本文尝试通过阅读源码的途径，了解elasticsearch节点启动的大体流程。

1. 读取配置创建运行环境

运行环境，这里指的是Environment对象，这个对象封装了Settings对象（es配置），data路径，plugins路径，modules路径，bin路径，libs路径，log路径等。

相关源码：

    /** Create an {@link Environment} for the command to use. Overrideable for tests. */
    protected Environment createEnv(final Terminal terminal, final Map<String, String> settings) throws UserException {
        final String esPathConf = System.getProperty("es.path.conf");
        if (esPathConf == null) {
            throw new UserException(ExitCodes.CONFIG, "the system property [es.path.conf] must be set");
        }
        return InternalSettingsPreparer.prepareEnvironment(Settings.EMPTY, terminal, settings, getConfigPath(esPathConf));
    }

可以看到这里读取了es.path.conf的配置，即es的各种配置文件所在路径。

个人认为这里没有必要读取es.path.conf的配置，这个配置实际上可以不要的，因为已经有es.home的配置了，并且Environment类的构造函数里有这样的代码：

Environment(final Settings settings, final Path configPath, final Path tmpPath)
    ...
    if (configPath != null) {
        configFile = configPath.normalize();
    } else {
        configFile = homeFile.resolve("config");
    }
    ...
}

默认的配置文件位置就是home路径下的config里面。

elasticsearch.yml配置文件的加载在下面这个方法里面：

public static Environment  prepareEnvironment(Settings input, Terminal terminal, Map<String, String> properties, Path configPath) {
    ...
    output = Settings.builder(); // start with a fresh output
    Path path = environment.configFile().resolve("elasticsearch.yml");
    if (Files.exists(path)) {
        try {
            output.loadFromPath(path);
        } catch (IOException e) {
            throw new SettingsException("Failed to load settings from " + path.toString(), e);
        }
    }
    ...
    return new Environment(output.build(), configPath);
}

加载出来的配置会封装成Settings对象设置到Environment里面。

2. 初始化节点

这块的代码都在Node类的下面这个构造函数里面

protected Node(final Environment environment, Collection<Class<? extends Plugin>> classpathPlugins) {
    ...
    logger.info("initializing ...");
    ...
    logger.info("initialized");
    ...
}

把一些细节省略之后，能很明显看到两条我们启动es的时候经常看到的日志。

下面去看里面一些关键性的细节，先整体看下这个构造函数：

    protected Node(final Environment environment, Collection<Class<? extends Plugin>> classpathPlugins) {
        final List<Closeable> resourcesToClose = new ArrayList<>(); // register everything we need to release in the case of an error
        boolean success = false;
        {
            // use temp logger just to say we are starting. we can't use it later on because the node name might not be set
            Logger logger = Loggers.getLogger(Node.class, NODE_NAME_SETTING.get(environment.settings()));
            logger.info("initializing ...");
        }
        try {
            ...
        } catch (IOException ex) {
            throw new ElasticsearchException("failed to bind service", ex);
        } finally {
            if (!success) {
                IOUtils.closeWhileHandlingException(resourcesToClose);
            }
        }
    }

具体细节在这个try里面，里面会打开很多资源，这些资源都会被添加到一个List里面，如果初始化失败，在finally里面会将这些资源关闭。

然后进入try里面去看具体初始化节点的流程。

配置节点

        originalSettings = environment.settings();
        Settings tmpSettings = Settings.builder().put(environment.settings())
            .put(Client.CLIENT_TYPE_SETTING_S.getKey(), CLIENT_TYPE).build();

        // create the node environment as soon as possible, to recover the node id and enable logging
        try {
            nodeEnvironment = new NodeEnvironment(tmpSettings, environment);
            resourcesToClose.add(nodeEnvironment);
        } catch (IOException ex) {
            throw new IllegalStateException("Failed to create node environment", ex);
        }

