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(RMStateStoreState.ACTIVE, EnumSet.of(RMStateStoreState.ACTIVE, RMStateStoreState.FENCED), RMStateStoreEventType.STORE_APP, new StoreAppTransition()) 跟着 StoreAppTransition 看看做了啥（发送 RMAppEventType.APP_NEW_SAVED 事件）
private static class StoreAppTransition implements MultipleArcTransition<RMStateStore, RMStateStoreEvent, RMStateStoreState> { @Override public RMStateStoreState transition(RMStateStore store, RMStateStoreEvent event) { if (!(event instanceof RMStateStoreAppEvent)) { // should never happen LOG.error("Illegal event type: " + event.getClass()); return RMStateStoreState.ACTIVE; } boolean isFenced = false; ApplicationStateData appState = ((RMStateStoreAppEvent) event).getAppState(); ApplicationId appId = appState.getApplicationSubmissionContext().getApplicationId(); LOG.info("Storing info for app: " + appId); try { store.storeApplicationStateInternal(appId, appState); // 这里发送了 RMAppEventType.APP_NEW_SAVED 事件 store.notifyApplication(new RMAppEvent(appId, RMAppEventType.APP_NEW_SAVED)); } catch (Exception e) { LOG.error("Error storing app: " + appId, e); isFenced = store.notifyStoreOperationFailedInternal(e); } return finalState(isFenced); }; } 二）RMAppImpl - APP_NEW_SAVED
我们再回到 RMAppImpl，找到对应的状态转移逻辑。
// 刚刚我们的状态是 NEW_SAVING，收到了 APP_NEW_SAVED 事件，执行 AddApplicationToSchedulerTransition() 后，转换为 SUBMITTED 状态 .addTransition(RMAppState.NEW_SAVING, RMAppState.SUBMITTED, RMAppEventType.APP_NEW_SAVED, new AddApplicationToSchedulerTransition()) 在 AddApplicationToSchedulerTransition() 中会发送 SchedulerEventType.APP_ADDED 事件。之后 RMAppImpl 转换为 RMAppState.SUBMITTED 状态。
SchedulerEventType.APP_ADDED 会被多个事件处理器捕获处理：
1）ResourceSchedulerWrapper 事件处理器，仅记录
} else if (schedulerEvent.getType() == SchedulerEventType.APP_ADDED && schedulerEvent instanceof AppAddedSchedulerEvent) { AppAddedSchedulerEvent appAddEvent = (AppAddedSchedulerEvent) schedulerEvent; String queueName = appAddEvent.getQueue(); appQueueMap.put(appAddEvent.getApplicationId(), queueName); } 2）各个 AbstractYarnScheduler 的实现类。以 CapacityScheduler 为例：
执行 addApplication()
case APP_ADDED: { AppAddedSchedulerEvent appAddedEvent = (AppAddedSchedulerEvent) event; String queueName = resolveReservationQueueName(appAddedEvent.getQueue(), appAddedEvent.getApplicationId(), appAddedEvent.getReservationID(), appAddedEvent.getIsAppRecovering()); if (queueName != null) { if (!appAddedEvent.getIsAppRecovering()) { addApplication(appAddedEvent.getApplicationId(), queueName, appAddedEvent.getUser(), appAddedEvent.getApplicatonPriority()); } else { addApplicationOnRecovery(appAddedEvent.getApplicationId(), queueName, appAddedEvent.getUser(), appAddedEvent.getApplicatonPriority()); } } } 在 addApplication() 中会提交 Application 并发送 RMAppEventType.APP_ACCEPTED 事件。
queue.submitApplication(applicationId, user, queueName); rmContext.getDispatcher().getEventHandler() .handle(new RMAppEvent(applicationId, RMAppEventType.APP_ACCEPTED)); 三）RMAppImpl - APP_ACCEPTED（重点）
