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.protobuf.ByteString;
  18 import java.io.IOException;
  19 import java.util.Arrays;
  20 import java.util.Collection;
  21 import java.util.Collections;
  22 import java.util.HashMap;
  23 import java.util.Iterator;
  24 import java.util.LinkedList;
  25 import java.util.List;
  26 import java.util.Map;
  27 import java.util.Map.Entry;
  28 import javax.annotation.Nullable;
  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.PeerInfo;
  34 import org.opendaylight.controller.cluster.raft.RaftActorContext;
  35 import org.opendaylight.controller.cluster.raft.RaftState;
  36 import org.opendaylight.controller.cluster.raft.ReplicatedLogEntry;
  37 import org.opendaylight.controller.cluster.raft.Snapshot;
  38 import org.opendaylight.controller.cluster.raft.VotingState;
  39 import org.opendaylight.controller.cluster.raft.base.messages.Replicate;
  40 import org.opendaylight.controller.cluster.raft.base.messages.SendHeartBeat;
  41 import org.opendaylight.controller.cluster.raft.base.messages.SendInstallSnapshot;
  42 import org.opendaylight.controller.cluster.raft.messages.AppendEntries;
  43 import org.opendaylight.controller.cluster.raft.messages.AppendEntriesReply;
  44 import org.opendaylight.controller.cluster.raft.messages.InstallSnapshot;
  45 import org.opendaylight.controller.cluster.raft.messages.InstallSnapshotReply;
  46 import org.opendaylight.controller.cluster.raft.messages.RaftRPC;
  47 import org.opendaylight.controller.cluster.raft.messages.RequestVoteReply;
  48 import org.opendaylight.controller.cluster.raft.messages.UnInitializedFollowerSnapshotReply;
  49 import scala.concurrent.duration.FiniteDuration;
  50
  51 /**
  52  * The behavior of a RaftActor when it is in the Leader state
  53  * <p/>
  54  * Leaders:
  55  * <ul>
  56  * <li> Upon election: send initial empty AppendEntries RPCs
  57  * (heartbeat) to each server; repeat during idle periods to
  58  * prevent election timeouts (§5.2)
  59  * <li> If command received from client: append entry to local log,
  60  * respond after entry applied to state machine (§5.3)
  61  * <li> If last log index ≥ nextIndex for a follower: send
  62  * AppendEntries RPC with log entries starting at nextIndex
  63  * <ul>
  64  * <li> If successful: update nextIndex and matchIndex for
  65  * follower (§5.3)
  66  * <li> If AppendEntries fails because of log inconsistency:
  67  * decrement nextIndex and retry (§5.3)
  68  * </ul>
  69  * <li> If there exists an N such that N > commitIndex, a majority
  70  * of matchIndex[i] ≥ N, and log[N].term == currentTerm:
  71  * set commitIndex = N (§5.3, §5.4).
  72  */
  73 public abstract class AbstractLeader extends AbstractRaftActorBehavior {
  74
  75     // The index of the first chunk that is sent when installing a snapshot
  76     public static final int FIRST_CHUNK_INDEX = 1;
  77
  78     // The index that the follower should respond with if it needs the install snapshot to be reset
  79     public static final int INVALID_CHUNK_INDEX = -1;
  80
  81     // This would be passed as the hash code of the last chunk when sending the first chunk
  82     public static final int INITIAL_LAST_CHUNK_HASH_CODE = -1;
  83
  84     private final Map<String, FollowerLogInformation> followerToLog = new HashMap<>();
  85     private final Map<String, FollowerToSnapshot> mapFollowerToSnapshot = new HashMap<>();
  86
  87     private Cancellable heartbeatSchedule = null;
  88
  89     private final Collection<ClientRequestTracker> trackerList = new LinkedList<>();
  90
  91     private int minReplicationCount;
  92
  93     private Optional<SnapshotHolder> snapshot;
  94
  95     protected AbstractLeader(RaftActorContext context, RaftState state) {
  96         super(context, state);
  97
  98         for(PeerInfo peerInfo: context.getPeers()) {
  99             FollowerLogInformation followerLogInformation = new FollowerLogInformationImpl(peerInfo, -1, context);
 100             followerToLog.put(peerInfo.getId(), followerLogInformation);
 101         }
 102
 103         LOG.debug("{}: Election: Leader has following peers: {}", logName(), getFollowerIds());
 104
 105         updateMinReplicaCount();
 106
 107         snapshot = Optional.absent();
 108
 109         // Immediately schedule a heartbeat
 110         // Upon election: send initial empty AppendEntries RPCs
 111         // (heartbeat) to each server; repeat during idle periods to
 112         // prevent election timeouts (§5.2)
 113         sendAppendEntries(0, false);
 114
 115         // It is important to schedule this heartbeat here
 116         scheduleHeartBeat(context.getConfigParams().getHeartBeatInterval());
 117     }
 118
 119     /**
 120      * Return an immutable collection of follower identifiers.
