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 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 leaderId){
 124         initialSyncStatusTracker.update(leaderId, 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             if(LOG.isDebugEnabled()) {
 153                 LOG.debug("{}: snapshot install is in progress, replying immediately with {}", logName(), reply);
 154             }
 155             sender.tell(reply, actor());
 156
 157             return this;
 158         }
 159
 160         // If we got here then we do appear to be talking to the leader
 161         leaderId = appendEntries.getLeaderId();
 162         leaderPayloadVersion = appendEntries.getPayloadVersion();
 163
 164         updateInitialSyncStatus(appendEntries.getLeaderCommit(), appendEntries.getLeaderId());
 165         // First check if the logs are in sync or not
 166         long lastIndex = lastIndex();
 167
 168         if (isOutOfSync(appendEntries)) {
 169             // We found that the log was out of sync so just send a negative
 170             // reply and return
 171
 172             LOG.debug("{}: Follower is out-of-sync, so sending negative reply, lastIndex: {}, lastTerm: {}",
 173                         logName(), lastIndex, lastTerm());
 174
 175             sender.tell(new AppendEntriesReply(context.getId(), currentTerm(), false, lastIndex,
 176                     lastTerm(), context.getPayloadVersion()), actor());
 177             return this;
 178         }
 179
 180         if (appendEntries.getEntries() != null && appendEntries.getEntries().size() > 0) {
 181
 182             LOG.debug("{}: Number of entries to be appended = {}", logName(),
 183                         appendEntries.getEntries().size());
 184
 185             // 3. If an existing entry conflicts with a new one (same index
 186             // but different terms), delete the existing entry and all that
 187             // follow it (§5.3)
 188             int addEntriesFrom = 0;
 189             if (context.getReplicatedLog().size() > 0) {
 190
 191                 // Find the entry up until the one that is not in the follower's log
 192                 for (int i = 0;i < appendEntries.getEntries().size(); i++, addEntriesFrom++) {
 193                     ReplicatedLogEntry matchEntry = appendEntries.getEntries().get(i);
 194
 195                     if(!isLogEntryPresent(matchEntry.getIndex())) {
 196                         // newEntry not found in the log
 197                         break;
 198                     }
 199
 200                     long existingEntryTerm = getLogEntryTerm(matchEntry.getIndex());
 201
 202                     LOG.debug("{}: matchEntry {} is present: existingEntryTerm: {}", logName(), matchEntry,
 203                             existingEntryTerm);
 204
 205                     // existingEntryTerm == -1 means it's in the snapshot and not in the log. We don't know
 206                     // what the term was so we'll assume it matches.
 207                     if(existingEntryTerm == -1 || existingEntryTerm == matchEntry.getTerm()) {
 208                         continue;
 209                     }
 210
 211                     if(!context.getRaftPolicy().applyModificationToStateBeforeConsensus()) {
 212
 213                         LOG.debug("{}: Removing entries from log starting at {}", logName(),
 214                                 matchEntry.getIndex());
 215
 216                         // Entries do not match so remove all subsequent entries
 217                         if(!context.getReplicatedLog().removeFromAndPersist(matchEntry.getIndex())) {
 218                             // Could not remove the entries - this means the matchEntry index must be in the
 219                             // snapshot and not the log. In this case the prior entries are part of the state
 220                             // so we must send back a reply to force a snapshot to completely re-sync the
 221                             // follower's log and state.