继续回到 RMAppImpl，执行 StartAppAttemptTransition()，创建 newAttempt，发送事件RMAppAttemptEventType.START
.addTransition(RMAppState.SUBMITTED, RMAppState.ACCEPTED, RMAppEventType.APP_ACCEPTED, new StartAppAttemptTransition()) private static final class StartAppAttemptTransition extends RMAppTransition { @Override public void transition(RMAppImpl app, RMAppEvent event) { app.createAndStartNewAttempt(false); }; } private void createAndStartNewAttempt(boolean transferStateFromPreviousAttempt) { createNewAttempt(); handler.handle(new RMAppStartAttemptEvent(currentAttempt.getAppAttemptId(), transferStateFromPreviousAttempt)); } 在 RMAppAttemptImpl 中会捕获这个事件，执行 AttemptStartedTransition()，其中会发送 SchedulerEventType.APP_ATTEMPT_ADDED 事件，由 AbstractYarnScheduler 实现类处理
.addTransition(RMAppAttemptState.NEW, RMAppAttemptState.SUBMITTED, RMAppAttemptEventType.START, new AttemptStartedTransition()) 如在 CapacityScheduler 中由 addApplicationAttempt 处理，会提交 ApplicationAttempt，并发送 RMAppAttemptEventType.ATTEMPT_ADDED 事件
private synchronized void addApplicationAttempt() { // 提交 attempt queue.submitApplicationAttempt(attempt, application.getUser()); // 发送 RMAppAttemptEventType.ATTEMPT_ADDED 事件 rmContext.getDispatcher().getEventHandler().handle( new RMAppAttemptEvent(applicationAttemptId, RMAppAttemptEventType.ATTEMPT_ADDED)); } RMAppAttemptImpl 收到 event 后继续处理，在 ScheduleTransition 会 allocate am container 资源。
.addTransition(RMAppAttemptState.SUBMITTED, EnumSet.of(RMAppAttemptState.LAUNCHED_UNMANAGED_SAVING, RMAppAttemptState.SCHEDULED), RMAppAttemptEventType.ATTEMPT_ADDED, new ScheduleTransition()) // AM resource has been checked when submission Allocation amContainerAllocation = appAttempt.scheduler.allocate( appAttempt.applicationAttemptId, Collections.singletonList(appAttempt.amReq), EMPTY_CONTAINER_RELEASE_LIST, amBlacklist.getBlacklistAdditions(), amBlacklist.getBlacklistRemovals(), null, null); ResourceScheduler 将资源返回给它之前，会向 RMContainerlmpl 发送一个 RMContainerEventType.ACQUIRED 事件。
在 RMContainerImpl 接到 RMContainerEventType.START，发送 RMAppAttemptEventType.CONTAINER_ALLOCATED 事件。
.addTransition(RMContainerState.NEW, RMContainerState.ALLOCATED, RMContainerEventType.START, new ContainerStartedTransition()) private static final class ContainerStartedTransition extends BaseTransition { @Override public void transition(RMContainerImpl container, RMContainerEvent event) { container.eventHandler.handle(new RMAppAttemptEvent( container.appAttemptId, RMAppAttemptEventType.CONTAINER_ALLOCATED)); } } 又回到RMAppAttemptImpl 后续状态机，执行 AMContainerAllocatedTransition，在其中又一次为 am allocate，和上一个状态中 allocate 仅参数不同，没搞懂为啥。这里如果发现 allocate container 资源还是 0，会退回上一步，状态还是 RMAppAttemptState.SCHEDULED 等待再次获取资源。如果正常获取到了资源，就会转为 RMAppAttemptState.ALLOCATED_SAVING 状态。