 121      *
 122      * @return Collection of follower IDs
 123      */
 124     public final Collection<String> getFollowerIds() {
 125         return followerToLog.keySet();
 126     }
 127
 128     public void addFollower(String followerId) {
 129         FollowerLogInformation followerLogInformation = new FollowerLogInformationImpl(
 130                 context.getPeerInfo(followerId), -1, context);
 131         followerToLog.put(followerId, followerLogInformation);
 132
 133         if(heartbeatSchedule == null) {
 134             scheduleHeartBeat(context.getConfigParams().getHeartBeatInterval());
 135         }
 136     }
 137
 138     public void removeFollower(String followerId) {
 139         followerToLog.remove(followerId);
 140         mapFollowerToSnapshot.remove(followerId);
 141     }
 142
 143     public void updateMinReplicaCount() {
 144         int numVoting = 0;
 145         for(PeerInfo peer: context.getPeers()) {
 146             if(peer.isVoting()) {
 147                 numVoting++;
 148             }
 149         }
 150
 151         minReplicationCount = getMajorityVoteCount(numVoting);
 152     }
 153
 154     protected int getMinIsolatedLeaderPeerCount(){
 155       //the isolated Leader peer count will be 1 less than the majority vote count.
 156         //this is because the vote count has the self vote counted in it
 157         //for e.g
 158         //0 peers = 1 votesRequired , minIsolatedLeaderPeerCount = 0
 159         //2 peers = 2 votesRequired , minIsolatedLeaderPeerCount = 1
 160         //4 peers = 3 votesRequired, minIsolatedLeaderPeerCount = 2
 161
 162         return minReplicationCount > 0 ? (minReplicationCount - 1) : 0;
 163     }
 164
 165     @VisibleForTesting
 166     void setSnapshot(@Nullable Snapshot snapshot) {
 167         if(snapshot != null) {
 168             this.snapshot = Optional.of(new SnapshotHolder(snapshot));
 169         } else {
 170             this.snapshot = Optional.absent();
 171         }
 172     }
 173
 174     @Override
 175     protected RaftActorBehavior handleAppendEntries(ActorRef sender,
 176         AppendEntries appendEntries) {
 177
 178         LOG.debug("{}: handleAppendEntries: {}", logName(), appendEntries);
 179
 180         return this;
 181     }
 182
 183     @Override
 184     protected RaftActorBehavior handleAppendEntriesReply(ActorRef sender,
 185         AppendEntriesReply appendEntriesReply) {
 186
 187         if(LOG.isTraceEnabled()) {
 188             LOG.trace("{}: handleAppendEntriesReply: {}", logName(), appendEntriesReply);
 189         }
 190
 191         // Update the FollowerLogInformation
 192         String followerId = appendEntriesReply.getFollowerId();
 193         FollowerLogInformation followerLogInformation =
 194             followerToLog.get(followerId);
 195
 196         if(followerLogInformation == null){
 197             LOG.error("{}: handleAppendEntriesReply - unknown follower {}", logName(), followerId);
 198             return this;
 199         }
 200
 201         if(followerLogInformation.timeSinceLastActivity() >
 202                 context.getConfigParams().getElectionTimeOutInterval().toMillis()) {
 203             LOG.warn("{} : handleAppendEntriesReply delayed beyond election timeout, " +
 204                             "appendEntriesReply : {}, timeSinceLastActivity : {}, lastApplied : {}, commitIndex : {}",
 205                     logName(), appendEntriesReply, followerLogInformation.timeSinceLastActivity(),
 206                     context.getLastApplied(), context.getCommitIndex());
 207         }
 208
 209         followerLogInformation.markFollowerActive();
 210         followerLogInformation.setPayloadVersion(appendEntriesReply.getPayloadVersion());
 211         followerLogInformation.setRaftVersion(appendEntriesReply.getRaftVersion());
 212
 213         boolean updated = false;
 214         if (appendEntriesReply.isSuccess()) {
 215             updated = updateFollowerLogInformation(followerLogInformation, appendEntriesReply);
 216         } else {
 217             LOG.debug("{}: handleAppendEntriesReply: received unsuccessful reply: {}", logName(), appendEntriesReply);
 218
 219             long followerLastLogIndex = appendEntriesReply.getLogLastIndex();
 220             ReplicatedLogEntry followersLastLogEntry = context.getReplicatedLog().get(followerLastLogIndex);
 221             if(appendEntriesReply.isForceInstallSnapshot()) {
 222                 // Reset the followers match and next index. This is to signal that this follower has nothing
 223                 // in common with this Leader and so would require a snapshot to be installed
 224                 followerLogInformation.setMatchIndex(-1);
 225                 followerLogInformation.setNextIndex(-1);
 226
 227                 // Force initiate a snapshot capture
 228                 initiateCaptureSnapshot(followerId);
 229             } else if(followerLastLogIndex < 0 || (followersLastLogEntry != null &&
 230                     followersLastLogEntry.getTerm() == appendEntriesReply.getLogLastTerm())) {
 231                 // The follower's log is empty or the last entry is present in the leader's journal
 232                 // and the terms match so the follower is just behind the leader's journal from
 233                 // the last snapshot, if any. We'll catch up the follower quickly by starting at the
 234                 // follower's last log index.
 235
 236                 updated = updateFollowerLogInformation(followerLogInformation, appendEntriesReply);
 237             } else {
 238                 // TODO: When we find that the follower is out of sync with the
 239                 // Leader we simply decrement that followers next index by 1.
 240                 // Would it be possible to do better than this? The RAFT spec
 241                 // does not explicitly deal with it but may be something for us to
 242                 // think about.