 222
 223                             LOG.debug("{}: Could not remove entries - sending reply to force snapshot", logName());
 224                             sender.tell(new AppendEntriesReply(context.getId(), currentTerm(), false, lastIndex,
 225                                     lastTerm(), context.getPayloadVersion(), true), actor());
 226                             return this;
 227                         }
 228
 229                         break;
 230                     } else {
 231                         sender.tell(new AppendEntriesReply(context.getId(), currentTerm(), false, lastIndex,
 232                                 lastTerm(), context.getPayloadVersion(), true), actor());
 233                         return this;
 234                     }
 235                 }
 236             }
 237
 238             lastIndex = lastIndex();
 239             LOG.debug("{}: After cleanup, lastIndex: {}, entries to be added from: {}", logName(),
 240                     lastIndex, addEntriesFrom);
 241
 242             // 4. Append any new entries not already in the log
 243             for (int i = addEntriesFrom; i < appendEntries.getEntries().size(); i++) {
 244                 ReplicatedLogEntry entry = appendEntries.getEntries().get(i);
 245
 246                 LOG.debug("{}: Append entry to log {}", logName(), entry.getData());
 247
 248                 context.getReplicatedLog().appendAndPersist(entry, appendAndPersistCallback);
 249
 250                 if(entry.getData() instanceof ServerConfigurationPayload) {
 251                     context.updatePeerIds((ServerConfigurationPayload)entry.getData());
 252                 }
 253             }
 254
 255             LOG.debug("{}: Log size is now {}", logName(), context.getReplicatedLog().size());
 256         }
 257
 258         // 5. If leaderCommit > commitIndex, set commitIndex =
 259         // min(leaderCommit, index of last new entry)
 260
 261         lastIndex = lastIndex();
 262         long prevCommitIndex = context.getCommitIndex();
 263
 264         context.setCommitIndex(Math.min(appendEntries.getLeaderCommit(), lastIndex));
 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, " +
 277                         "appendEntries.getLeaderCommit(): {}," +
 278                         "context.getLastApplied(): {}, lastIndex(): {}", logName(),
 279                     appendEntries.getLeaderCommit(), context.getLastApplied(), lastIndex);
 280             }
 281
 282             applyLogToStateMachine(appendEntries.getLeaderCommit());
 283         }
 284
 285         AppendEntriesReply reply = new AppendEntriesReply(context.getId(), currentTerm(), true,
 286             lastIndex, lastTerm(), context.getPayloadVersion());
 287
 288         if(LOG.isTraceEnabled()) {
 289             LOG.trace("{}: handleAppendEntries returning : {}", logName(), reply);
 290         } else if(LOG.isDebugEnabled() && numLogEntries > 0) {
 291             LOG.debug("{}: handleAppendEntries returning : {}", logName(), reply);
 292         }
 293
 294         sender.tell(reply, actor());
 295
 296         if (!context.getSnapshotManager().isCapturing()) {
 297             super.performSnapshotWithoutCapture(appendEntries.getReplicatedToAllIndex());
 298         }
 299
 300         return this;
 301     }
 302
 303     private boolean isOutOfSync(AppendEntries appendEntries) {
 304
 305         long prevLogTerm = getLogEntryTerm(appendEntries.getPrevLogIndex());
 306         boolean prevEntryPresent = isLogEntryPresent(appendEntries.getPrevLogIndex());
 307         long lastIndex = lastIndex();
 308         int numLogEntries = appendEntries.getEntries() != null ? appendEntries.getEntries().size() : 0;
 309         boolean outOfSync = true;
 310
 311         if (lastIndex == -1 && appendEntries.getPrevLogIndex() != -1) {
 312
 313             // The follower's log is out of sync because the leader does have
 314             // an entry at prevLogIndex and this follower has no entries in
 315             // it's log.
 316
 317             LOG.debug("{}: The followers log is empty and the senders prevLogIndex is {}",
 318                         logName(), appendEntries.getPrevLogIndex());
 319         } else if (lastIndex > -1 && appendEntries.getPrevLogIndex() != -1 && !prevEntryPresent) {
 320
 321             // The follower's log is out of sync because the Leader's
 322             // prevLogIndex entry was not found in it's log
 323
 324             LOG.debug("{}: The log is not empty but the prevLogIndex {} was not found in it - lastIndex: {}, snapshotIndex: {}",
 325                         logName(), appendEntries.getPrevLogIndex(), lastIndex, 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(
 333                     "{}: Cannot append entries because previous entry term {}  is not equal to append entries prevLogTerm {}",
 334                     logName(), prevLogTerm, appendEntries.getPrevLogTerm());
 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(
 342                     "{}: Cannot append entries because the replicatedToAllIndex {} does not appear to be in the in-memory journal",
 343                     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(
 348                     "{}: Cannot append entries because the calculated previousIndex {} was not found in the in-memory journal",
