opendaylight/md-sal/sal-akka-raft/src/main/java/org/opendaylight/controller/cluster/raft/behaviors/AbstractLeader.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.Cancellable;
  14 import com.google.common.annotations.VisibleForTesting;
  15 import com.google.common.base.Optional;
  16 import com.google.common.base.Preconditions;
  17 import com.google.common.collect.ImmutableMap;
  18 import com.google.common.collect.ImmutableMap.Builder;
  19 import com.google.protobuf.ByteString;
  20 import java.io.IOException;
  21 import java.util.Collection;
  22 import java.util.Collections;
  23 import java.util.HashMap;
  24 import java.util.Iterator;
  25 import java.util.LinkedList;
  26 import java.util.List;
  27 import java.util.Map;
  28 import java.util.Map.Entry;
  29 import org.opendaylight.controller.cluster.raft.ClientRequestTracker;
  30 import org.opendaylight.controller.cluster.raft.ClientRequestTrackerImpl;
  31 import org.opendaylight.controller.cluster.raft.FollowerLogInformation;
  32 import org.opendaylight.controller.cluster.raft.FollowerLogInformationImpl;
  33 import org.opendaylight.controller.cluster.raft.RaftActorContext;
  34 import org.opendaylight.controller.cluster.raft.RaftState;
  35 import org.opendaylight.controller.cluster.raft.ReplicatedLogEntry;
  36 import org.opendaylight.controller.cluster.raft.base.messages.CaptureSnapshot;
  37 import org.opendaylight.controller.cluster.raft.base.messages.Replicate;
  38 import org.opendaylight.controller.cluster.raft.base.messages.SendHeartBeat;
  39 import org.opendaylight.controller.cluster.raft.base.messages.SendInstallSnapshot;
  40 import org.opendaylight.controller.cluster.raft.messages.AppendEntries;
  41 import org.opendaylight.controller.cluster.raft.messages.AppendEntriesReply;
  42 import org.opendaylight.controller.cluster.raft.messages.InstallSnapshot;
  43 import org.opendaylight.controller.cluster.raft.messages.InstallSnapshotReply;
  44 import org.opendaylight.controller.cluster.raft.messages.RaftRPC;
  45 import org.opendaylight.controller.cluster.raft.messages.RequestVoteReply;
  46 import scala.concurrent.duration.FiniteDuration;
  47
  48 /**
  49  * The behavior of a RaftActor when it is in the Leader state
  50  * <p/>
  51  * Leaders:
  52  * <ul>
  53  * <li> Upon election: send initial empty AppendEntries RPCs
  54  * (heartbeat) to each server; repeat during idle periods to
  55  * prevent election timeouts (§5.2)
  56  * <li> If command received from client: append entry to local log,
  57  * respond after entry applied to state machine (§5.3)
  58  * <li> If last log index ≥ nextIndex for a follower: send
  59  * AppendEntries RPC with log entries starting at nextIndex
  60  * <ul>
  61  * <li> If successful: update nextIndex and matchIndex for
  62  * follower (§5.3)
  63  * <li> If AppendEntries fails because of log inconsistency:
  64  * decrement nextIndex and retry (§5.3)
  65  * </ul>
  66  * <li> If there exists an N such that N > commitIndex, a majority
  67  * of matchIndex[i] ≥ N, and log[N].term == currentTerm:
  68  * set commitIndex = N (§5.3, §5.4).
  69  */
  70 public abstract class AbstractLeader extends AbstractRaftActorBehavior {
  71
  72     // The index of the first chunk that is sent when installing a snapshot
  73     public static final int FIRST_CHUNK_INDEX = 1;
  74
  75     // The index that the follower should respond with if it needs the install snapshot to be reset
  76     public static final int INVALID_CHUNK_INDEX = -1;
  77
  78     // This would be passed as the hash code of the last chunk when sending the first chunk
  79     public static final int INITIAL_LAST_CHUNK_HASH_CODE = -1;
  80
  81     private final Map<String, FollowerLogInformation> followerToLog;
  82     private final Map<String, FollowerToSnapshot> mapFollowerToSnapshot = new HashMap<>();
  83
  84     private Cancellable heartbeatSchedule = null;
  85
  86     private final Collection<ClientRequestTracker> trackerList = new LinkedList<>();
  87
  88     protected final int minReplicationCount;
  89
  90     protected final int minIsolatedLeaderPeerCount;
  91
  92     private Optional<ByteString> snapshot;
  93
  94     private long replicatedToAllIndex = -1;
  95
  96     public AbstractLeader(RaftActorContext context) {
  97         super(context);
  98
  99         final Builder<String, FollowerLogInformation> ftlBuilder = ImmutableMap.builder();
 100         for (String followerId : context.getPeerAddresses().keySet()) {
 101             FollowerLogInformation followerLogInformation =
 102                 new FollowerLogInformationImpl(followerId,
 103                     context.getCommitIndex(), -1,
 104                     context.getConfigParams().getElectionTimeOutInterval());
 105
 106             ftlBuilder.put(followerId, followerLogInformation);
 107         }
 108         followerToLog = ftlBuilder.build();
 109
 110         leaderId = context.getId();
 111
 112         LOG.debug("{}: Election: Leader has following peers: {}", context.getId(), getFollowerIds());
 113
 114         minReplicationCount = getMajorityVoteCount(getFollowerIds().size());
 115
 116         // the isolated Leader peer count will be 1 less than the majority vote count.