读取Environment对象里面的配置，创建节点自己的配置NodeEnvironment，里面的配置包括但不限于以下这些信息：

• node.max_local_storage_nodes，本地最大存储节点数量，默认1，即本地最多只能同时启动一个能存储数据的节点；
• node.id.seed，生成节点id的随机数种子，默认是0；
• 节点数据存放路径nodePath，默认是{esHomePath}/data/nodes/0,{esHomePath}/data/nodes/1等；
• indice数据存放路径，默认是{nodePath}/indices；

同时这里面随机生成了节点id，读取并打印出了磁盘、jvm信息，这里的heap size是默认值1/4物理内存：

[2018-10-19T17:52:35,480][INFO ][o.e.e.NodeEnvironment    ] [0aIXj0y] using [1] data paths, mounts [[(F:)]], net usable_space [215gb], net total_space [231gb], types [NTFS]
[2018-10-19T17:53:12,209][INFO ][o.e.e.NodeEnvironment    ] [0aIXj0y] heap size [7.9gb], compressed ordinary object pointers [true]

生成节点名称

        final boolean hadPredefinedNodeName = NODE_NAME_SETTING.exists(tmpSettings);
        final String nodeId = nodeEnvironment.nodeId();
        tmpSettings = addNodeNameIfNeeded(tmpSettings, nodeId);

检查node.name是否有配置，如果用户没有配置，则取nodeId前7位作为节点名称。

加载plugins和modules

this.pluginsService = new PluginsService(tmpSettings, environment.configFile(), environment.modulesFile(), environment.pluginsFile(), classpathPlugins);

在PluginService的构造函数里加载了所有的plugins和modules，过程中会打印出如下日志：

[2018-10-22T15:42:43,978][INFO ][o.e.p.PluginsService     ] [localhost-debug] loaded module [aggs-matrix-stats]
[2018-10-22T15:42:43,980][INFO ][o.e.p.PluginsService     ] [localhost-debug] loaded module [analysis-common]
[2018-10-22T15:42:43,981][INFO ][o.e.p.PluginsService     ] [localhost-debug] loaded module [ingest-common]
[2018-10-22T15:42:43,982][INFO ][o.e.p.PluginsService     ] [localhost-debug] loaded module [lang-expression]
[2018-10-22T15:42:43,983][INFO ][o.e.p.PluginsService     ] [localhost-debug] loaded module [lang-mustache]
[2018-10-22T15:42:43,983][INFO ][o.e.p.PluginsService     ] [localhost-debug] loaded module [lang-painless]
[2018-10-22T15:42:43,984][INFO ][o.e.p.PluginsService     ] [localhost-debug] loaded module [mapper-extras]
[2018-10-22T15:42:43,985][INFO ][o.e.p.PluginsService     ] [localhost-debug] loaded module [parent-join]
[2018-10-22T15:42:43,986][INFO ][o.e.p.PluginsService     ] [localhost-debug] loaded module [percolator]
[2018-10-22T15:42:43,987][INFO ][o.e.p.PluginsService     ] [localhost-debug] loaded module [rank-eval]
[2018-10-22T15:42:43,988][INFO ][o.e.p.PluginsService     ] [localhost-debug] loaded module [reindex]
[2018-10-22T15:42:43,989][INFO ][o.e.p.PluginsService     ] [localhost-debug] loaded module [repository-url]
[2018-10-22T15:42:43,990][INFO ][o.e.p.PluginsService     ] [localhost-debug] loaded module [transport-netty4]
[2018-10-22T15:42:43,991][INFO ][o.e.p.PluginsService     ] [localhost-debug] loaded module [tribe]
[2018-10-22T15:42:43,992][INFO ][o.e.p.PluginsService     ] [localhost-debug] loaded module [x-pack-core]
[2018-10-22T15:42:43,993][INFO ][o.e.p.PluginsService     ] [localhost-debug] loaded module [x-pack-deprecation]
[2018-10-22T15:42:43,994][INFO ][o.e.p.PluginsService     ] [localhost-debug] loaded module [x-pack-graph]
[2018-10-22T15:42:43,994][INFO ][o.e.p.PluginsService     ] [localhost-debug] loaded module [x-pack-logstash]
[2018-10-22T15:42:43,995][INFO ][o.e.p.PluginsService     ] [localhost-debug] loaded module [x-pack-ml]
[2018-10-22T15:42:43,996][INFO ][o.e.p.PluginsService     ] [localhost-debug] loaded module [x-pack-monitoring]
[2018-10-22T15:42:43,998][INFO ][o.e.p.PluginsService     ] [localhost-debug] loaded module [x-pack-rollup]
[2018-10-22T15:42:43,999][INFO ][o.e.p.PluginsService     ] [localhost-debug] loaded module [x-pack-security]
[2018-10-22T15:42:44,000][INFO ][o.e.p.PluginsService     ] [localhost-debug] loaded module [x-pack-sql]
[2018-10-22T15:42:44,001][INFO ][o.e.p.PluginsService     ] [localhost-debug] loaded module [x-pack-upgrade]
[2018-10-22T15:42:44,002][INFO ][o.e.p.PluginsService     ] [localhost-debug] loaded module [x-pack-watcher]
[2018-10-22T15:42:45,130][INFO ][o.e.p.PluginsService     ] [localhost-debug] no plugins loaded

