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+
+# 简介
+
+Yarn采用了基于事件驱动的并发模型：
+
+- 所有状态机都实现了EventHandler接口，很多服务（类名通常带有Service后缀）也实现了该接口，它们都是事件处理器。
+- 需要异步处理的事件由中央异步调度器（类名通常带有Dispatcher后缀）统一接收/派发，需要同步处理的事件直接交给相应的事件处理器。
+
+![pic](https://pan.zeekling.cn/zeekling/hadoop/event/state_event_001.png)
+
+某些事件处理器不仅处理事件，也会向中央异步调度器发送事件。
+
+
+# 事件处理器定义
+
+事件处理器定义如下：
+
+```java
+@SuppressWarnings("rawtypes")
+@Public
+@Evolving
+public interface EventHandler<T extends Event> {
+
+  void handle(T event);
+
+}
+```
+
+只有一个handler函数，如参是事件：
+
+
+# 中央处理器AsyncDispatcher 
+
+AsyncDispatcher 实现了接口Dispatcher，Dispatcher中定义了事件Dispatcher的接口。主要提供两个功能：
+- 注册不同类型的事件，主要包含事件类型和事件处理器。
+- 获取事件处理器，用来派发事件，等待异步执行真正的EventHandler。
+
+```java
+@Public
+@Evolving
+public interface Dispatcher {
+
+  EventHandler<Event> getEventHandler();
+
+  void register(Class<? extends Enum> eventType, EventHandler handler);
+
+}
+```
+
+AsyncDispatcher实现了Dispatcher接口，也扩展了AbstractService，表明AsyncDispatcher也是一个服务，
+是一个典型的生产者消费这模型。
+
+```java
+public class AsyncDispatcher extends AbstractService implements Dispatcher {
+ ...
+}
+```
+
+# 事件处理器的注册
+
+事件注册就是将事件写入到eventDispatchers里面，eventDispatchers的定义：`Map<Class<? extends Enum>, EventHandler> eventDispatchers`，键是事件类型，value是事件的处理器。
+
+对于同一事件类型注册多次handler处理函数时，将使用MultiListenerHandler代替，MultiListenerHandler里面保存了多个handler，调用handler函数时，会依次调用每个handler。
+
+```java 
+public void register(Class<? extends Enum> eventType,
+      EventHandler handler) {
+    /* check to see if we have a listener registered */
+    EventHandler<Event> registeredHandler = (EventHandler<Event>) eventDispatchers.get(eventType);
+    LOG.info("Registering " + eventType + " for " + handler.getClass());
+    if (registeredHandler == null) {
+      eventDispatchers.put(eventType, handler);
+    } else if (!(registeredHandler instanceof MultiListenerHandler)){
+      /* for multiple listeners of an event add the multiple listener handler */
+      MultiListenerHandler multiHandler = new MultiListenerHandler();
+      multiHandler.addHandler(registeredHandler);
+      multiHandler.addHandler(handler);
+      eventDispatchers.put(eventType, multiHandler);
+    } else {
+      /* already a multilistener, just add to it */
+      MultiListenerHandler multiHandler
+      = (MultiListenerHandler) registeredHandler;
+      multiHandler.addHandler(handler);
+    }
+  }
+```
+
+
+# 事件处理
+
+AsyncDispatcher#getEventHandler()是异步派发的关键：
+
+```java 
+private final EventHandler<Event> handlerInstance = new GenericEventHandler();
+
+// 省略.....