 117         // this is because the vote count has the self vote counted in it
 118         // for e.g
 119         // 0 peers = 1 votesRequired , minIsolatedLeaderPeerCount = 0
 120         // 2 peers = 2 votesRequired , minIsolatedLeaderPeerCount = 1
 121         // 4 peers = 3 votesRequired, minIsolatedLeaderPeerCount = 2
 122         minIsolatedLeaderPeerCount = minReplicationCount > 0 ? (minReplicationCount - 1) : 0;
 123
 124         snapshot = Optional.absent();
 125
 126         // Immediately schedule a heartbeat
 127         // Upon election: send initial empty AppendEntries RPCs
 128         // (heartbeat) to each server; repeat during idle periods to
 129         // prevent election timeouts (§5.2)
 130         sendAppendEntries(0);
 131     }
 132
 133     /**
 134      * Return an immutable collection of follower identifiers.
 135      *
 136      * @return Collection of follower IDs
 137      */
 138     protected final Collection<String> getFollowerIds() {
 139         return followerToLog.keySet();
 140     }
 141
 142     private Optional<ByteString> getSnapshot() {
 143         return snapshot;
 144     }
 145
 146     @VisibleForTesting
 147     void setSnapshot(Optional<ByteString> snapshot) {
 148         this.snapshot = snapshot;
 149     }
 150
 151     @Override
 152     protected RaftActorBehavior handleAppendEntries(ActorRef sender,
 153         AppendEntries appendEntries) {
 154
 155         if(LOG.isDebugEnabled()) {
 156             LOG.debug("{}: handleAppendEntries: {}", context.getId(), appendEntries);
 157         }
 158
 159         return this;
 160     }
 161
 162     @Override
 163     protected RaftActorBehavior handleAppendEntriesReply(ActorRef sender,
 164         AppendEntriesReply appendEntriesReply) {
 165
 166         if(! appendEntriesReply.isSuccess()) {
 167             if(LOG.isDebugEnabled()) {
 168                 LOG.debug("{}: handleAppendEntriesReply: {}", context.getId(), appendEntriesReply);
 169             }
 170         }
 171
 172         // Update the FollowerLogInformation
 173         String followerId = appendEntriesReply.getFollowerId();
 174         FollowerLogInformation followerLogInformation =
 175             followerToLog.get(followerId);
 176
 177         if(followerLogInformation == null){
 178             LOG.error("{}: handleAppendEntriesReply - unknown follower {}", context.getId(), followerId);
 179             return this;
 180         }
 181
 182         followerLogInformation.markFollowerActive();
 183
 184         if (appendEntriesReply.isSuccess()) {
 185             followerLogInformation
 186                 .setMatchIndex(appendEntriesReply.getLogLastIndex());
 187             followerLogInformation
 188                 .setNextIndex(appendEntriesReply.getLogLastIndex() + 1);
 189         } else {
 190
 191             // TODO: When we find that the follower is out of sync with the
 192             // Leader we simply decrement that followers next index by 1.
 193             // Would it be possible to do better than this? The RAFT spec
 194             // does not explicitly deal with it but may be something for us to
 195             // think about
 196
 197             followerLogInformation.decrNextIndex();
 198         }
 199
 200         // Now figure out if this reply warrants a change in the commitIndex
 201         // If there exists an N such that N > commitIndex, a majority
 202         // of matchIndex[i] ≥ N, and log[N].term == currentTerm:
 203         // set commitIndex = N (§5.3, §5.4).
