本节开始，将对 ResourceManager 中一些常见行为进行分析探究，看某些具体关键的行为，在 RM 中是如何流转的。本节将深入源码探究「启动 ApplicationMaster」的具体流程。

一、整体流程

本小节介绍从应用程序提交到启动 ApplicationMaster 的整个过程，期间涉及 Client、RMService、 RMAppManager、RMApplmpl、RMAppAttemptImpl、RMNode、ResourceScheduler 等几个主要组件。当客户端调用 RPC 函数 ApplicationClientProtocol#submitApplication 后， ResourceManager 端的处理过程如下图所示。

二、具体流程分析

接下来跟随上面的流程图，我们深入源码具体分析每一步都是如何执行的：
最开始由客户端发起任务提交 submitApplication()，经过 ClientRMService 和 RMAppManager 发送 RMAppEventType.START 事件，之后交由 RMAppImpl 处理。

  protected void submitApplication(
      ApplicationSubmissionContext submissionContext, long submitTime,
      String user) throws YarnException {
    ApplicationId applicationId = submissionContext.getApplicationId();

    RMAppImpl application =
        createAndPopulateNewRMApp(submissionContext, submitTime, user, false);
    Credentials credentials = null;
    try {
      credentials = parseCredentials(submissionContext);
      if (UserGroupInformation.isSecurityEnabled()) {
        this.rmContext.getDelegationTokenRenewer()
            .addApplicationAsync(applicationId, credentials,
                submissionContext.getCancelTokensWhenComplete(),
                application.getUser());
      } else {
        // Dispatcher is not yet started at this time, so these START events
        // enqueued should be guaranteed to be first processed when dispatcher
        // gets started.
        // 这里发送 RMAppEventType.START 事件
        this.rmContext.getDispatcher().getEventHandler()
            .handle(new RMAppEvent(applicationId, RMAppEventType.START));
      }

RMAppImpl 这东西是个状态机，收到事件之后会自己转换状态并且处理相应的逻辑。
（状态机还不熟悉的同学，可翻到我前面的文章进行学习《2-4 Yarn 基础库 - 状态机库》）

截取一部分状态转换代码：

  private static final StateMachineFactory<RMAppImpl,
                                           RMAppState,
                                           RMAppEventType,
                                           RMAppEvent> stateMachineFactory
                               = new StateMachineFactory<RMAppImpl,
                                           RMAppState,
                                           RMAppEventType,
                                           RMAppEvent>(RMAppState.NEW)


     // Transitions from NEW state
    .addTransition(RMAppState.NEW, RMAppState.NEW,
        RMAppEventType.NODE_UPDATE, new RMAppNodeUpdateTransition())
     // 收到 RMAppEventType.START 事件
    .addTransition(RMAppState.NEW, RMAppState.NEW_SAVING,
        RMAppEventType.START, new RMAppNewlySavingTransition())
    .addTransition(RMAppState.NEW, EnumSet.of(RMAppState.SUBMITTED,
            RMAppState.ACCEPTED, RMAppState.FINISHED, RMAppState.FAILED,
            RMAppState.KILLED, RMAppState.FINAL_SAVING),
        RMAppEventType.RECOVER, new RMAppRecoveredTransition())
    .addTransition(RMAppState.NEW, RMAppState.KILLED, RMAppEventType.KILL,
        new AppKilledTransition())
    .addTransition(RMAppState.NEW, RMAppState.FINAL_SAVING,
        RMAppEventType.APP_REJECTED,
        new FinalSavingTransition(new AppRejectedTransition(),
          RMAppState.FAILED))

一）RMAppImpl - START

收到 RMAppEventType.START 事件之后，会执行 RMAppNewlySavingTransition()。

  private static final class RMAppNewlySavingTransition extends RMAppTransition {
    @Override
    public void transition(RMAppImpl app, RMAppEvent event) {

      // If recovery is enabled then store the application information in a
      // non-blocking call so make sure that RM has stored the information
      // needed to restart the AM after RM restart without further client
      // communication
      LOG.info("Storing application with id " + app.applicationId);
      app.rmContext.getStateStore().storeNewApplication(app);
    }
  }

跟下去会发现它发出 RMStateStoreEventType.STORE_APP 事件，去 RMStateStore 中找一下对应的事件处理。发现也是个状态机：

.addTransition(RMStat