 243
 244                 followerLogInformation.decrNextIndex();
 245             }
 246         }
 247
 248         // Now figure out if this reply warrants a change in the commitIndex
 249         // If there exists an N such that N > commitIndex, a majority
 250         // of matchIndex[i] ≥ N, and log[N].term == currentTerm:
 251         // set commitIndex = N (§5.3, §5.4).
 252         for (long N = context.getCommitIndex() + 1; ; N++) {
 253             int replicatedCount = 1;
 254
 255             for (FollowerLogInformation info : followerToLog.values()) {
 256                 final PeerInfo peerInfo = context.getPeerInfo(info.getId());
 257                 if(info.getMatchIndex() >= N && (peerInfo != null && peerInfo.isVoting())) {
 258                     replicatedCount++;
 259                 }
 260             }
 261
 262             if (replicatedCount >= minReplicationCount) {
 263                 ReplicatedLogEntry replicatedLogEntry = context.getReplicatedLog().get(N);
 264                 if (replicatedLogEntry == null) {
 265                     break;
 266                 }
 267
 268                 // Don't update the commit index if the log entry is from a previous term, as per §5.4.1:
 269                 // "Raft never commits log entries from previous terms by counting replicas".
 270                 // However we keep looping so we can make progress when new entries in the current term
 271                 // reach consensus, as per §5.4.1: "once an entry from the current term is committed by
 272                 // counting replicas, then all prior entries are committed indirectly".
 273                 if (replicatedLogEntry.getTerm() == currentTerm()) {
 274                     context.setCommitIndex(N);
 275                 }
 276             } else {
 277                 break;
 278             }
 279         }
 280
 281         // Apply the change to the state machine
 282         if (context.getCommitIndex() > context.getLastApplied()) {
 283             if(LOG.isDebugEnabled()) {
 284                 LOG.debug("{}: handleAppendEntriesReply from {}: applying to log - commitIndex: {}, lastAppliedIndex: {}",
 285                         logName(), followerId, context.getCommitIndex(), context.getLastApplied());
 286             }
 287
 288             applyLogToStateMachine(context.getCommitIndex());
 289         }
 290
 291         if (!context.getSnapshotManager().isCapturing()) {
 292             purgeInMemoryLog();
 293         }
 294
 295         //Send the next log entry immediately, if possible, no need to wait for heartbeat to trigger that event
 296         sendUpdatesToFollower(followerId, followerLogInformation, false, !updated);
 297         return this;
 298     }
 299
 300     private boolean updateFollowerLogInformation(FollowerLogInformation followerLogInformation,
 301             AppendEntriesReply appendEntriesReply) {
 302         boolean updated = followerLogInformation.setMatchIndex(appendEntriesReply.getLogLastIndex());
 303         updated = followerLogInformation.setNextIndex(appendEntriesReply.getLogLastIndex() + 1) || updated;
 304
 305         if(updated && LOG.isDebugEnabled()) {
 306             LOG.debug("{}: handleAppendEntriesReply - FollowerLogInformation for {} updated: matchIndex: {}, nextIndex: {}",
 307                     logName(), followerLogInformation.getId(), followerLogInformation.getMatchIndex(),
 308                     followerLogInformation.getNextIndex());
 309         }
 310         return updated;
 311     }
 312
 313     private void purgeInMemoryLog() {
 314         //find the lowest index across followers which has been replicated to all.
 315         // lastApplied if there are no followers, so that we keep clearing the log for single-node
 316         // we would delete the in-mem log from that index on, in-order to minimize mem usage
 317         // we would also share this info thru AE with the followers so that they can delete their log entries as well.
 318         long minReplicatedToAllIndex = followerToLog.isEmpty() ? context.getLastApplied() : Long.MAX_VALUE;
 319         for (FollowerLogInformation info : followerToLog.values()) {
 320             minReplicatedToAllIndex = Math.min(minReplicatedToAllIndex, info.getMatchIndex());
 321         }
 322
 323         super.performSnapshotWithoutCapture(minReplicatedToAllIndex);
 324     }
 325
 326     @Override
 327     protected ClientRequestTracker removeClientRequestTracker(long logIndex) {
 328         final Iterator<ClientRequestTracker> it = trackerList.iterator();
 329         while (it.hasNext()) {
 330             final ClientRequestTracker t = it.next();
 331             if (t.getIndex() == logIndex) {
 332                 it.remove();
 333                 return t;
 334             }
 335         }
 336
 337         return null;
 338     }
 339
 340     @Override
 341     protected ClientRequestTracker findClientRequestTracker(long logIndex) {
 342         for (ClientRequestTracker tracker : trackerList) {
 343             if (tracker.getIndex() == logIndex) {
 344                 return tracker;
 345             }
 346         }
 347         return null;
 348     }
 349
 350     @Override
 351     protected RaftActorBehavior handleRequestVoteReply(ActorRef sender,
 352         RequestVoteReply requestVoteReply) {
 353         return this;
 354     }
 355
 356     protected void beforeSendHeartbeat(){}
 357
 358     @Override
 359     public RaftActorBehavior handleMessage(ActorRef sender, Object originalMessage) {
 360         Preconditions.checkNotNull(sender, "sender should not be null");
 361
