeaderId().equals(appendEntries.getLeaderId())) {
 142             log.debug("{}: snapshot install is in progress but the prior snapshot leaderId {} does not match the "
 143                 + "AppendEntries leaderId {}", logName(), snapshotTracker.getLeaderId(), appendEntries.getLeaderId());
 144             snapshotTracker = null;
 145         }
 146
 147         if (snapshotTracker != null || context.getSnapshotManager().isApplying()) {
 148             // if snapshot install is in progress, follower should just acknowledge append entries with a reply.
 149             AppendEntriesReply reply = new AppendEntriesReply(context.getId(), currentTerm(), true,
 150                     lastIndex(), lastTerm(), context.getPayloadVersion());
 151
 152             log.debug("{}: snapshot install is in progress, replying immediately with {}", logName(), reply);
 153             sender.tell(reply, actor());
 154
 155             return this;
 156         }
 157
 158         // If we got here then we do appear to be talking to the leader
 159         leaderId = appendEntries.getLeaderId();
 160         leaderPayloadVersion = appendEntries.getPayloadVersion();
 161
 162         updateInitialSyncStatus(appendEntries.getLeaderCommit(), appendEntries.getLeaderId());
 163         // First check if the logs are in sync or not
 164         long lastIndex = lastIndex();
 165
 166         if (isOutOfSync(appendEntries)) {
 167             // We found that the log was out of sync so just send a negative
 168             // reply and return
 169
 170             log.debug("{}: Follower is out-of-sync, so sending negative reply, lastIndex: {}, lastTerm: {}",
 171                         logName(), lastIndex, lastTerm());
 172
 173             sender.tell(new AppendEntriesReply(context.getId(), currentTerm(), false, lastIndex,
 174                     lastTerm(), context.getPayloadVersion()), actor());
 175             return this;
 176         }
 177
 178         if (appendEntries.getEntries() != null && appendEntries.getEntries().size() > 0) {
 179
 180             log.debug("{}: Number of entries to be appended = {}", logName(),
 181                         appendEntries.getEntries().size());
 182
 183             // 3. If an existing entry conflicts with a new one (same index
 184             // but different terms), delete the existing entry and all that
 185             // follow it (§5.3)
 186             int addEntriesFrom = 0;
 187             if (context.getReplicatedLog().size() > 0) {
 188
 189                 // Find the entry up until the one that is not in the follower's log
 190                 for (int i = 0;i < appendEntries.getEntries().size(); i++, addEntriesFrom++) {
 191                     ReplicatedLogEntry matchEntry = appendEntries.getEntries().get(i);
 192
 193                     if (!isLogEntryPresent(matchEntry.getIndex())) {
 194                         // newEntry not found in the log
 195                         break;
 196                     }
 197
 198                     long existingEntryTerm = getLogEntryTerm(matchEntry.getIndex());
 199
 200                     log.debug("{}: matchEntry {} is present: existingEntryTerm: {}", logName(), matchEntry,
 201                             existingEntryTerm);
 202
 203                     // existingEntryTerm == -1 means it's in the snapshot and not in the log. We don't know
 204                     // what the term was so we'll assume it matches.
 205                     if (existingEntryTerm == -1 || existingEntryTerm == matchEntry.getTerm()) {
 206                         continue;
 207                     }
 208
 209                     if (!context.getRaftPolicy().applyModificationToStateBeforeConsensus()) {
 210
 211                         log.debug("{}: Removing entries from log starting at {}", logName(),
 212                                 matchEntry.getIndex());
 213
 214                         // Entries do not match so remove all subsequent entries
 215                         if (!context.getReplicatedLog().removeFromAndPersist(matchEntry.getIndex())) {
 216                             // Could not remove the entries - this means the matchEntry index must be in the
 217                             // snapshot and not the log. In this case the prior entries are part of the state
 218                             // so we must send back a reply to force a snapshot to completely re-sync the
 219                             // follower's log and state.
