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