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