 204         for (long N = context.getCommitIndex() + 1; ; N++) {
 205             int replicatedCount = 1;
 206
 207             for (FollowerLogInformation info : followerToLog.values()) {
 208                 if (info.getMatchIndex() >= N) {
 209                     replicatedCount++;
 210                 }
 211             }
 212
 213             if (replicatedCount >= minReplicationCount) {
 214                 ReplicatedLogEntry replicatedLogEntry = context.getReplicatedLog().get(N);
 215                 if (replicatedLogEntry != null &&
 216                     replicatedLogEntry.getTerm() == currentTerm()) {
 217                     context.setCommitIndex(N);
 218                 }
 219             } else {
 220                 break;
 221             }
 222         }
 223
 224         // Apply the change to the state machine
 225         if (context.getCommitIndex() > context.getLastApplied()) {
 226             applyLogToStateMachine(context.getCommitIndex());
 227         }
 228
 229         if (!context.isSnapshotCaptureInitiated()) {
 230             purgeInMemoryLog();
 231         }
 232
 233         //Send the next log entry immediately, if possible, no need to wait for heartbeat to trigger that event
 234         sendUpdatesToFollower(followerId, followerLogInformation, false);
 235         return this;
 236     }
 237
 238     private void purgeInMemoryLog() {
 239         //find the lowest index across followers which has been replicated to all. -1 if there are no followers.
 240         // we would delete the in-mem log from that index on, in-order to minimize mem usage
 241         // we would also share this info thru AE with the followers so that they can delete their log entries as well.
 242         long minReplicatedToAllIndex = followerToLog.isEmpty() ? -1 : Long.MAX_VALUE;
 243         for (FollowerLogInformation info : followerToLog.values()) {
 244             minReplicatedToAllIndex = Math.min(minReplicatedToAllIndex, info.getMatchIndex());
 245         }
 246
 247         replicatedToAllIndex = fakeSnapshot(minReplicatedToAllIndex, replicatedToAllIndex);
 248     }
 249
 250     @Override
 251     protected ClientRequestTracker removeClientRequestTracker(long logIndex) {
 252         final Iterator<ClientRequestTracker> it = trackerList.iterator();
 253         while (it.hasNext()) {
 254             final ClientRequestTracker t = it.next();
 255             if (t.getIndex() == logIndex) {
 256                 it.remove();
 257                 return t;
 258             }
 259         }
 260
 261         return null;
 262     }
 263
 264     @Override
 265     protected ClientRequestTracker findClientRequestTracker(long logIndex) {
 266         for (ClientRequestTracker tracker : trackerList) {
 267             if (tracker.getIndex() == logIndex) {
 268                 return tracker;
 269             }
 270         }
 271         return null;
 272     }
 273
 274     @Override
 275     protected RaftActorBehavior handleRequestVoteReply(ActorRef sender,
 276         RequestVoteReply requestVoteReply) {
 277         return this;
 278     }
 279
 280     @Override
 281     public RaftState state() {
 282         return RaftState.Leader;
 283     }
 284
 285     @Override
 286     public RaftActorBehavior handleMessage(ActorRef sender, Object originalMessage) {
 287         Preconditions.checkNotNull(sender, "sender should not be null");
 288
 289         Object message = fromSerializableMessage(originalMessage);
 290
 291         if (message instanceof RaftRPC) {
 292             RaftRPC rpc = (RaftRPC) message;
 293             // If RPC request or response contains term T > currentTerm:
 294             // set currentTerm = T, convert to follower (§5.1)
 295             // This applies to all RPC messages and responses
 296             if (rpc.getTerm() > context.getTermInformation().getCurrentTerm()) {
 297                 LOG.debug("{}: Term {} in \"{}\" message is greater than leader's term {}", context.getId(),
 298                         rpc.getTerm(), rpc, context.getTermInformation().getCurrentTerm());
 299
 300                 context.getTermInformation().updateAndPersist(rpc.getTerm(), null);
 301
 302                 return switchBehavior(new Follower(context));
 303             }
 304         }
 305
 306         try {
 307             if (message instanceof SendHeartBeat) {
 308                 sendHeartBeat();
 309                 return this;
 310
 311             } else if(message instanceof SendInstallSnapshot) {
 312                 // received from RaftActor
 313                 setSnapshot(Optional.of(((SendInstallSnapshot) message).getSnapshot()));
 314                 sendInstallSnapshot();
 315
 316             } else if (message instanceof Replicate) {
 317                 replicate((Replicate) message);
 318
 319             } else if (message instanceof InstallSnapshotReply){
 320                 handleInstallSnapshotReply((InstallSnapshotReply) message);
 321
 322             }
 323         } finally {
 324             scheduleHeartBeat(context.getConfigParams().getHeartBeatInterval());
 325         }
 326
 327         return super.handleMessage(sender, message);
 328     }
 329
 330     private void handleInstallSnapshotReply(InstallSnapshotReply reply) {
 331         String followerId = reply.getFollowerId();
 332         FollowerToSnapshot followerToSnapshot = mapFollowerToSnapshot.get(followerId);
