/*** A module for loading classes for node discovery.*/

The discovery module is responsible for discovering nodes within a cluster, as well as electing a master node.Note, Elasticsearch is a peer to peer based system, nodes communicate with one another directly if operations are delegated / broadcast. All the main APIs (index, delete, search) do not communicate with the master node. The responsibility of the master node is to maintain the global cluster state, and act if nodes join or leave the cluster by reassigning shards. Each time a cluster state is changed, the state is made known to the other nodes in the cluster (the manner depends on the actual discovery implementation).

            final DiscoveryModule discoveryModule = new DiscoveryModule(this.settings, threadPool, transportService, namedWriteableRegistry,networkService, clusterService.getMasterService(), clusterService.getClusterApplierService(),clusterService.getClusterSettings(), pluginsService.filterPlugins(DiscoveryPlugin.class),clusterModule.getAllocationService(), environment.configFile());this.nodeService = new NodeService(settings, threadPool, monitorService, discoveryModule.getDiscovery(),transportService, indicesService, pluginsService, circuitBreakerService, scriptModule.getScriptService(),httpServerTransport, ingestService, clusterService, settingsModule.getSettingsFilter(), responseCollectorService,searchTransportService);

/*** A pluggable module allowing to implement discovery of other nodes, publishing of the cluster* state to all nodes, electing a master of the cluster that raises cluster state change* events.*/

    public SingleNodeDiscovery(final Settings settings, final TransportService transportService,final MasterService masterService, final ClusterApplier clusterApplier) {super(Objects.requireNonNull(settings));this.transportService = Objects.requireNonNull(transportService);masterService.setClusterStateSupplier(() -> clusterState);this.clusterApplier = clusterApplier;}

   @Overrideprotected synchronized void doStart() {// set initial stateDiscoveryNode localNode = transportService.getLocalNode();clusterState = createInitialState(localNode);clusterApplier.setInitialState(clusterState);}

 public ZenDiscovery(Settings settings, ThreadPool threadPool, TransportService transportService,NamedWriteableRegistry namedWriteableRegistry, MasterService masterService, ClusterApplier clusterApplier,ClusterSettings clusterSettings, UnicastHostsProvider hostsProvider, AllocationService allocationService,Collection<BiConsumer<DiscoveryNode, ClusterState>> onJoinValidators) {super(settings);this.onJoinValidators = addBuiltInJoinValidators(onJoinValidators);this.masterService = masterService;this.clusterApplier = clusterApplier;this.transportService = transportService;this.discoverySettings = new DiscoverySettings(settings, clusterSettings);this.zenPing = newZenPing(settings, threadPool, transportService, hostsProvider);this.electMaster = new ElectMasterService(settings);this.pingTimeout = PING_TIMEOUT_SETTING.get(settings);this.joinTimeout = JOIN_TIMEOUT_SETTING.get(settings);this.joinRetryAttempts = JOIN_RETRY_ATTEMPTS_SETTING.get(settings);this.joinRetryDelay = JOIN_RETRY_DELAY_SETTING.get(settings);this.maxPingsFromAnotherMaster = MAX_PINGS_FROM_ANOTHER_MASTER_SETTING.get(settings);this.sendLeaveRequest = SEND_LEAVE_REQUEST_SETTING.get(settings);this.threadPool = threadPool;ClusterName clusterName = ClusterName.CLUSTER_NAME_SETTING.get(settings);this.committedState = new AtomicReference<>();this.masterElectionIgnoreNonMasters = MASTER_ELECTION_IGNORE_NON_MASTER_PINGS_SETTING.get(settings);this.masterElectionWaitForJoinsTimeout = MASTER_ELECTION_WAIT_FOR_JOINS_TIMEOUT_SETTING.get(settings);logger.debug("using ping_timeout [{}], join.timeout [{}], master_election.ignore_non_master [{}]",this.pingTimeout, joinTimeout, masterElectionIgnoreNonMasters);clusterSettings.addSettingsUpdateConsumer(ElectMasterService.DISCOVERY_ZEN_MINIMUM_MASTER_NODES_SETTING,this::handleMinimumMasterNodesChanged, (value) -> {final ClusterState clusterState = this.clusterState();int masterNodes = clusterState.nodes().getMasterNodes().size();// the purpose of this validation is to make sure that the master doesn't step down// due to a change in master nodes, which also means that there is no way to revert// an accidental change. Since we validate using the current cluster state (and// not the one from which the settings come from) we have to be careful and only// validate if the local node is already a master. Doing so all the time causes// subtle issues. For example, a node that joins a cluster has no nodes in its// current cluster state. When it receives a cluster state from the master with// a dynamic minimum master nodes setting int it, we must make sure we don't reject// it.if (clusterState.nodes().isLocalNodeElectedMaster() && value > masterNodes) {throw new IllegalArgumentException("cannot set "+ ElectMasterService.DISCOVERY_ZEN_MINIMUM_MASTER_NODES_SETTING.getKey() + " to more than the current" +" master nodes count [" + masterNodes + "]");}});this.masterFD = new MasterFaultDetection(settings, threadPool, transportService, this::clusterState, masterService, clusterName);this.masterFD.addListener(new MasterNodeFailureListener());this.nodesFD = new NodesFaultDetection(settings, threadPool, transportService, this::clusterState, clusterName);this.nodesFD.addListener(new NodeFaultDetectionListener());this.pendingStatesQueue = new PendingClusterStatesQueue(logger, MAX_PENDING_CLUSTER_STATES_SETTING.get(settings));this.publishClusterState =new PublishClusterStateAction(settings,transportService,namedWriteableRegistry,this,discoverySettings);this.membership = new MembershipAction(settings, transportService, new MembershipListener(), onJoinValidators);this.joinThreadControl = new JoinThreadControl();this.nodeJoinController = new NodeJoinController(masterService, allocationService, electMaster, settings);this.nodeRemovalExecutor = new NodeRemovalClusterStateTaskExecutor(allocationService, electMaster, this::submitRejoin, logger);masterService.setClusterStateSupplier(this::clusterState);transportService.registerRequestHandler(DISCOVERY_REJOIN_ACTION_NAME, RejoinClusterRequest::new, ThreadPool.Names.SAME, new RejoinClusterRequestHandler());}

    @Overrideprotected void doStart() {DiscoveryNode localNode = transportService.getLocalNode();assert localNode != null;synchronized (stateMutex) {// set initial stateassert committedState.get() == null;assert localNode != null;ClusterState.Builder builder = ClusterState.builder(ClusterName.CLUSTER_NAME_SETTING.get(settings));ClusterState initialState = builder.blocks(ClusterBlocks.builder().addGlobalBlock(STATE_NOT_RECOVERED_BLOCK).addGlobalBlock(discoverySettings.getNoMasterBlock())).nodes(DiscoveryNodes.builder().add(localNode).localNodeId(localNode.getId())).build();committedState.set(initialState);clusterApplier.setInitialState(initialState);nodesFD.setLocalNode(localNode);joinThreadControl.start();}zenPing.start();}