 220
 221                             log.debug("{}: Could not remove entries - sending reply to force snapshot", logName());
 222                             sender.tell(new AppendEntriesReply(context.getId(), currentTerm(), false, lastIndex,
 223                                     lastTerm(), context.getPayloadVersion(), true), actor());
 224                             return this;
 225                         }
 226
 227                         break;
 228                     } else {
 229                         sender.tell(new AppendEntriesReply(context.getId(), currentTerm(), false, lastIndex,
 230                                 lastTerm(), context.getPayloadVersion(), true), actor());
 231                         return this;
 232                     }
 233                 }
 234             }
 235
 236             lastIndex = lastIndex();
 237             log.debug("{}: After cleanup, lastIndex: {}, entries to be added from: {}", logName(),
 238                     lastIndex, addEntriesFrom);
 239
 240             // 4. Append any new entries not already in the log
 241             for (int i = addEntriesFrom; i < appendEntries.getEntries().size(); i++) {
 242                 ReplicatedLogEntry entry = appendEntries.getEntries().get(i);
 243
 244                 log.debug("{}: Append entry to log {}", logName(), entry.getData());
 245
 246                 context.getReplicatedLog().appendAndPersist(entry, appendAndPersistCallback);
 247
 248                 if (entry.getData() instanceof ServerConfigurationPayload) {
 249                     context.updatePeerIds((ServerConfigurationPayload)entry.getData());
 250                 }
 251             }
 252
 253             log.debug("{}: Log size is now {}", logName(), context.getReplicatedLog().size());
 254         }
 255
 256         // 5. If leaderCommit > commitIndex, set commitIndex =
 257         // min(leaderCommit, index of last new entry)
 258
 259         lastIndex = lastIndex();
 260         long prevCommitIndex = context.getCommitIndex();
 261
 262         if (appendEntries.getLeaderCommit() > prevCommitIndex) {
 263             context.setCommitIndex(Math.min(appendEntries.getLeaderCommit(), lastIndex));
 264         }
 265
 266         if (prevCommitIndex != context.getCommitIndex()) {
 267             log.debug("{}: Commit index set to {}", logName(), context.getCommitIndex());
 268         }
 269
 270         // If commitIndex > lastApplied: increment lastApplied, apply
 271         // log[lastApplied] to state machine (§5.3)
 272         // check if there are any entries to be applied. last-applied can be equal to last-index
 273         if (appendEntries.getLeaderCommit() > context.getLastApplied()
 274                 && context.getLastApplied() < lastIndex) {
 275             if (log.isDebugEnabled()) {
 276                 log.debug("{}: applyLogToStateMachine, appendEntries.getLeaderCommit(): {},"
 277                         + "context.getLastApplied(): {}, lastIndex(): {}", logName(),
 278                     appendEntries.getLeaderCommit(), context.getLastApplied(), lastIndex);
 279             }
 280
 281             applyLogToStateMachine(appendEntries.getLeaderCommit());
 282         }
 283
 284         AppendEntriesReply reply = new AppendEntriesReply(context.getId(), currentTerm(), true,
 285             lastIndex, lastTerm(), context.getPayloadVersion());
 286
 287         if (log.isTraceEnabled()) {
 288             log.trace("{}: handleAppendEntries returning : {}", logName(), reply);
 289         } else if (log.isDebugEnabled() && numLogEntries > 0) {
 290             log.debug("{}: handleAppendEntries returning : {}", logName(), reply);
 291         }
 292
 293         sender.tell(reply, actor());
 294
 295         if (!context.getSnapshotManager().isCapturing()) {
 296             super.performSnapshotWithoutCapture(appendEntries.getReplicatedToAllIndex());
 297         }
 298
 299         return this;
 300     }
 301
 302     private boolean isOutOfSync(AppendEntries appendEntries) {
 303
 304         long prevLogTerm = getLogEntryTerm(appendEntries.getPrevLogIndex());
 305         boolean prevEntryPresent = isLogEntryPresent(appendEntries.getPrevLogIndex());
 306         long lastIndex = lastIndex();
 307         int numLogEntries = appendEntries.getEntries() != null ? appendEntries.getEntries().size() : 0;
 308         boolean outOfSync = true;
 309
 310         if (lastIndex == -1 && appendEntries.getPrevLogIndex() != -1) {
 311
 312             // The follower's log is out of sync because the leader does have
 313             // an entry at prevLogIndex and this follower has no entries in
 314             // it's log.