 333
 334         if (followerToSnapshot == null) {
 335             LOG.error("{}: FollowerId {} in InstallSnapshotReply not known to Leader",
 336                     context.getId(), followerId);
 337             return;
 338         }
 339
 340         FollowerLogInformation followerLogInformation = followerToLog.get(followerId);
 341         followerLogInformation.markFollowerActive();
 342
 343         if (followerToSnapshot.getChunkIndex() == reply.getChunkIndex()) {
 344             boolean wasLastChunk = false;
 345             if (reply.isSuccess()) {
 346                 if(followerToSnapshot.isLastChunk(reply.getChunkIndex())) {
 347                     //this was the last chunk reply
 348                     if(LOG.isDebugEnabled()) {
 349                         LOG.debug("{}: InstallSnapshotReply received, " +
 350                                 "last chunk received, Chunk:{}. Follower:{} Setting nextIndex:{}",
 351                                 context.getId(), reply.getChunkIndex(), followerId,
 352                             context.getReplicatedLog().getSnapshotIndex() + 1
 353                         );
 354                     }
 355
 356                     followerLogInformation.setMatchIndex(
 357                         context.getReplicatedLog().getSnapshotIndex());
 358                     followerLogInformation.setNextIndex(
 359                         context.getReplicatedLog().getSnapshotIndex() + 1);
 360                     mapFollowerToSnapshot.remove(followerId);
 361
 362                     if(LOG.isDebugEnabled()) {
 363                         LOG.debug("{}: followerToLog.get(followerId).getNextIndex()=" +
 364                                 context.getId(), followerToLog.get(followerId).getNextIndex());
 365                     }
 366
 367                     if (mapFollowerToSnapshot.isEmpty()) {
 368                         // once there are no pending followers receiving snapshots
 369                         // we can remove snapshot from the memory
 370                         setSnapshot(Optional.<ByteString>absent());
 371                     }
 372                     wasLastChunk = true;
 373
 374                 } else {
 375                     followerToSnapshot.markSendStatus(true);
 376                 }
 377             } else {
 378                 LOG.info("{}: InstallSnapshotReply received sending snapshot chunk failed, Will retry, Chunk: {}",
 379                         context.getId(), reply.getChunkIndex());
 380
 381                 followerToSnapshot.markSendStatus(false);
 382             }
 383
 384             if (!wasLastChunk && followerToSnapshot.canSendNextChunk()) {
 385                 ActorSelection followerActor = context.getPeerActorSelection(followerId);
 386                 if(followerActor != null) {
 387                     sendSnapshotChunk(followerActor, followerId);
 388                 }
 389             }
 390
 391         } else {
 392             LOG.error("{}: Chunk index {} in InstallSnapshotReply from follower {} does not match expected index {}",
 393                     context.getId(), reply.getChunkIndex(), followerId,
 394                     followerToSnapshot.getChunkIndex());
 395
 396             if(reply.getChunkIndex() == INVALID_CHUNK_INDEX){
 397                 // Since the Follower did not find this index to be valid we should reset the follower snapshot
 398                 // so that Installing the snapshot can resume from the beginning
 399                 followerToSnapshot.reset();
 400             }
 401         }
 402     }
 403
 404     private void replicate(Replicate replicate) {
 405         long logIndex = replicate.getReplicatedLogEntry().getIndex();
 406
 407         if(LOG.isDebugEnabled()) {
 408             LOG.debug("{}: Replicate message {}", context.getId(), logIndex);
 409         }
 410
 411         // Create a tracker entry we will use this later to notify the
 412         // client actor
 413         trackerList.add(
 414             new ClientRequestTrackerImpl(replicate.getClientActor(),
 415                 replicate.getIdentifier(),
 416                 logIndex)
 417         );
 418
 419         if (followerToLog.isEmpty()) {
 420             context.setCommitIndex(logIndex);
 421             applyLogToStateMachine(logIndex);
 422         } else {
 423             sendAppendEntries(0);
 424         }
 425     }
 426
 427     private void sendAppendEntries(long timeSinceLastActivityInterval) {
 428         // Send an AppendEntries to all followers
 429         for (Entry<String, FollowerLogInformation> e : followerToLog.entrySet()) {
 430             final String followerId = e.getKey();
 431             final FollowerLogInformation followerLogInformation = e.getValue();
 432             // This checks helps not to send a repeat message to the follower
 433             if(!followerLogInformation.isFollowerActive() ||
 434                     followerLogInformation.timeSinceLastActivity() >= timeSinceLastActivityInterval) {
 435                 sendUpdatesToFollower(followerId, followerLogInformation, true);
 436             }
 437         }
 438     }
 439
 440     /**
 441      *
 442      * This method checks if any update needs to be sent to the given follower. This includes append log entries,
 443      * sending next snapshot chunk, and initiating a snapshot.