 362         Object message = fromSerializableMessage(originalMessage);
 363
 364         if (message instanceof RaftRPC) {
 365             RaftRPC rpc = (RaftRPC) message;
 366             // If RPC request or response contains term T > currentTerm:
 367             // set currentTerm = T, convert to follower (§5.1)
 368             // This applies to all RPC messages and responses
 369             if (rpc.getTerm() > context.getTermInformation().getCurrentTerm()) {
 370                 LOG.debug("{}: Term {} in \"{}\" message is greater than leader's term {} - switching to Follower",
 371                         logName(), rpc.getTerm(), rpc, context.getTermInformation().getCurrentTerm());
 372
 373                 context.getTermInformation().updateAndPersist(rpc.getTerm(), null);
 374
 375                 return internalSwitchBehavior(RaftState.Follower);
 376             }
 377         }
 378
 379         if (message instanceof SendHeartBeat) {
 380             beforeSendHeartbeat();
 381             sendHeartBeat();
 382             scheduleHeartBeat(context.getConfigParams().getHeartBeatInterval());
 383         } else if(message instanceof SendInstallSnapshot) {
 384             // received from RaftActor
 385             setSnapshot(((SendInstallSnapshot) message).getSnapshot());
 386             sendInstallSnapshot();
 387         } else if (message instanceof Replicate) {
 388             replicate((Replicate) message);
 389         } else if (message instanceof InstallSnapshotReply) {
 390             handleInstallSnapshotReply((InstallSnapshotReply) message);
 391         } else {
 392             return super.handleMessage(sender, message);
 393         }
 394
 395         return this;
 396     }
 397
 398     private void handleInstallSnapshotReply(InstallSnapshotReply reply) {
 399         LOG.debug("{}: handleInstallSnapshotReply: {}", logName(), reply);
 400
 401         String followerId = reply.getFollowerId();
 402         FollowerToSnapshot followerToSnapshot = mapFollowerToSnapshot.get(followerId);
 403
 404         if (followerToSnapshot == null) {
 405             LOG.error("{}: FollowerToSnapshot not found for follower {} in InstallSnapshotReply",
 406                     logName(), followerId);
 407             return;
 408         }
 409
 410         FollowerLogInformation followerLogInformation = followerToLog.get(followerId);
 411         if(followerLogInformation == null) {
 412             // This can happen during AddServer if it times out.
 413             LOG.error("{}: FollowerLogInformation not found for follower {} in InstallSnapshotReply",
 414                     logName(), followerId);
 415             mapFollowerToSnapshot.remove(followerId);
 416             return;
 417         }
 418
 419         followerLogInformation.markFollowerActive();
 420
 421         if (followerToSnapshot.getChunkIndex() == reply.getChunkIndex()) {
 422             boolean wasLastChunk = false;
 423             if (reply.isSuccess()) {
 424                 if(followerToSnapshot.isLastChunk(reply.getChunkIndex())) {
 425                     //this was the last chunk reply
 426                     if(LOG.isDebugEnabled()) {
 427                         LOG.debug("{}: InstallSnapshotReply received, " +
 428                                 "last chunk received, Chunk: {}. Follower: {} Setting nextIndex: {}",
 429                                 logName(), reply.getChunkIndex(), followerId,
 430                             context.getReplicatedLog().getSnapshotIndex() + 1
 431                         );
 432                     }
 433
 434                     long followerMatchIndex = snapshot.get().getLastIncludedIndex();
 435                     followerLogInformation.setMatchIndex(followerMatchIndex);
 436                     followerLogInformation.setNextIndex(followerMatchIndex + 1);
 437                     mapFollowerToSnapshot.remove(followerId);
 438
 439                     LOG.debug("{}: follower: {}, matchIndex set to {}, nextIndex set to {}",
 440                         logName(), followerId, followerLogInformation.getMatchIndex(),
 441                         followerLogInformation.getNextIndex());
 442
 443                     if (mapFollowerToSnapshot.isEmpty()) {
 444                         // once there are no pending followers receiving snapshots
 445                         // we can remove snapshot from the memory
 446                         setSnapshot(null);
 447                     }
 448                     wasLastChunk = true;
 449                     if(context.getPeerInfo(followerId).getVotingState() == VotingState.VOTING_NOT_INITIALIZED){
 450                         UnInitializedFollowerSnapshotReply unInitFollowerSnapshotSuccess =
 451                                              new UnInitializedFollowerSnapshotReply(followerId);
 452                         context.getActor().tell(unInitFollowerSnapshotSuccess, context.getActor());
 453                         LOG.debug("Sent message UnInitializedFollowerSnapshotReply to self");
 454                     }
 455                 } else {
 456                     followerToSnapshot.markSendStatus(true);
 457                 }
 458             } else {
 459                 LOG.info("{}: InstallSnapshotReply received sending snapshot chunk failed, Will retry, Chunk: {}",
 460                         logName(), reply.getChunkIndex());
 461
 462                 followerToSnapshot.markSendStatus(false);
 463             }
 464
 465             if (wasLastChunk && !context.getSnapshotManager().isCapturing()) {
 466                 // Since the follower is now caught up try to purge the log.
