order

This paper mainly studies the LagDetector of elastic search

LagDetector

elasticsearch-7.0.1/server/src/main/java/org/elasticsearch/cluster/coordination/LagDetector.java

/**
 * A publication can succeed and complete before all nodes have applied the published state and acknowledged it; however we need every node
 * eventually either to apply the published state (or a later state) or be removed from the cluster. This component achieves this by
 * removing any lagging nodes from the cluster after a timeout.
 */
public class LagDetector {

    private static final Logger logger = LogManager.getLogger(LagDetector.class);

    // the timeout for each node to apply a cluster state update after the leader has applied it, before being removed from the cluster
    public static final Setting<TimeValue> CLUSTER_FOLLOWER_LAG_TIMEOUT_SETTING =
        Setting.timeSetting("cluster.follower_lag.timeout",
            TimeValue.timeValueMillis(90000), TimeValue.timeValueMillis(1), Setting.Property.NodeScope);

    private final TimeValue clusterStateApplicationTimeout;
    private final Consumer<DiscoveryNode> onLagDetected;
    private final Supplier<DiscoveryNode> localNodeSupplier;
    private final ThreadPool threadPool;
    private final Map<DiscoveryNode, NodeAppliedStateTracker> appliedStateTrackersByNode = newConcurrentMap();

    public LagDetector(final Settings settings, final ThreadPool threadPool, final Consumer<DiscoveryNode> onLagDetected,
                       final Supplier<DiscoveryNode> localNodeSupplier) {
        this.threadPool = threadPool;
        this.clusterStateApplicationTimeout = CLUSTER_FOLLOWER_LAG_TIMEOUT_SETTING.get(settings);
        this.onLagDetected = onLagDetected;
        this.localNodeSupplier = localNodeSupplier;
    }

    public void setTrackedNodes(final Iterable<DiscoveryNode> discoveryNodes) {
        final Set<DiscoveryNode> discoveryNodeSet = new HashSet<>();
        discoveryNodes.forEach(discoveryNodeSet::add);
        discoveryNodeSet.remove(localNodeSupplier.get());
        appliedStateTrackersByNode.keySet().retainAll(discoveryNodeSet);
        discoveryNodeSet.forEach(node -> appliedStateTrackersByNode.putIfAbsent(node, new NodeAppliedStateTracker(node)));
    }

    public void clearTrackedNodes() {
        appliedStateTrackersByNode.clear();
    }

    public void setAppliedVersion(final DiscoveryNode discoveryNode, final long appliedVersion) {
        final NodeAppliedStateTracker nodeAppliedStateTracker = appliedStateTrackersByNode.get(discoveryNode);
        if (nodeAppliedStateTracker == null) {
            // Received an ack from a node that a later publication has removed (or we are no longer master). No big deal.
            logger.trace("node {} applied version {} but this node's version is not being tracked", discoveryNode, appliedVersion);
        } else {
            nodeAppliedStateTracker.increaseAppliedVersion(appliedVersion);
        }
    }

    public void startLagDetector(final long version) {
        final List<NodeAppliedStateTracker> laggingTrackers
            = appliedStateTrackersByNode.values().stream().filter(t -> t.appliedVersionLessThan(version)).collect(Collectors.toList());

        if (laggingTrackers.isEmpty()) {
            logger.trace("lag detection for version {} is unnecessary: {}", version, appliedStateTrackersByNode.values());
        } else {
            logger.debug("starting lag detector for version {}: {}", version, laggingTrackers);

            threadPool.scheduleUnlessShuttingDown(clusterStateApplicationTimeout, Names.GENERIC, new Runnable() {
                @Override
                public void run() {
                    laggingTrackers.forEach(t -> t.checkForLag(version));
                }

                @Override
                public String toString() {
                    return "lag detector for version " + version + " on " + laggingTrackers;
                }
            });
        }
    }

    @Override
    public String toString() {
        return "LagDetector{" +
            "clusterStateApplicationTimeout=" + clusterStateApplicationTimeout +
            ", appliedStateTrackersByNode=" + appliedStateTrackersByNode.values() +
            '}';
    }

    // for assertions
    Set<DiscoveryNode> getTrackedNodes() {
        return Collections.unmodifiableSet(appliedStateTrackersByNode.keySet());
    }

    private class NodeAppliedStateTracker {
        private final DiscoveryNode discoveryNode;
        private final AtomicLong appliedVersion = new AtomicLong();

        NodeAppliedStateTracker(final DiscoveryNode discoveryNode) {
            this.discoveryNode = discoveryNode;
        }

        void increaseAppliedVersion(long appliedVersion) {
            long maxAppliedVersion = this.appliedVersion.updateAndGet(v -> Math.max(v, appliedVersion));
            logger.trace("{} applied version {}, max now {}", this, appliedVersion, maxAppliedVersion);
        }

        boolean appliedVersionLessThan(final long version) {
            return appliedVersion.get() < version;
        }

        @Override
        public String toString() {
            return "NodeAppliedStateTracker{" +
                "discoveryNode=" + discoveryNode +
                ", appliedVersion=" + appliedVersion +
                '}';
        }

        void checkForLag(final long version) {
            if (appliedStateTrackersByNode.get(discoveryNode) != this) {
                logger.trace("{} no longer active when checking version {}", this, version);
                return;
            }

            long appliedVersion = this.appliedVersion.get();
            if (version <= appliedVersion) {
                logger.trace("{} satisfied when checking version {}, node applied version {}", this, version, appliedVersion);
                return;
            }

            logger.debug("{}, detected lag at version {}, node has only applied version {}", this, version, appliedVersion);
            onLagDetected.accept(discoveryNode);
        }
    }
}

• The LagDetector is used to detect and remove the lagging nodes. Its constructor reads the configuration of cluster.follower'lag.timeout. The default value is 9000ms, and the minimum value is 1ms
• startLagDetector first obtains the laggingTrackers whose appliedVersion is less than the specified version from the appliedStateTrackersByNode, and then delays the clusterStateApplicationTimeout to perform the detection. Its run method will traverse laggingTrackers, one by one, the nodeappliedstatetracker.checkforag method
• The checkForLag method of NodeAppliedStateTracker first determines version and finally calls onLagDetected.accept(discoveryNode).