 444      * @return true if any update is sent, false otherwise
 445      */
 446
 447     private void sendUpdatesToFollower(String followerId, FollowerLogInformation followerLogInformation,
 448                                           boolean sendHeartbeat) {
 449
 450         ActorSelection followerActor = context.getPeerActorSelection(followerId);
 451         if (followerActor != null) {
 452             long followerNextIndex = followerLogInformation.getNextIndex();
 453             boolean isFollowerActive = followerLogInformation.isFollowerActive();
 454
 455             if (mapFollowerToSnapshot.get(followerId) != null) {
 456                 // if install snapshot is in process , then sent next chunk if possible
 457                 if (isFollowerActive && mapFollowerToSnapshot.get(followerId).canSendNextChunk()) {
 458                     sendSnapshotChunk(followerActor, followerId);
 459                 } else if(sendHeartbeat) {
 460                     // we send a heartbeat even if we have not received a reply for the last chunk
 461                     sendAppendEntriesToFollower(followerActor, followerLogInformation.getNextIndex(),
 462                         Collections.<ReplicatedLogEntry>emptyList(), followerId);
 463                 }
 464             } else {
 465                 long leaderLastIndex = context.getReplicatedLog().lastIndex();
 466                 long leaderSnapShotIndex = context.getReplicatedLog().getSnapshotIndex();
 467                 if (isFollowerActive &&
 468                     context.getReplicatedLog().isPresent(followerNextIndex)) {
 469                     // FIXME : Sending one entry at a time
 470                     final List<ReplicatedLogEntry> entries = context.getReplicatedLog().getFrom(followerNextIndex, 1);
 471
 472                     sendAppendEntriesToFollower(followerActor, followerNextIndex, entries, followerId);
 473
 474                 } else if (isFollowerActive && followerNextIndex >= 0 &&
 475                     leaderLastIndex >= followerNextIndex) {
 476                     // if the followers next index is not present in the leaders log, and
 477                     // if the follower is just not starting and if leader's index is more than followers index
 478                     // then snapshot should be sent
 479
 480                     if (LOG.isDebugEnabled()) {
 481                         LOG.debug("InitiateInstallSnapshot to follower:{}," +
 482                                 "follower-nextIndex:{}, leader-snapshot-index:{},  " +
 483                                 "leader-last-index:{}", followerId,
 484                             followerNextIndex, leaderSnapShotIndex, leaderLastIndex
 485                         );
 486                     }
 487
 488                     // Send heartbeat to follower whenever install snapshot is initiated.
 489                     sendAppendEntriesToFollower(followerActor, followerLogInformation.getNextIndex(),
 490                             Collections.<ReplicatedLogEntry>emptyList(), followerId);
 491
 492                     initiateCaptureSnapshot(followerId, followerNextIndex);
 493
 494                 } else if(sendHeartbeat) {
 495                     //we send an AppendEntries, even if the follower is inactive
 496                     // in-order to update the followers timestamp, in case it becomes active again
 497                     sendAppendEntriesToFollower(followerActor, followerLogInformation.getNextIndex(),
 498                         Collections.<ReplicatedLogEntry>emptyList(), followerId);
 499                 }
 500
 501             }
 502         }
 503     }
 504
 505     private void sendAppendEntriesToFollower(ActorSelection followerActor, long followerNextIndex,
 506         List<ReplicatedLogEntry> entries, String followerId) {
 507         AppendEntries appendEntries = new AppendEntries(currentTerm(), context.getId(),
 508             prevLogIndex(followerNextIndex),
 509             prevLogTerm(followerNextIndex), entries,
 510             context.getCommitIndex(), replicatedToAllIndex);
 511
 512         if(!entries.isEmpty()) {
 513             LOG.debug("{}: Sending AppendEntries to follower {}: {}", context.getId(), followerId,
 514                     appendEntries);
 515         }
 516
 517         followerActor.tell(appendEntries.toSerializable(), actor());
 518     }
 519
 520     /**
 521      * /**
 522      * Install Snapshot works as follows
 523      * 1. Leader initiates the capture snapshot by sending a CaptureSnapshot message to actor
 524      * 2. RaftActor on receipt of the CaptureSnapshotReply (from Shard), stores the received snapshot in the replicated log
 525      * and makes a call to Leader's handleMessage , with SendInstallSnapshot message.