 349                     logName(), appendEntries.getEntries().get(0).getIndex() - 1);
 350         } else {
 351             outOfSync = false;
 352         }
 353         return outOfSync;
 354     }
 355
 356     @Override
 357     protected RaftActorBehavior handleAppendEntriesReply(ActorRef sender,
 358         AppendEntriesReply appendEntriesReply) {
 359         return this;
 360     }
 361
 362     @Override
 363     protected RaftActorBehavior handleRequestVoteReply(ActorRef sender,
 364         RequestVoteReply requestVoteReply) {
 365         return this;
 366     }
 367
 368     @Override
 369     public RaftActorBehavior handleMessage(ActorRef sender, Object message) {
 370         if (message instanceof ElectionTimeout || message instanceof TimeoutNow) {
 371             return handleElectionTimeout(message);
 372         }
 373
 374         if (!(message instanceof RaftRPC)) {
 375             // The rest of the processing requires the message to be a RaftRPC
 376             return null;
 377         }
 378
 379         final RaftRPC rpc = (RaftRPC) message;
 380         // If RPC request or response contains term T > currentTerm:
 381         // set currentTerm = T, convert to follower (§5.1)
 382         // This applies to all RPC messages and responses
 383         if (rpc.getTerm() > context.getTermInformation().getCurrentTerm()) {
 384             LOG.debug("{}: Term {} in \"{}\" message is greater than follower's term {} - updating term",
 385                 logName(), rpc.getTerm(), rpc, context.getTermInformation().getCurrentTerm());
 386
 387             context.getTermInformation().updateAndPersist(rpc.getTerm(), null);
 388         }
 389
 390         if (rpc instanceof InstallSnapshot) {
 391             handleInstallSnapshot(sender, (InstallSnapshot) rpc);
 392             restartLastLeaderMessageTimer();
 393             scheduleElection(electionDuration());
 394             return this;
 395         }
 396
 397         if (!(rpc instanceof RequestVote) || canGrantVote((RequestVote) rpc)) {
 398             restartLastLeaderMessageTimer();
 399             scheduleElection(electionDuration());
 400         }
 401
 402         return super.handleMessage(sender, rpc);
 403     }
 404
 405     private RaftActorBehavior handleElectionTimeout(Object message) {
 406         // If the message is ElectionTimeout, verify we haven't actually seen a message from the leader
 407         // during the election timeout interval. It may that the election timer expired b/c this actor
 408         // was busy and messages got delayed, in which case leader messages would be backed up in the
 409         // queue but would be processed before the ElectionTimeout message and thus would restart the
 410         // lastLeaderMessageTimer.
 411         long lastLeaderMessageInterval = lastLeaderMessageTimer.elapsed(TimeUnit.MILLISECONDS);
 412         long electionTimeoutInMillis = context.getConfigParams().getElectionTimeOutInterval().toMillis();
 413         boolean noLeaderMessageReceived = !lastLeaderMessageTimer.isRunning() ||
 414                 lastLeaderMessageInterval >= electionTimeoutInMillis;
 415
 416         if(canStartElection()) {
 417             if(message instanceof TimeoutNow) {
 418                 LOG.debug("{}: Received TimeoutNow - switching to Candidate", logName());
 419                 return internalSwitchBehavior(RaftState.Candidate);
 420             } else if(noLeaderMessageReceived) {
 421                 // Check the cluster state to see if the leader is known to be up before we go to Candidate.
 422                 // However if we haven't heard from the leader in a long time even though the cluster state
 423                 // indicates it's up then something is wrong - leader might be stuck indefinitely - so switch
 424                 // to Candidate,
 425                 long maxElectionTimeout = electionTimeoutInMillis * MAX_ELECTION_TIMEOUT_FACTOR;
 426                 if(isLeaderAvailabilityKnown() && lastLeaderMessageInterval < maxElectionTimeout) {
 427                     LOG.debug("{}: Received ElectionTimeout but leader appears to be available", logName());
 428                     scheduleElection(electionDuration());
 429                 } else {
 430                     LOG.debug("{}: Received ElectionTimeout - switching to Candidate", logName());
 431                     return internalSwitchBehavior(RaftState.Candidate);
 432                 }
 433             } else {
 434                 LOG.debug("{}: Received ElectionTimeout but lastLeaderMessageInterval {} < election timeout {}",
 435                         logName(), lastLeaderMessageInterval, context.getConfigParams().getElectionTimeOutInterval());
 436                 scheduleElection(electionDuration());
 437             }
 438         } else if(message instanceof ElectionTimeout) {
 439             if(noLeaderMessageReceived) {
 440                 setLeaderId(null);
 441             }
 442
 443             scheduleElection(electionDuration());
 444         }
 445
 446         return this;
 447     }
 448
 449     private boolean isLeaderAvailabilityKnown() {
 450         if(leaderId == null) {
 451             return false;
 452         }
 453
 454         Optional<Cluster> cluster = context.getCluster();