 467                 purgeInMemoryLog();
 468             } else if (!wasLastChunk && followerToSnapshot.canSendNextChunk()) {
 469                 ActorSelection followerActor = context.getPeerActorSelection(followerId);
 470                 if(followerActor != null) {
 471                     sendSnapshotChunk(followerActor, followerId);
 472                 }
 473             }
 474
 475         } else {
 476             LOG.error("{}: Chunk index {} in InstallSnapshotReply from follower {} does not match expected index {}",
 477                     logName(), reply.getChunkIndex(), followerId,
 478                     followerToSnapshot.getChunkIndex());
 479
 480             if(reply.getChunkIndex() == INVALID_CHUNK_INDEX){
 481                 // Since the Follower did not find this index to be valid we should reset the follower snapshot
 482                 // so that Installing the snapshot can resume from the beginning
 483                 followerToSnapshot.reset();
 484             }
 485         }
 486     }
 487
 488     private void replicate(Replicate replicate) {
 489         long logIndex = replicate.getReplicatedLogEntry().getIndex();
 490
 491         LOG.debug("{}: Replicate message: identifier: {}, logIndex: {}", logName(),
 492                 replicate.getIdentifier(), logIndex);
 493
 494         // Create a tracker entry we will use this later to notify the
 495         // client actor
 496         trackerList.add(
 497             new ClientRequestTrackerImpl(replicate.getClientActor(),
 498                 replicate.getIdentifier(),
 499                 logIndex)
 500         );
 501
 502         boolean applyModificationToState = followerToLog.isEmpty()
 503                 || context.getRaftPolicy().applyModificationToStateBeforeConsensus();
 504
 505         if(applyModificationToState){
 506             context.setCommitIndex(logIndex);
 507             applyLogToStateMachine(logIndex);
 508         }
 509
 510         if (!followerToLog.isEmpty()) {
 511             sendAppendEntries(0, false);
 512         }
 513     }
 514
 515     protected void sendAppendEntries(long timeSinceLastActivityInterval, boolean isHeartbeat) {
 516         // Send an AppendEntries to all followers
 517         for (Entry<String, FollowerLogInformation> e : followerToLog.entrySet()) {
 518             final String followerId = e.getKey();
 519             final FollowerLogInformation followerLogInformation = e.getValue();
 520             // This checks helps not to send a repeat message to the follower
 521             if(!followerLogInformation.isFollowerActive() ||
 522                     followerLogInformation.timeSinceLastActivity() >= timeSinceLastActivityInterval) {
 523                 sendUpdatesToFollower(followerId, followerLogInformation, true, isHeartbeat);
 524             }
 525         }
 526     }
 527
 528     /**
 529      *
 530      * This method checks if any update needs to be sent to the given follower. This includes append log entries,
 531      * sending next snapshot chunk, and initiating a snapshot.
 532      * @return true if any update is sent, false otherwise
 533      */
 534
 535     private void sendUpdatesToFollower(String followerId, FollowerLogInformation followerLogInformation,
 536                                        boolean sendHeartbeat, boolean isHeartbeat) {
 537
 538         ActorSelection followerActor = context.getPeerActorSelection(followerId);
 539         if (followerActor != null) {
 540             long followerNextIndex = followerLogInformation.getNextIndex();
 541             boolean isFollowerActive = followerLogInformation.isFollowerActive();
 542             boolean sendAppendEntries = false;
 543             List<ReplicatedLogEntry> entries = Collections.emptyList();
 544
 545             if (mapFollowerToSnapshot.get(followerId) != null) {
 546                 // if install snapshot is in process , then sent next chunk if possible
 547                 if (isFollowerActive && mapFollowerToSnapshot.get(followerId).canSendNextChunk()) {
 548                     sendSnapshotChunk(followerActor, followerId);
 549                 } else if(sendHeartbeat) {
 550                     // we send a heartbeat even if we have not received a reply for the last chunk
 551                     sendAppendEntries = true;
 552                 }
 553             } else {
 554                 long leaderLastIndex = context.getReplicatedLog().lastIndex();
 555                 long leaderSnapShotIndex = context.getReplicatedLog().getSnapshotIndex();
 556
 557                 if((!isHeartbeat && LOG.isDebugEnabled()) || LOG.isTraceEnabled()) {
 558                     LOG.debug("{}: Checking sendAppendEntries for follower {}: active: {}, followerNextIndex: {}, leaderLastIndex: {}, leaderSnapShotIndex: {}",
 559                             logName(), followerId, isFollowerActive, followerNextIndex, leaderLastIndex, leaderSnapShotIndex);
 560                 }
 561
 562                 if (isFollowerActive && context.getReplicatedLog().isPresent(followerNextIndex)) {
 563
 564                     LOG.debug("{}: sendAppendEntries: {} is present for follower {}", logName(),
 565                             followerNextIndex, followerId);
 566
 567                     if(followerLogInformation.okToReplicate()) {
 568                         // Try to send all the entries in the journal but not exceeding the max data size
 569                         // for a single AppendEntries message.
 570                         int maxEntries = (int) context.getReplicatedLog().size();
 571                         entries = context.getReplicatedLog().getFrom(followerNextIndex, maxEntries,
 572                                 context.getConfigParams().getSnapshotChunkSize());
 573                         sendAppendEntries = true;
 574                     }
 575                 } else if (isFollowerActive && followerNextIndex >= 0 &&
 576                     leaderLastIndex > followerNextIndex && !context.getSnapshotManager().isCapturing()) {
 577                     // if the followers next index is not present in the leaders log, and
 578                     // if the follower is just not starting and if leader's index is more than followers index
 579                     // then snapshot should be sent
 580
 581                     if (LOG.isDebugEnabled()) {
 582                         LOG.debug(String.format("%s: InitiateInstallSnapshot to follower: %s," +
 583                                     "follower-nextIndex: %d, leader-snapshot-index: %d,  " +
 584                                     "leader-last-index: %d", logName(), followerId,
 585                                     followerNextIndex, leaderSnapShotIndex, leaderLastIndex));
 586                     }
 587
 588                     // Send heartbeat to follower whenever install snapshot is initiated.