初始化各模块

后面长达几百行的代码都是在构造各个模块及组件（AbstractComponent类），如ScriptModule、AnalysisModule、SearchModule、IndicesService、ClusterService、TransportService等。

打印日志如下：

[2018-10-24T16:10:08,595][DEBUG][o.e.a.ActionModule       ] Using REST wrapper from plugin org.elasticsearch.xpack.security.Security
[2018-10-24T16:10:08,856][INFO ][o.e.d.DiscoveryModule    ] [localhost-debug] using discovery type [zen]

各个模块及组件分别承担什么功能，此处暂时不深究，实际上根据类名大致也能猜测出其作用。

3. 启动节点

这块代码在Node类的start()方法里面。

启动各组件

    /**
     * Start the node. If the node is already started, this method is no-op.
     */
    public Node start() throws NodeValidationException {
        if (!lifecycle.moveToStarted()) {
            return this;
        }

        Logger logger = Loggers.getLogger(Node.class, NODE_NAME_SETTING.get(settings));
        logger.info("starting ...");
        pluginLifecycleComponents.forEach(LifecycleComponent::start);

        injector.getInstance(MappingUpdatedAction.class).setClient(client);
        injector.getInstance(IndicesService.class).start();
        injector.getInstance(IndicesClusterStateService.class).start();
        injector.getInstance(SnapshotsService.class).start();
        injector.getInstance(SnapshotShardsService.class).start();
        injector.getInstance(RoutingService.class).start();
        injector.getInstance(SearchService.class).start();
        nodeService.getMonitorService().start();
                final ClusterService clusterService = injector.getInstance(ClusterService.class);

        final NodeConnectionsService nodeConnectionsService = injector.getInstance(NodeConnectionsService.class);
        nodeConnectionsService.start();
        clusterService.setNodeConnectionsService(nodeConnectionsService);

        injector.getInstance(ResourceWatcherService.class).start();
        injector.getInstance(GatewayService.class).start();
        Discovery discovery = injector.getInstance(Discovery.class);
        clusterService.getMasterService().setClusterStatePublisher(discovery::publish);

        // Start the transport service now so the publish address will be added to the local disco node in ClusterService
        TransportService transportService = injector.getInstance(TransportService.class);
        transportService.getTaskManager().setTaskResultsService(injector.getInstance(TaskResultsService.class));
        transportService.start();
        assert localNodeFactory.getNode() != null;
        assert transportService.getLocalNode().equals(localNodeFactory.getNode())
            : "transportService has a different local node than the factory provided";
        final MetaData onDiskMetadata;
        try {
            // we load the global state here (the persistent part of the cluster state stored on disk) to
            // pass it to the bootstrap checks to allow plugins to enforce certain preconditions based on the recovered state.
            if (DiscoveryNode.isMasterNode(settings) || DiscoveryNode.isDataNode(settings)) {
                onDiskMetadata = injector.getInstance(GatewayMetaState.class).loadMetaState();
            } else {
                onDiskMetadata = MetaData.EMPTY_META_DATA;
            }
            assert onDiskMetadata != null : "metadata is null but shouldn't"; // this is never null
        } catch (IOException e) {
            throw new UncheckedIOException(e);
        }
        ...
    }

