ZenNodeDiscovery

上篇博客就寫了nodeDiscovery服務的引用，這裏就詳細分解一下ZenNodeDiscovery。

AbstractLifecycleComponent

@SuppressWarnings({"unchecked"})
    @Override
    public T start() {
        if (!lifecycle.canMoveToStarted()) {
            return (T) this;
        }
        for (LifecycleListener listener : listeners) {
            listener.beforeStart();
        }
        doStart();
        lifecycle.moveToStarted();
        for (LifecycleListener listener : listeners) {
            listener.afterStart();
        }
        return (T) this;
    }

    protected abstract void doStart();

ZenDiscovery

@Override
    protected void doStart() {
        nodesFD.setLocalNode(clusterService.localNode());
        joinThreadControl.start();
        pingService.start();
        this.nodeJoinController = new NodeJoinController(clusterService, routingService, discoverySettings, settings);

        // start the join thread from a cluster state update. See {@link JoinThreadControl} for details.
        clusterService.submitStateUpdateTask("initial_join", new ClusterStateUpdateTask() {

            @Override
            public boolean runOnlyOnMaster() {
                return false;
            }

            @Override
            public ClusterState execute(ClusterState currentState) throws Exception {
                // do the join on a different thread, the DiscoveryService waits for 30s anyhow till it is discovered
                joinThreadControl.startNewThreadIfNotRunning();
                return currentState;
            }

            @Override
            public void onFailure(String source, @org.elasticsearch.common.Nullable Throwable t) {
                logger.warn("failed to start initial join process", t);
            }
        });
    }