 589                     sendAppendEntries = true;
 590                     if (canInstallSnapshot(followerNextIndex)) {
 591                         initiateCaptureSnapshot(followerId);
 592                     }
 593
 594                 } else if(sendHeartbeat) {
 595                     // we send an AppendEntries, even if the follower is inactive
 596                     // in-order to update the followers timestamp, in case it becomes active again
 597                     sendAppendEntries = true;
 598                 }
 599
 600             }
 601
 602             if(sendAppendEntries) {
 603                 sendAppendEntriesToFollower(followerActor, followerNextIndex,
 604                         entries, followerId);
 605             }
 606         }
 607     }
 608
 609     private void sendAppendEntriesToFollower(ActorSelection followerActor, long followerNextIndex,
 610         List<ReplicatedLogEntry> entries, String followerId) {
 611         AppendEntries appendEntries = new AppendEntries(currentTerm(), context.getId(),
 612             prevLogIndex(followerNextIndex),
 613             prevLogTerm(followerNextIndex), entries,
 614             context.getCommitIndex(), super.getReplicatedToAllIndex(), context.getPayloadVersion());
 615
 616         if(!entries.isEmpty() || LOG.isTraceEnabled()) {
 617             LOG.debug("{}: Sending AppendEntries to follower {}: {}", logName(), followerId,
 618                     appendEntries);
 619         }
 620
 621         followerActor.tell(appendEntries, actor());
 622     }
 623
 624     /**
 625      * Install Snapshot works as follows
 626      * 1. Leader initiates the capture snapshot by sending a CaptureSnapshot message to actor
 627      * 2. RaftActor on receipt of the CaptureSnapshotReply (from Shard), stores the received snapshot in the replicated log
 628      * and makes a call to Leader's handleMessage , with SendInstallSnapshot message.
 629      * 3. Leader , picks the snapshot from im-mem ReplicatedLog and sends it in chunks to the Follower
 630      * 4. On complete, Follower sends back a InstallSnapshotReply.
 631      * 5. On receipt of the InstallSnapshotReply for the last chunk, Leader marks the install complete for that follower
 632      * and replenishes the memory by deleting the snapshot in Replicated log.
 633      * 6. If another follower requires a snapshot and a snapshot has been collected (via CaptureSnapshotReply)
 634      * then send the existing snapshot in chunks to the follower.
 635      * @param followerId
 636      */
 637     public boolean initiateCaptureSnapshot(String followerId) {
 638         if (snapshot.isPresent()) {
 639             // if a snapshot is present in the memory, most likely another install is in progress
 640             // no need to capture snapshot.
 641             // This could happen if another follower needs an install when one is going on.
 642             final ActorSelection followerActor = context.getPeerActorSelection(followerId);
 643             sendSnapshotChunk(followerActor, followerId);
 644             return true;
 645         } else {
 646             return context.getSnapshotManager().captureToInstall(context.getReplicatedLog().last(),
 647                     this.getReplicatedToAllIndex(), followerId);
 648         }
 649     }
 650
 651     private boolean canInstallSnapshot(long nextIndex){
 652         // If the follower's nextIndex is -1 then we might as well send it a snapshot
 653         // Otherwise send it a snapshot only if the nextIndex is not present in the log but is present
 654         // in the snapshot
 655         return (nextIndex == -1 ||
 656                 (!context.getReplicatedLog().isPresent(nextIndex)
 657                         && context.getReplicatedLog().isInSnapshot(nextIndex)));
 658
 659     }
 660
 661
 662     private void sendInstallSnapshot() {
 663         LOG.debug("{}: sendInstallSnapshot", logName());
 664         for (Entry<String, FollowerLogInformation> e : followerToLog.entrySet()) {
 665             String followerId = e.getKey();
 666             ActorSelection followerActor = context.getPeerActorSelection(followerId);
 667             FollowerLogInformation followerLogInfo = e.getValue();
 668
 669             if (followerActor != null) {
 670                 long nextIndex = followerLogInfo.getNextIndex();
 671                 if (context.getPeerInfo(followerId).getVotingState() == VotingState.VOTING_NOT_INITIALIZED ||
 672                         canInstallSnapshot(nextIndex)) {
 673                     sendSnapshotChunk(followerActor, followerId);
 674                 }
 675             }
 676         }
 677     }
 678
 679     /**
 680      *  Sends a snapshot chunk to a given follower
 681      *  InstallSnapshot should qualify as a heartbeat too.
 682      */
 683     private void sendSnapshotChunk(ActorSelection followerActor, String followerId) {
 684         try {
 685             if (snapshot.isPresent()) {
 686                 byte[] nextSnapshotChunk = getNextSnapshotChunk(followerId, snapshot.get().getSnapshotBytes());
 687
 688                 // Note: the previous call to getNextSnapshotChunk has the side-effect of adding
 689                 // followerId to the followerToSnapshot map.