 526      * 3. Leader , picks the snapshot from im-mem ReplicatedLog and sends it in chunks to the Follower
 527      * 4. On complete, Follower sends back a InstallSnapshotReply.
 528      * 5. On receipt of the InstallSnapshotReply for the last chunk, Leader marks the install complete for that follower
 529      * and replenishes the memory by deleting the snapshot in Replicated log.
 530      * 6. If another follower requires a snapshot and a snapshot has been collected (via CaptureSnapshotReply)
 531      * then send the existing snapshot in chunks to the follower.
 532      * @param followerId
 533      * @param followerNextIndex
 534      */
 535     private void initiateCaptureSnapshot(String followerId, long followerNextIndex) {
 536         if(LOG.isDebugEnabled()) {
 537             LOG.debug("{}: initiateCaptureSnapshot, followers {}", context.getId(), followerToLog.keySet());
 538         }
 539
 540         if (!context.getReplicatedLog().isPresent(followerNextIndex) &&
 541                 context.getReplicatedLog().isInSnapshot(followerNextIndex)) {
 542
 543             if (snapshot.isPresent()) {
 544                 // if a snapshot is present in the memory, most likely another install is in progress
 545                 // no need to capture snapshot.
 546                 // This could happen if another follower needs an install when one is going on.
 547                 final ActorSelection followerActor = context.getPeerActorSelection(followerId);
 548                 sendSnapshotChunk(followerActor, followerId);
 549
 550             } else if (!context.isSnapshotCaptureInitiated()) {
 551
 552                 LOG.info("{}: Initiating Snapshot Capture to Install Snapshot, Leader:{}", context.getId(), getLeaderId());
 553                 ReplicatedLogEntry lastAppliedEntry = context.getReplicatedLog().get(context.getLastApplied());
 554                 long lastAppliedIndex = -1;
 555                 long lastAppliedTerm = -1;
 556
 557                 if (lastAppliedEntry != null) {
 558                     lastAppliedIndex = lastAppliedEntry.getIndex();
 559                     lastAppliedTerm = lastAppliedEntry.getTerm();
 560                 } else if (context.getReplicatedLog().getSnapshotIndex() > -1)  {
 561                     lastAppliedIndex = context.getReplicatedLog().getSnapshotIndex();
 562                     lastAppliedTerm = context.getReplicatedLog().getSnapshotTerm();
 563                 }
 564
 565                 boolean isInstallSnapshotInitiated = true;
 566                 actor().tell(new CaptureSnapshot(lastIndex(), lastTerm(),
 567                                 lastAppliedIndex, lastAppliedTerm, isInstallSnapshotInitiated),
 568                         actor());
 569                 context.setSnapshotCaptureInitiated(true);
 570             }
 571         }
 572     }
 573
 574
 575     private void sendInstallSnapshot() {
 576         for (Entry<String, FollowerLogInformation> e : followerToLog.entrySet()) {
 577             ActorSelection followerActor = context.getPeerActorSelection(e.getKey());
 578
 579             if (followerActor != null) {
 580                 long nextIndex = e.getValue().getNextIndex();
 581
 582                 if (!context.getReplicatedLog().isPresent(nextIndex) &&
 583                     context.getReplicatedLog().isInSnapshot(nextIndex)) {
 584                     sendSnapshotChunk(followerActor, e.getKey());
 585                 }
 586             }
 587         }
 588     }
 589
 590     /**
 591      *  Sends a snapshot chunk to a given follower
 592      *  InstallSnapshot should qualify as a heartbeat too.
 593      */
 594     private void sendSnapshotChunk(ActorSelection followerActor, String followerId) {
 595         try {
 596             if (snapshot.isPresent()) {
 597                 ByteString nextSnapshotChunk = getNextSnapshotChunk(followerId,snapshot.get());
 598
 599                 // Note: the previous call to getNextSnapshotChunk has the side-effect of adding
 600                 // followerId to the followerToSnapshot map.