.addTransition(RMAppAttemptState.SCHEDULED, EnumSet.of(RMAppAttemptState.ALLOCATED_SAVING, RMAppAttemptState.SCHEDULED), RMAppAttemptEventType.CONTAINER_ALLOCATED, new AMContainerAllocatedTransition()) Allocation amContainerAllocation = appAttempt.scheduler.allocate(appAttempt.applicationAttemptId, EMPTY_CONTAINER_REQUEST_LIST, EMPTY_CONTAINER_RELEASE_LIST, null, null, null, null); 日志记录完成后，RMStateStore 向 RMAppAttemptImpl 发送 RMAppAttemptEventType.ATTEMPT_NEW_SAVED 事件。
RMAppAttemptImpl 后续向 ApplicationMasterLauncher 发 送 AMLauncherEventType.LAUNCH 事件（实际执行是在 AMLauncher 中），并将状态从 ALLOCATED_SAVING 转移为 ALLOCATED。
.addTransition(RMAppAttemptState.ALLOCATED_SAVING, RMAppAttemptState.ALLOCATED, RMAppAttemptEventType.ATTEMPT_NEW_SAVED, new AttemptStoredTransition()) ApplicationMasterLauncher 收到 AMLauncherEventType.LAUNCH 事件后，会将该事件放到事件队列中，等待 AMLauncher 线程池中的线程处理该事件。它将与对应的 NodeManager 通信，启动 ApplicationMaster，一旦成功启动后，将向 RMAppAttemptImpl 发送 RMAppAttemptEventType.LAUNCHED 事件。
public void run() { switch (eventType) { case LAUNCH: try { LOG.info("Launching master" + application.getAppAttemptId()); launch(); handler.handle(new RMAppAttemptEvent(application.getAppAttemptId(), RMAppAttemptEventType.LAUNCHED)); RMAppAttemptImpl 收到 RMAppAttemptEventType.LAUNCHED 事件后，会向 AMLivelinessMonitor 注册，以监控运行状态。RMAppAttemptImpl 状态从 ALLOCATED 转移为 LAUNCHED。
之后，NodeManager 通过心跳机制汇报 ApplicationMaster 所在 Container 已经成功启动，收到该信息后，ResourceScheduler 将发送一个 RMContainerEventType.LAUNCHED 事件，RMContainerImpl 收到该事件后，会从 ContainerAllocationExpirer 监控列表中移除。
启动的 ApplicationMaster 通过RPC 函数 ApplicationMasterProtocol#registerApplicationMaster 向 ResourceManager 注册，ResourceManager 中的 ApplicationMasterService 服务接收到该请求后，发送 RMAppAttemptEventType.REGISTERED 事件。
// ApplicationMasterService#registerApplicationMaster LOG.info("AM registration " + applicationAttemptId); this.rmContext .getDispatcher() .getEventHandler() .handle( // 这里发送 RMAppAttemptEventType.REGISTERED 事件 new RMAppAttemptRegistrationEvent(applicationAttemptId, request .getHost(), request.getRpcPort(), request.getTrackingUrl())); RMAppAttemptImpl 收到该事件后，首先保存该 ApplicationMaster 的基本信息（比如所在 host、启用的 RPC 端口号等），然后向 RMApplmpl 发送一个 RMAppEventType.ATTEMPT_REGISTERED 事件。RMAppAttemptImpl 状态从 LAUNCHED 转移为 RUNNING。
.addTransition(RMAppAttemptState.LAUNCHED, RMAppAttemptState.RUNNING, RMAppAttemptEventType.REGISTERED, REGISTERED_TRANSITION) // AMRegisteredTransition appAttempt.eventHandler.handle(new RMAppEvent(appAttempt .getAppAttemptId().getApplicationId(), RMAppEventType.ATTEMPT_REGISTERED)); 四）RMAppImpl - ATTEMPT_REGISTERED
RMAppImpl 收到 RMAppEventType.ATTEMPT_REGISTERED 事件后，将状态从 ACCEPTED 转换为 RUNNING。
.addTransition(RMAppState.ACCEPTED, RMAppState.RUNNING, RMAppEventType.ATTEMPT_REGISTERED, new RMAppStateUpdateTransition( YarnApplicationState.RUNNING)) 到这里，启动 ApplicationMaster 的整体流程分析完毕！
三、总结
本篇文章分析了从应用程序提交到启动 ApplicationMaster 的整个过程，分析具体过程看的可能会有些繁琐。但只要抓住核心本质，就很容易捋清楚。重点就是事件处理和状态机，这两个部件理解清楚，就很容易看明白程序的流转。
实际逻辑无非就是几个服务之间互相发送对应的事件，接收到事件后会执行启动服务、记录日志、监控状态，然后再发送个新的事件。
本身不难，但需要耐下心来一点点去梳理。