 455         if(!cluster.isPresent()) {
 456             return false;
 457         }
 458
 459         ActorSelection leaderActor = context.getPeerActorSelection(leaderId);
 460         if(leaderActor == null) {
 461             return false;
 462         }
 463
 464         Address leaderAddress = leaderActor.anchorPath().address();
 465
 466         CurrentClusterState state = cluster.get().state();
 467         Set<Member> unreachable = state.getUnreachable();
 468
 469         LOG.debug("{}: Checking for leader {} in the cluster unreachable set {}", logName(), leaderAddress,
 470                 unreachable);
 471
 472         for(Member m: unreachable) {
 473             if(leaderAddress.equals(m.address())) {
 474                 LOG.info("{}: Leader {} is unreachable", logName(), leaderAddress);
 475                 return false;
 476             }
 477         }
 478
 479         for(Member m: state.getMembers()) {
 480             if(leaderAddress.equals(m.address())) {
 481                 if(m.status() == MemberStatus.up() || m.status() == MemberStatus.weaklyUp()) {
 482                     LOG.debug("{}: Leader {} cluster status is {} - leader is available", logName(),
 483                             leaderAddress, m.status());
 484                     return true;
 485                 } else {
 486                     LOG.debug("{}: Leader {} cluster status is {} - leader is unavailable", logName(),
 487                             leaderAddress, m.status());
 488                     return false;
 489                 }
 490             }
 491         }
 492
 493         LOG.debug("{}: Leader {} not found in the cluster member set", logName(), leaderAddress);
 494
 495         return false;
 496     }
 497
 498     private void handleInstallSnapshot(final ActorRef sender, InstallSnapshot installSnapshot) {
 499
 500         LOG.debug("{}: handleInstallSnapshot: {}", logName(), installSnapshot);
 501
 502         leaderId = installSnapshot.getLeaderId();
 503
 504         if(snapshotTracker == null){
 505             snapshotTracker = new SnapshotTracker(LOG, installSnapshot.getTotalChunks(), installSnapshot.getLeaderId());
 506         }
 507
 508         updateInitialSyncStatus(installSnapshot.getLastIncludedIndex(), installSnapshot.getLeaderId());
 509
 510         try {
 511             final InstallSnapshotReply reply = new InstallSnapshotReply(
 512                     currentTerm(), context.getId(), installSnapshot.getChunkIndex(), true);
 513
 514             if(snapshotTracker.addChunk(installSnapshot.getChunkIndex(), installSnapshot.getData(),
 515                     installSnapshot.getLastChunkHashCode())){
 516                 Snapshot snapshot = Snapshot.create(snapshotTracker.getSnapshot(),
 517                         new ArrayList<ReplicatedLogEntry>(),
 518                         installSnapshot.getLastIncludedIndex(),
 519                         installSnapshot.getLastIncludedTerm(),
 520                         installSnapshot.getLastIncludedIndex(),
 521                         installSnapshot.getLastIncludedTerm(),
 522                         context.getTermInformation().getCurrentTerm(),
 523                         context.getTermInformation().getVotedFor(),
 524                         installSnapshot.getServerConfig().orNull());
 525
 526                 ApplySnapshot.Callback applySnapshotCallback = new ApplySnapshot.Callback() {
 527                     @Override
 528                     public void onSuccess() {
 529                         LOG.debug("{}: handleInstallSnapshot returning: {}", logName(), reply);
 530
 531                         sender.tell(reply, actor());
 532                     }
 533
 534                     @Override
 535                     public void onFailure() {
 536                         sender.tell(new InstallSnapshotReply(currentTerm(), context.getId(), -1, false), actor());
 537                     }
 538                 };
 539
 540                 actor().tell(new ApplySnapshot(snapshot, applySnapshotCallback), actor());
 541
 542                 snapshotTracker = null;
 543             } else {
 544                 LOG.debug("{}: handleInstallSnapshot returning: {}", logName(), reply);
 545
 546                 sender.tell(reply, actor());
 547             }
 548         } catch (SnapshotTracker.InvalidChunkException e) {
 549             LOG.debug("{}: Exception in InstallSnapshot of follower", logName(), e);
 550
 551             sender.tell(new InstallSnapshotReply(currentTerm(), context.getId(),
 552                     -1, false), actor());
 553             snapshotTracker = null;
 554
 555         } catch (Exception e){
 556             LOG.error("{}: Exception in InstallSnapshot of follower", logName(), e);
 557
 558             //send reply with success as false. The chunk will be sent again on failure
 559             sender.tell(new InstallSnapshotReply(currentTerm(), context.getId(),
 560                     installSnapshot.getChunkIndex(), false), actor());
 561
 562         }
 563     }
 564
 565     @Override
 566     public void close() {
 567         stopElection();
 568     }
 569
 570     @VisibleForTesting
 571     SnapshotTracker getSnapshotTracker(){
 572         return snapshotTracker;
 573     }
 574 }