 315
 316             log.debug("{}: The followers log is empty and the senders prevLogIndex is {}",
 317                         logName(), appendEntries.getPrevLogIndex());
 318         } else if (lastIndex > -1 && appendEntries.getPrevLogIndex() != -1 && !prevEntryPresent) {
 319
 320             // The follower's log is out of sync because the Leader's
 321             // prevLogIndex entry was not found in it's log
 322
 323             log.debug("{}: The log is not empty but the prevLogIndex {} was not found in it - "
 324                     + "lastIndex: {}, snapshotIndex: {}", logName(), appendEntries.getPrevLogIndex(), lastIndex,
 325                     context.getReplicatedLog().getSnapshotIndex());
 326         } else if (lastIndex > -1 && prevEntryPresent && prevLogTerm != appendEntries.getPrevLogTerm()) {
 327
 328             // The follower's log is out of sync because the Leader's
 329             // prevLogIndex entry does exist in the follower's log but it has
 330             // a different term in it
 331
 332             log.debug("{}: The prevLogIndex {} was found in the log but the term {} is not equal to the append entries"
 333                       + "prevLogTerm {} - lastIndex: {}, snapshotIndex: {}", logName(), appendEntries.getPrevLogIndex(),
 334                       prevLogTerm, appendEntries.getPrevLogTerm(), lastIndex,
 335                       context.getReplicatedLog().getSnapshotIndex());
 336         } else if (appendEntries.getPrevLogIndex() == -1 && appendEntries.getPrevLogTerm() == -1
 337                 && appendEntries.getReplicatedToAllIndex() != -1
 338                 && !isLogEntryPresent(appendEntries.getReplicatedToAllIndex())) {
 339             // This append entry comes from a leader who has it's log aggressively trimmed and so does not have
 340             // the previous entry in it's in-memory journal
 341
 342             log.debug("{}: Cannot append entries because the replicatedToAllIndex {} does not appear to be in the"
 343                     + " in-memory journal", logName(), appendEntries.getReplicatedToAllIndex());
 344         } else if (appendEntries.getPrevLogIndex() == -1 && appendEntries.getPrevLogTerm() == -1
 345                 && appendEntries.getReplicatedToAllIndex() != -1 && numLogEntries > 0
 346                 && !isLogEntryPresent(appendEntries.getEntries().get(0).getIndex() - 1)) {
 347             log.debug("{}: Cannot append entries because the calculated previousIndex {} was not found in the "
 348                     + " in-memory journal", logName(), appendEntries.getEntries().get(0).getIndex() - 1);
 349         } else {
 350             outOfSync = false;
 351         }
 352         return outOfSync;
 353     }
 354
 355     @Override
 356     protected RaftActorBehavior handleAppendEntriesReply(ActorRef sender,
 357         AppendEntriesReply appendEntriesReply) {
 358         return this;
 359     }
 360
 361     @Override
 362     protected RaftActorBehavior handleRequestVoteReply(ActorRef sender,
 363         RequestVoteReply requestVoteReply) {
 364         return this;
 365     }
 366
 367     @Override
 368     public RaftActorBehavior handleMessage(ActorRef sender, Object message) {
 369         if (message instanceof ElectionTimeout || message instanceof TimeoutNow) {
 370             return handleElectionTimeout(message);
 371         }
 372
 373         if (!(message instanceof RaftRPC)) {
 374             // The rest of the processing requires the message to be a RaftRPC
 375             return null;
 376         }
 377
 378         final RaftRPC rpc = (RaftRPC) message;
 379         // If RPC request or response contains term T > currentTerm:
 380         // set currentTerm = T, convert to follower (§5.1)
 381         // This applies to all RPC messages and responses
 382         if (rpc.getTerm() > context.getTermInformation().getCurrentTerm()) {
 383             log.debug("{}: Term {} in \"{}\" message is greater than follower's term {} - updating term",
 384                 logName(), rpc.getTerm(), rpc, context.getTermInformation().getCurrentTerm());
 385
 386             context.getTermInformation().updateAndPersist(rpc.getTerm(), null);
 387         }
 388