Coordinator

elasticsearch-7.0.1/server/src/main/java/org/elasticsearch/cluster/coordination/Coordinator.java

public class Coordinator extends AbstractLifecycleComponent implements Discovery {
	//......

    public Coordinator(String nodeName, Settings settings, ClusterSettings clusterSettings, TransportService transportService,
                       NamedWriteableRegistry namedWriteableRegistry, AllocationService allocationService, MasterService masterService,
                       Supplier<CoordinationState.PersistedState> persistedStateSupplier, SeedHostsProvider seedHostsProvider,
                       ClusterApplier clusterApplier, Collection<BiConsumer<DiscoveryNode, ClusterState>> onJoinValidators, Random random) {
        this.settings = settings;
        this.transportService = transportService;
        this.masterService = masterService;
        this.allocationService = allocationService;
        this.onJoinValidators = JoinTaskExecutor.addBuiltInJoinValidators(onJoinValidators);
        this.singleNodeDiscovery = DiscoveryModule.SINGLE_NODE_DISCOVERY_TYPE.equals(DiscoveryModule.DISCOVERY_TYPE_SETTING.get(settings));
        this.joinHelper = new JoinHelper(settings, allocationService, masterService, transportService,
            this::getCurrentTerm, this::getStateForMasterService, this::handleJoinRequest, this::joinLeaderInTerm, this.onJoinValidators);
        this.persistedStateSupplier = persistedStateSupplier;
        this.noMasterBlockService = new NoMasterBlockService(settings, clusterSettings);
        this.lastKnownLeader = Optional.empty();
        this.lastJoin = Optional.empty();
        this.joinAccumulator = new InitialJoinAccumulator();
        this.publishTimeout = PUBLISH_TIMEOUT_SETTING.get(settings);
        this.random = random;
        this.electionSchedulerFactory = new ElectionSchedulerFactory(settings, random, transportService.getThreadPool());
        this.preVoteCollector = new PreVoteCollector(transportService, this::startElection, this::updateMaxTermSeen);
        configuredHostsResolver = new SeedHostsResolver(nodeName, settings, transportService, seedHostsProvider);
        this.peerFinder = new CoordinatorPeerFinder(settings, transportService,
            new HandshakingTransportAddressConnector(settings, transportService), configuredHostsResolver);
        this.publicationHandler = new PublicationTransportHandler(transportService, namedWriteableRegistry,
            this::handlePublishRequest, this::handleApplyCommit);
        this.leaderChecker = new LeaderChecker(settings, transportService, getOnLeaderFailure());
        this.followersChecker = new FollowersChecker(settings, transportService, this::onFollowerCheckRequest, this::removeNode);
        this.nodeRemovalExecutor = new NodeRemovalClusterStateTaskExecutor(allocationService, logger);
        this.clusterApplier = clusterApplier;
        masterService.setClusterStateSupplier(this::getStateForMasterService);
        this.reconfigurator = new Reconfigurator(settings, clusterSettings);
        this.clusterBootstrapService = new ClusterBootstrapService(settings, transportService, this::getFoundPeers,
            this::isInitialConfigurationSet, this::setInitialConfiguration);
        this.discoveryUpgradeService = new DiscoveryUpgradeService(settings, transportService,
            this::isInitialConfigurationSet, joinHelper, peerFinder::getFoundPeers, this::setInitialConfiguration);
        this.lagDetector = new LagDetector(settings, transportService.getThreadPool(), n -> removeNode(n, "lagging"),
            transportService::getLocalNode);
        this.clusterFormationFailureHelper = new ClusterFormationFailureHelper(settings, this::getClusterFormationState,
            transportService.getThreadPool(), joinHelper::logLastFailedJoinAttempt);
    }

    private void removeNode(DiscoveryNode discoveryNode, String reason) {
        synchronized (mutex) {
            if (mode == Mode.LEADER) {
                masterService.submitStateUpdateTask("node-left",
                    new NodeRemovalClusterStateTaskExecutor.Task(discoveryNode, reason),
                    ClusterStateTaskConfig.build(Priority.IMMEDIATE),
                    nodeRemovalExecutor,
                    nodeRemovalExecutor);
            }
        }
    }

	//......
}

• The Coordinator's constructor creates a LagDetector, whose consumer < discoverynode > executes the removeNode method, which will execute NodeRemovalClusterStateTaskExecutor.Task when the current mode is LEADER

Summary

• The LagDetector is used to detect and remove the lagging nodes. Its constructor reads the configuration of cluster.follower'lag.timeout. The default value is 9000ms, and the minimum value is 1ms
• startLagDetector first obtains the laggingTrackers whose appliedVersion is less than the specified version from appliedStateTrackersByNode, and then delays the clusterStateApplicationTimeout to perform the detection. Its run method will traverse laggingTrackers, one by one, the NodeAppliedStateTracker.checkforag method. The checkforag method of NodeAppliedStateTracker first performs the version judgment, Finally, call onLagDetected.accept(discoveryNode).
• The Coordinator's constructor creates a LagDetector, whose consumer < discoverynode > executes the removeNode method, which will execute NodeRemovalClusterStateTaskExecutor.Task when the current mode is LEADER

doc

• LagDetector