1a721335068336c7b9d00e95da720d2fba48ee80
[controller.git] / 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 }

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