 690                 FollowerToSnapshot followerToSnapshot = mapFollowerToSnapshot.get(followerId);
 691
 692                 followerActor.tell(
 693                     new InstallSnapshot(currentTerm(), context.getId(),
 694                         snapshot.get().getLastIncludedIndex(),
 695                         snapshot.get().getLastIncludedTerm(),
 696                         nextSnapshotChunk,
 697                         followerToSnapshot.incrementChunkIndex(),
 698                         followerToSnapshot.getTotalChunks(),
 699                         Optional.of(followerToSnapshot.getLastChunkHashCode())
 700                     ).toSerializable(followerToLog.get(followerId).getRaftVersion()),
 701                     actor()
 702                 );
 703
 704                 if(LOG.isDebugEnabled()) {
 705                     LOG.debug("{}: InstallSnapshot sent to follower {}, Chunk: {}/{}",
 706                             logName(), followerActor.path(), followerToSnapshot.getChunkIndex(),
 707                             followerToSnapshot.getTotalChunks());
 708                 }
 709             }
 710         } catch (IOException e) {
 711             LOG.error("{}: InstallSnapshot failed for Leader.", logName(), e);
 712         }
 713     }
 714
 715     /**
 716      * Acccepts snaphot as ByteString, enters into map for future chunks
 717      * creates and return a ByteString chunk
 718      */
 719     private byte[] getNextSnapshotChunk(String followerId, ByteString snapshotBytes) throws IOException {
 720         FollowerToSnapshot followerToSnapshot = mapFollowerToSnapshot.get(followerId);
 721         if (followerToSnapshot == null) {
 722             followerToSnapshot = new FollowerToSnapshot(snapshotBytes);
 723             mapFollowerToSnapshot.put(followerId, followerToSnapshot);
 724         }
 725         byte[] nextChunk = followerToSnapshot.getNextChunk();
 726
 727         LOG.debug("{}: next snapshot chunk size for follower {}: {}", logName(), followerId, nextChunk.length);
 728
 729         return nextChunk;
 730     }
 731
 732     private void sendHeartBeat() {
 733         if (!followerToLog.isEmpty()) {
 734             LOG.trace("{}: Sending heartbeat", logName());
 735             sendAppendEntries(context.getConfigParams().getHeartBeatInterval().toMillis(), true);
 736         }
 737     }
 738
 739     private void stopHeartBeat() {
 740         if (heartbeatSchedule != null && !heartbeatSchedule.isCancelled()) {
 741             heartbeatSchedule.cancel();
 742         }
 743     }
 744
 745     private void scheduleHeartBeat(FiniteDuration interval) {
 746         if (followerToLog.isEmpty()) {
 747             // Optimization - do not bother scheduling a heartbeat as there are
 748             // no followers
 749             return;
 750         }
 751
 752         stopHeartBeat();
 753
 754         // Schedule a heartbeat. When the scheduler triggers a SendHeartbeat
 755         // message is sent to itself.
 756         // Scheduling the heartbeat only once here because heartbeats do not
 757         // need to be sent if there are other messages being sent to the remote
 758         // actor.
 759         heartbeatSchedule = context.getActorSystem().scheduler().scheduleOnce(
 760             interval, context.getActor(), SendHeartBeat.INSTANCE,
 761             context.getActorSystem().dispatcher(), context.getActor());
 762     }
 763
 764     @Override
 765     public void close() {
 766         stopHeartBeat();
 767     }
 768
 769     @Override
 770     public final String getLeaderId() {
 771         return context.getId();
 772     }
 773
 774     @Override
 775     public final short getLeaderPayloadVersion() {
 776         return context.getPayloadVersion();
 777     }
 778
 779     protected boolean isLeaderIsolated() {
 780         int minPresent = getMinIsolatedLeaderPeerCount();
 781         for (FollowerLogInformation followerLogInformation : followerToLog.values()) {
 782             if (followerLogInformation.isFollowerActive()) {
 783                 --minPresent;
 784                 if (minPresent == 0) {
 785                     break;
 786                 }
 787             }
 788         }
 789         return (minPresent != 0);
 790     }
 791
 792     /**
 793      * Encapsulates the snapshot bytestring and handles the logic of sending
 794      * snapshot chunks
 795      */
 796     protected class FollowerToSnapshot {
 797         private final ByteString snapshotBytes;
 798         private int offset = 0;
 799         // the next snapshot chunk is sent only if the replyReceivedForOffset matches offset
 800         private int replyReceivedForOffset;
 801         // if replyStatus is false, the previous chunk is attempted
 802         private boolean replyStatus = false;
 803         private int chunkIndex;
 804         private final int totalChunks;
 805         private int lastChunkHashCode = AbstractLeader.INITIAL_LAST_CHUNK_HASH_CODE;
 806         private int nextChunkHashCode = AbstractLeader.INITIAL_LAST_CHUNK_HASH_CODE;
 807
 808         public FollowerToSnapshot(ByteString snapshotBytes) {
 809             this.snapshotBytes = snapshotBytes;
 810             int size = snapshotBytes.size();
 811             totalChunks = ( size / context.getConfigParams().getSnapshotChunkSize()) +
 812                 ((size % context.getConfigParams().getSnapshotChunkSize()) > 0 ? 1 : 0);
 813             if(LOG.isDebugEnabled()) {
 814                 LOG.debug("{}: Snapshot {} bytes, total chunks to send:{}",
 815                         logName(), size, totalChunks);