 601                 FollowerToSnapshot followerToSnapshot = mapFollowerToSnapshot.get(followerId);
 602
 603                 followerActor.tell(
 604                     new InstallSnapshot(currentTerm(), context.getId(),
 605                         context.getReplicatedLog().getSnapshotIndex(),
 606                         context.getReplicatedLog().getSnapshotTerm(),
 607                         nextSnapshotChunk,
 608                             followerToSnapshot.incrementChunkIndex(),
 609                             followerToSnapshot.getTotalChunks(),
 610                         Optional.of(followerToSnapshot.getLastChunkHashCode())
 611                     ).toSerializable(),
 612                     actor()
 613                 );
 614                 LOG.info("{}: InstallSnapshot sent to follower {}, Chunk: {}/{}",
 615                         context.getId(), followerActor.path(),
 616                         followerToSnapshot.getChunkIndex(),
 617                         followerToSnapshot.getTotalChunks());
 618             }
 619         } catch (IOException e) {
 620             LOG.error("{}: InstallSnapshot failed for Leader.", context.getId(), e);
 621         }
 622     }
 623
 624     /**
 625      * Acccepts snaphot as ByteString, enters into map for future chunks
 626      * creates and return a ByteString chunk
 627      */
 628     private ByteString getNextSnapshotChunk(String followerId, ByteString snapshotBytes) throws IOException {
 629         FollowerToSnapshot followerToSnapshot = mapFollowerToSnapshot.get(followerId);
 630         if (followerToSnapshot == null) {
 631             followerToSnapshot = new FollowerToSnapshot(snapshotBytes);
 632             mapFollowerToSnapshot.put(followerId, followerToSnapshot);
 633         }
 634         ByteString nextChunk = followerToSnapshot.getNextChunk();
 635         if (LOG.isDebugEnabled()) {
 636             LOG.debug("{}: Leader's snapshot nextChunk size:{}", context.getId(), nextChunk.size());
 637         }
 638         return nextChunk;
 639     }
 640
 641     private void sendHeartBeat() {
 642         if (!followerToLog.isEmpty()) {
 643             sendAppendEntries(context.getConfigParams().getHeartBeatInterval().toMillis());
 644         }
 645     }
 646
 647     private void stopHeartBeat() {
 648         if (heartbeatSchedule != null && !heartbeatSchedule.isCancelled()) {
 649             heartbeatSchedule.cancel();
 650         }
 651     }
 652
 653     private void scheduleHeartBeat(FiniteDuration interval) {
 654         if (followerToLog.isEmpty()) {
 655             // Optimization - do not bother scheduling a heartbeat as there are
 656             // no followers
 657             return;
 658         }
 659
 660         stopHeartBeat();
 661
 662         // Schedule a heartbeat. When the scheduler triggers a SendHeartbeat
 663         // message is sent to itself.
 664         // Scheduling the heartbeat only once here because heartbeats do not
 665         // need to be sent if there are other messages being sent to the remote
 666         // actor.
 667         heartbeatSchedule = context.getActorSystem().scheduler().scheduleOnce(
 668             interval, context.getActor(), new SendHeartBeat(),
 669             context.getActorSystem().dispatcher(), context.getActor());
 670     }
 671
 672     @Override
 673     public void close() throws Exception {
 674         stopHeartBeat();
 675     }
 676
 677     @Override
 678     public String getLeaderId() {
 679         return context.getId();
 680     }
 681
 682     protected boolean isLeaderIsolated() {
 683         int minPresent = minIsolatedLeaderPeerCount;
 684         for (FollowerLogInformation followerLogInformation : followerToLog.values()) {
 685             if (followerLogInformation.isFollowerActive()) {
 686                 --minPresent;
 687                 if (minPresent == 0) {
 688                     break;
 689                 }
 690             }
 691         }
 692         return (minPresent != 0);
 693     }
 694
 695     /**
 696      * Encapsulates the snapshot bytestring and handles the logic of sending
 697      * snapshot chunks
 698      */
 699     protected class FollowerToSnapshot {
 700         private final ByteString snapshotBytes;
 701         private int offset = 0;
 702         // the next snapshot chunk is sent only if the replyReceivedForOffset matches offset
 703         private int replyReceivedForOffset;
 704         // if replyStatus is false, the previous chunk is attempted
 705         private boolean replyStatus = false;
 706         private int chunkIndex;
 707         private final int totalChunks;
 708         private int lastChunkHashCode = AbstractLeader.INITIAL_LAST_CHUNK_HASH_CODE;
 709         private int nextChunkHashCode = AbstractLeader.INITIAL_LAST_CHUNK_HASH_CODE;
 710
 711         public FollowerToSnapshot(ByteString snapshotBytes) {
 712             this.snapshotBytes = snapshotBytes;
 713             int size = snapshotBytes.size();
 714             totalChunks = ( size / context.getConfigParams().getSnapshotChunkSize()) +