这块start了很多组件（Service，都是AbstractComponent的子类），具体逻辑暂时不深究，打印日志如下：

[2018-10-24T16:10:16,609][INFO ][o.e.n.Node               ] [localhost-debug] starting ...
[2018-10-24T16:11:14,864][INFO ][o.e.t.TransportService   ] [localhost-debug] publish_address {xx.xx.xx.xx:9300}, bound_addresses {xx.xx.xx.xx:9300}

验证node信息

    validateNodeBeforeAcceptingRequests(new BootstrapContext(settings, onDiskMetadata), transportService.boundAddress(), pluginsService
            .filterPlugins(Plugin
            .class)
            .stream()
            .flatMap(p -> p.getBootstrapChecks().stream()).collect(Collectors.toList()));

这块代码对node进行启动验证，日志如下：

[2018-10-24T16:33:31,983][INFO ][o.e.b.BootstrapChecks    ] [localhost-debug] bound or publishing to a non-loopback address, enforcing bootstrap checks

如果修改了elasticsearch.yml里面network.host或者其它方式(如http.host,transport.host)的host的配置，而不是默认的127.0.0.1，那么bootstrap check如果检测到问题就不是warning而是error了。

这里可能会有一些问题报出来。比如配置-Xms和-Xmx的值不相同就会有问题，还有一些其它如file descriptors之类的问题，网上都能找到解决办法。

join cluster

discovery.startInitialJoin();

这行代码尝试把当前node加入cluster，不同的Discovery有不同的实现，默认实现是ZenDiscovery。

        if (initialStateTimeout.millis() > 0) {
            final ThreadPool thread = injector.getInstance(ThreadPool.class);
            ClusterState clusterState = clusterService.state();
            ClusterStateObserver observer = new ClusterStateObserver(clusterState, clusterService, null, logger, thread.getThreadContext());
            if (clusterState.nodes().getMasterNodeId() == null) {
                logger.debug("waiting to join the cluster. timeout [{}]", initialStateTimeout);
                final CountDownLatch latch = new CountDownLatch(1);
                observer.waitForNextChange(new ClusterStateObserver.Listener() {
                    @Override
                    public void onNewClusterState(ClusterState state) { latch.countDown(); }

                    @Override
                    public void onClusterServiceClose() {
                        latch.countDown();
                    }

                    @Override
                    public void onTimeout(TimeValue timeout) {
                        logger.warn("timed out while waiting for initial discovery state - timeout: {}",
                            initialStateTimeout);
                        latch.countDown();
                    }
                }, state -> state.nodes().getMasterNodeId() != null, initialStateTimeout);

                try {
                    latch.await();
                } catch (InterruptedException e) {
                    throw new ElasticsearchTimeoutException("Interrupted while waiting for initial discovery state");
                }
            }
        }

加入cluster需要先找到master节点，找到master节点需要时间，这里会进行等待，等到master节点出现，或超时（默认30秒）结束。

启用http

        if (NetworkModule.HTTP_ENABLED.get(settings)) {
            injector.getInstance(HttpServerTransport.class).start();
        }

这里根据配置启用http，默认是开启的，打印日志如下：

[2018-10-24T19:13:07,544][INFO ][o.e.x.s.t.n.SecurityNetty4HttpServerTransport] [localhost-debug] publish_address {xx.xx.xx.xx:9200}, bound_addresses {xx.xx.xx.xx:9200}

到这里当前节点就已经成功启动了，此时仍然有可能还没有选出master节点，没有加入cluster，但不要紧，虽然不能提供集群服务，但凭借此单节点还是能提供一些服务的（可以尝试发送http请求试验）。