 816             }
 817             replyReceivedForOffset = -1;
 818             chunkIndex = AbstractLeader.FIRST_CHUNK_INDEX;
 819         }
 820
 821         public ByteString getSnapshotBytes() {
 822             return snapshotBytes;
 823         }
 824
 825         public int incrementOffset() {
 826             if(replyStatus) {
 827                 // if prev chunk failed, we would want to sent the same chunk again
 828                 offset = offset + context.getConfigParams().getSnapshotChunkSize();
 829             }
 830             return offset;
 831         }
 832
 833         public int incrementChunkIndex() {
 834             if (replyStatus) {
 835                 // if prev chunk failed, we would want to sent the same chunk again
 836                 chunkIndex =  chunkIndex + 1;
 837             }
 838             return chunkIndex;
 839         }
 840
 841         public int getChunkIndex() {
 842             return chunkIndex;
 843         }
 844
 845         public int getTotalChunks() {
 846             return totalChunks;
 847         }
 848
 849         public boolean canSendNextChunk() {
 850             // we only send a false if a chunk is sent but we have not received a reply yet
 851             return replyReceivedForOffset == offset;
 852         }
 853
 854         public boolean isLastChunk(int chunkIndex) {
 855             return totalChunks == chunkIndex;
 856         }
 857
 858         public void markSendStatus(boolean success) {
 859             if (success) {
 860                 // if the chunk sent was successful
 861                 replyReceivedForOffset = offset;
 862                 replyStatus = true;
 863                 lastChunkHashCode = nextChunkHashCode;
 864             } else {
 865                 // if the chunk sent was failure
 866                 replyReceivedForOffset = offset;
 867                 replyStatus = false;
 868             }
 869         }
 870
 871         public byte[] getNextChunk() {
 872             int snapshotLength = getSnapshotBytes().size();
 873             int start = incrementOffset();
 874             int size = context.getConfigParams().getSnapshotChunkSize();
 875             if (context.getConfigParams().getSnapshotChunkSize() > snapshotLength) {
 876                 size = snapshotLength;
 877             } else if ((start + context.getConfigParams().getSnapshotChunkSize()) > snapshotLength) {
 878                 size = snapshotLength - start;
 879             }
 880
 881             byte[] nextChunk = new byte[size];
 882             getSnapshotBytes().copyTo(nextChunk, start, 0, size);
 883             nextChunkHashCode = Arrays.hashCode(nextChunk);
 884
 885             LOG.debug("{}: Next chunk: total length={}, offset={}, size={}, hashCode={}", logName(),
 886                     snapshotLength, start, size, nextChunkHashCode);
 887             return nextChunk;
 888         }
 889
 890         /**
 891          * reset should be called when the Follower needs to be sent the snapshot from the beginning
 892          */
 893         public void reset(){
 894             offset = 0;
 895             replyStatus = false;
 896             replyReceivedForOffset = offset;
 897             chunkIndex = AbstractLeader.FIRST_CHUNK_INDEX;
 898             lastChunkHashCode = AbstractLeader.INITIAL_LAST_CHUNK_HASH_CODE;
 899         }
 900
 901         public int getLastChunkHashCode() {
 902             return lastChunkHashCode;
 903         }
 904     }
 905
 906     // called from example-actor for printing the follower-states
 907     public String printFollowerStates() {
 908         final StringBuilder sb = new StringBuilder();
 909
 910         sb.append('[');
 911         for (FollowerLogInformation followerLogInformation : followerToLog.values()) {
 912             sb.append('{');
 913             sb.append(followerLogInformation.getId());
 914             sb.append(" state:");
 915             sb.append(followerLogInformation.isFollowerActive());
 916             sb.append("},");
 917         }
 918         sb.append(']');
 919
 920         return sb.toString();
 921     }
 922
 923     @VisibleForTesting
 924     public FollowerLogInformation getFollower(String followerId) {
 925         return followerToLog.get(followerId);
 926     }
 927
 928     @VisibleForTesting
 929     protected void setFollowerSnapshot(String followerId, FollowerToSnapshot snapshot) {
 930         mapFollowerToSnapshot.put(followerId, snapshot);
 931     }
 932
 933     @VisibleForTesting
 934     public int followerSnapshotSize() {
 935         return mapFollowerToSnapshot.size();
 936     }
 937
 938     @VisibleForTesting
 939     public int followerLogSize() {
 940         return followerToLog.size();
 941     }
 942
 943     private static class SnapshotHolder {
 944         private final long lastIncludedTerm;
 945         private final long lastIncludedIndex;
 946         private final ByteString snapshotBytes;
 947
 948         SnapshotHolder(Snapshot snapshot) {
 949             this.lastIncludedTerm = snapshot.getLastAppliedTerm();
 950             this.lastIncludedIndex = snapshot.getLastAppliedIndex();
 951             this.snapshotBytes = ByteString.copyFrom(snapshot.getState());
 952         }
 953
 954         long getLastIncludedTerm() {
 955             return lastIncludedTerm;
 956         }
 957
 958         long getLastIncludedIndex() {
 959             return lastIncludedIndex;
 960         }
 961
 962         ByteString getSnapshotBytes() {
 963             return snapshotBytes;
 964         }
 965     }
 966 }