 715                 ((size % context.getConfigParams().getSnapshotChunkSize()) > 0 ? 1 : 0);
 716             if(LOG.isDebugEnabled()) {
 717                 LOG.debug("{}: Snapshot {} bytes, total chunks to send:{}",
 718                         context.getId(), size, totalChunks);
 719             }
 720             replyReceivedForOffset = -1;
 721             chunkIndex = AbstractLeader.FIRST_CHUNK_INDEX;
 722         }
 723
 724         public ByteString getSnapshotBytes() {
 725             return snapshotBytes;
 726         }
 727
 728         public int incrementOffset() {
 729             if(replyStatus) {
 730                 // if prev chunk failed, we would want to sent the same chunk again
 731                 offset = offset + context.getConfigParams().getSnapshotChunkSize();
 732             }
 733             return offset;
 734         }
 735
 736         public int incrementChunkIndex() {
 737             if (replyStatus) {
 738                 // if prev chunk failed, we would want to sent the same chunk again
 739                 chunkIndex =  chunkIndex + 1;
 740             }
 741             return chunkIndex;
 742         }
 743
 744         public int getChunkIndex() {
 745             return chunkIndex;
 746         }
 747
 748         public int getTotalChunks() {
 749             return totalChunks;
 750         }
 751
 752         public boolean canSendNextChunk() {
 753             // we only send a false if a chunk is sent but we have not received a reply yet
 754             return replyReceivedForOffset == offset;
 755         }
 756
 757         public boolean isLastChunk(int chunkIndex) {
 758             return totalChunks == chunkIndex;
 759         }
 760
 761         public void markSendStatus(boolean success) {
 762             if (success) {
 763                 // if the chunk sent was successful
 764                 replyReceivedForOffset = offset;
 765                 replyStatus = true;
 766                 lastChunkHashCode = nextChunkHashCode;
 767             } else {
 768                 // if the chunk sent was failure
 769                 replyReceivedForOffset = offset;
 770                 replyStatus = false;
 771             }
 772         }
 773
 774         public ByteString getNextChunk() {
 775             int snapshotLength = getSnapshotBytes().size();
 776             int start = incrementOffset();
 777             int size = context.getConfigParams().getSnapshotChunkSize();
 778             if (context.getConfigParams().getSnapshotChunkSize() > snapshotLength) {
 779                 size = snapshotLength;
 780             } else {
 781                 if ((start + context.getConfigParams().getSnapshotChunkSize()) > snapshotLength) {
 782                     size = snapshotLength - start;
 783                 }
 784             }
 785
 786             if(LOG.isDebugEnabled()) {
 787                 LOG.debug("{}: Next chunk: length={}, offset={},size={}", context.getId(),
 788                     snapshotLength, start, size);
 789             }
 790             ByteString substring = getSnapshotBytes().substring(start, start + size);
 791             nextChunkHashCode = substring.hashCode();
 792             return substring;
 793         }
 794
 795         /**
 796          * reset should be called when the Follower needs to be sent the snapshot from the beginning
 797          */
 798         public void reset(){
 799             offset = 0;
 800             replyStatus = false;
 801             replyReceivedForOffset = offset;
 802             chunkIndex = AbstractLeader.FIRST_CHUNK_INDEX;
 803             lastChunkHashCode = AbstractLeader.INITIAL_LAST_CHUNK_HASH_CODE;
 804         }
 805
 806         public int getLastChunkHashCode() {
 807             return lastChunkHashCode;
 808         }
 809     }
 810
 811     // called from example-actor for printing the follower-states
 812     public String printFollowerStates() {
 813         final StringBuilder sb = new StringBuilder();
 814
 815         sb.append('[');
 816         for (FollowerLogInformation followerLogInformation : followerToLog.values()) {
 817             sb.append('{');
 818             sb.append(followerLogInformation.getId());
 819             sb.append(" state:");
 820             sb.append(followerLogInformation.isFollowerActive());
 821             sb.append("},");
 822         }
 823         sb.append(']');
 824
 825         return sb.toString();
 826     }
 827
 828     @VisibleForTesting
 829     public FollowerLogInformation getFollower(String followerId) {
 830         return followerToLog.get(followerId);
 831     }
 832
 833     @VisibleForTesting
 834     protected void setFollowerSnapshot(String followerId, FollowerToSnapshot snapshot) {
 835         mapFollowerToSnapshot.put(followerId, snapshot);
 836     }
 837
 838     @VisibleForTesting
 839     public int followerSnapshotSize() {
 840         return mapFollowerToSnapshot.size();
 841     }
 842
 843     @VisibleForTesting
 844     public int followerLogSize() {
 845         return followerToLog.size();
 846     }
 847 }