 389         if (rpc instanceof InstallSnapshot) {
 390             handleInstallSnapshot(sender, (InstallSnapshot) rpc);
 391             restartLastLeaderMessageTimer();
 392             scheduleElection(electionDuration());
 393             return this;
 394         }
 395
 396         if (!(rpc instanceof RequestVote) || canGrantVote((RequestVote) rpc)) {
 397             restartLastLeaderMessageTimer();
 398             scheduleElection(electionDuration());
 399         }
 400
 401         return super.handleMessage(sender, rpc);
 402     }
 403
 404     private RaftActorBehavior handleElectionTimeout(Object message) {
 405         // If the message is ElectionTimeout, verify we haven't actually seen a message from the leader
 406         // during the election timeout interval. It may that the election timer expired b/c this actor
 407         // was busy and messages got delayed, in which case leader messages would be backed up in the
 408         // queue but would be processed before the ElectionTimeout message and thus would restart the
 409         // lastLeaderMessageTimer.
 410         long lastLeaderMessageInterval = lastLeaderMessageTimer.elapsed(TimeUnit.MILLISECONDS);
 411         long electionTimeoutInMillis = context.getConfigParams().getElectionTimeOutInterval().toMillis();
 412         boolean noLeaderMessageReceived = !lastLeaderMessageTimer.isRunning()
 413                 || lastLeaderMessageInterval >= electionTimeoutInMillis;
 414
 415         if (canStartElection()) {
 416             if (message instanceof TimeoutNow) {
 417                 log.debug("{}: Received TimeoutNow - switching to Candidate", logName());
 418                 return internalSwitchBehavior(RaftState.Candidate);
 419             } else if (noLeaderMessageReceived) {
 420                 // Check the cluster state to see if the leader is known to be up before we go to Candidate.
 421                 // However if we haven't heard from the leader in a long time even though the cluster state
 422                 // indicates it's up then something is wrong - leader might be stuck indefinitely - so switch
 423                 // to Candidate,
 424                 long maxElectionTimeout = electionTimeoutInMillis * MAX_ELECTION_TIMEOUT_FACTOR;
 425                 if (isLeaderAvailabilityKnown() && lastLeaderMessageInterval < maxElectionTimeout) {
 426                     log.debug("{}: Received ElectionTimeout but leader appears to be available", logName());
 427                     scheduleElection(electionDuration());
 428                 } else {
 429                     log.debug("{}: Received ElectionTimeout - switching to Candidate", logName());
 430                     return internalSwitchBehavior(RaftState.Candidate);
 431                 }
 432             } else {
 433                 log.debug("{}: Received ElectionTimeout but lastLeaderMessageInterval {} < election timeout {}",
 434                         logName(), lastLeaderMessageInterval, context.getConfigParams().getElectionTimeOutInterval());
 435                 scheduleElection(electionDuration());
 436             }
 437         } else if (message instanceof ElectionTimeout) {
 438             if (noLeaderMessageReceived) {
 439                 setLeaderId(null);
 440             }
 441
 442             scheduleElection(electionDuration());
 443         }
 444
 445         return this;
 446     }
 447
 448     private boolean isLeaderAvailabilityKnown() {
 449         if (leaderId == null) {
 450             return false;
 451         }
 452
 453         Optional<Cluster> cluster = context.getCluster();
 454         if (!cluster.isPresent()) {
 455             return false;
 456         }
 457
 458         ActorSelection leaderActor = context.getPeerActorSelection(leaderId);
 459         if (leaderActor == null) {
 460             return false;
 461         }
 462
 463         Address leaderAddress = leaderActor.anchorPath().address();
 464
 465         CurrentClusterState state = cluster.get().state();
 466         Set<Member> unreachable = state.getUnreachable();
 467
 468         log.debug("{}: Checking for leader {} in the cluster unreachable set {}", logName(), leaderAddress,
 469                 unreachable);
 470
 471         for (Member m: unreachable) {
 472             if (leaderAddress.equals(m.address())) {
 473                 log.info("{}: Leader {} is unreachable", logName(), leaderAddress);
 474                 return false;
 475             }
 476         }
 477
 478         for (Member m: state.getMembers()) {
 479             if (leaderAddress.equals(m.address())) {
 480                 if (m.status() == MemberStatus.up() || m.status() == MemberStatus.weaklyUp()) {
 481                     log.debug("{}: Leader {} cluster status is {} - leader is available", logName(),
 482                             leaderAddress, m.status());
 483                     return true;
 484                 } else {
 485                     log.debug("{}: Leader {} cluster status is {} - leader is unavailable", logName(),
 486                             leaderAddress, m.status());
 487                     return false;
 488                 }
 489             }
 490         }
 491
 492         log.debug("{}: Leader {} not found in the cluster member set", logName(), leaderAddress);
 493
 494         return false;
 495     }
 496
 497     private void handleInstallSnapshot(final ActorRef sender, InstallSnapshot installSnapshot) {
 498
 499         log.debug("{}: handleInstallSnapshot: {}", logName(), installSnapshot);
 500
 501         leaderId = installSnapshot.getLeaderId();
 502
 503         if (snapshotTracker == null) {
 504             snapshotTracker = new SnapshotTracker(log, installSnapshot.getTotalChunks(), installSnapshot.getLeaderId());
 505         }
 506
 507         updateInitialSyncStatus(installSnapshot.getLastIncludedIndex(), installSnapshot.getLeaderId());
 508
 509         try {
 510             final InstallSnapshotReply reply = new InstallSnapshotReply(
 511                     currentTerm(), context.getId(), installSnapshot.getChunkIndex(), true);
 512
 513             if (snapshotTracker.addChunk(installSnapshot.getChunkIndex(), installSnapshot.getData(),
 514                     installSnapshot.getLastChunkHashCode())) {
 515                 Snapshot snapshot = Snapshot.create(snapshotTracker.getSnapshot(),
 516                         new ArrayList<>(),
 517                         installSnapshot.getLastIncludedIndex(),
 518                         installSnapshot.getLastIncludedTerm(),
 519                         installSnapshot.getLastIncludedIndex(),
 520                         installSnapshot.getLastIncludedTerm(),
 521                         context.getTermInformation().getCurrentTerm(),
 522                         context.getTermInformation().getVotedFor(),
 523                         installSnapshot.getServerConfig().orNull());
 524
 525                 ApplySnapshot.Callback applySnapshotCallback = new ApplySnapshot.Callback() {
 526                     @Override
 527                     public void onSuccess() {
 528                         log.debug("{}: handleInstallSnapshot returning: {}", logName(), reply);
 529
 530                         sender.tell(reply, actor());
 531                     }
 532
 533                     @Override
 534                     public void onFailure() {
 535                         sender.tell(new InstallSnapshotReply(currentTerm(), context.getId(), -1, false), actor());
 536                     }
 537                 };
 538
 539                 actor().tell(new ApplySnapshot(snapshot, applySnapshotCallback), actor());
 540
 541                 snapshotTracker = null;
 542             } else {
 543                 log.debug("{}: handleInstallSnapshot returning: {}", logName(), reply);
 544
 545                 sender.tell(reply, actor());
 546             }
 547         } catch (SnapshotTracker.InvalidChunkException e) {
 548             log.debug("{}: Exception in InstallSnapshot of follower", logName(), e);
 549
 550             sender.tell(new InstallSnapshotReply(currentTerm(), context.getId(),
 551                     -1, false), actor());
 552             snapshotTracker = null;
 553
 554         }
 555     }
 556
 557     @Override
 558     public void close() {
 559         stopElection();
 560     }
 561
 562     @VisibleForTesting
 563     SnapshotTracker getSnapshotTracker() {
 564         return snapshotTracker;
 565     }
 566 }