+
+@Override
+public EventHandler<Event> getEventHandler() {
+   return handlerInstance;
+}
+```
+
+## GenericEventHandler：一个特殊的事件处理器
+
+GenericEventHandler是一个特殊的事件处理器，用于接受各种事件。由指定线程处理接收到的事件。
+
+```java
+public void handle(Event event) {
+  if (blockNewEvents) {
+    return;
+  }
+  drained = false;
+  /* all this method does is enqueue all the events onto the queue */
+  int qSize = eventQueue.size();
+  if (qSize != 0 && qSize % 1000 == 0
+      && lastEventQueueSizeLogged != qSize) {
+    lastEventQueueSizeLogged = qSize;
+    LOG.info("Size of event-queue is " + qSize);
+  }
+  if (qSize != 0 && qSize % detailsInterval == 0
+          && lastEventDetailsQueueSizeLogged != qSize) {
+    lastEventDetailsQueueSizeLogged = qSize;
+    printEventQueueDetails();
+    printTrigger = true;
+  }
+  int remCapacity = eventQueue.remainingCapacity();
+  if (remCapacity < 1000) {
+    LOG.warn("Very low remaining capacity in the event-queue: "
+        + remCapacity);
+  }
+  try {
+    eventQueue.put(event);
+  } catch (InterruptedException e) {
+    if (!stopped) {
+      LOG.warn("AsyncDispatcher thread interrupted", e);
+    }
+    // Need to reset drained flag to true if event queue is empty,
+    // otherwise dispatcher will hang on stop.
+    drained = eventQueue.isEmpty();
+    throw new YarnRuntimeException(e);
+  }
+};
+```
+
+
+
+- blockNewEvents: 是否阻塞事件处理，只有当中央处理器停止之后才会停止接受事件。
+
+- eventQueue：将接收到的请求放置到当前阻塞队列里面。方便指定线程及时处理。
+
+  
+
+## 事件处理线程
+
+在服务启动时（serviceStart函数）创建一个线程，会循环处理接受到的事件。核心处理逻辑在函数dispatch里面。
+
+```java
+Runnable createThread() {
+  return new Runnable() {
+    @Override
+    public void run() {
+      while (!stopped && !Thread.currentThread().isInterrupted()) {
+        drained = eventQueue.isEmpty();
+        // 省略。。。
+        Event event;
+        try {
+          event = eventQueue.take();
+        } catch(InterruptedException ie) {
+          if (!stopped) {
+            LOG.warn("AsyncDispatcher thread interrupted", ie);
+          }
+          return;
+        }
+        if (event != null) {
+          // 省略。。。
+          dispatch(event);
+          // 省略。。。
+        }
+      }
+    }
+  };
+}
+```
+
+
+
+### dispatch详解
+
+- 从已经注册的eventDispatchers列表里面查找当前事件对应的处理器，调用当前处理器的handler函数。
+- 如果当前handler处理出现异常时，默认会退出RM。
+
+```java
+protected void dispatch(Event event) {
+  //all events go thru this loop
+  LOG.debug("Dispatching the event {}.{}", event.getClass().getName(),
+      event);
+
+  Class<? extends Enum> type = event.getType().getDeclaringClass();
+
+  try{
+    EventHandler handler = eventDispatchers.get(type);
+    if(handler != null) {
+      handler.handle(event);
+    } else {
+      throw new Exception("No handler for registered for " + type);
+    }
+  } catch (Throwable t) {
+    //TODO Maybe log the state of the queue
+    LOG.error(FATAL, "Error in dispatcher thread", t);
+    // If serviceStop is called, we should exit this thread gracefully.
+    if (exitOnDispatchException
+        && (ShutdownHookManager.get().isShutdownInProgress()) == false
+        && stopped == false) {
+      stopped = true;
+      Thread shutDownThread = new Thread(createShutDownThread());
+      shutDownThread.setName("AsyncDispatcher ShutDown handler");
+      shutDownThread.start();
+    }
+  }
+}
+```
+
+
+
+
+
+# 状态机
+
+状态转换由成员变量StateMachine管理，所有的StateMachine都由StateMachineFactory进行管理。由addTransition函数实现状态机。
+
+```java
+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())
+    .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))
+
+    .addTransition(
+        RMAppState.KILLED,
+        RMAppState.KILLED,
+        EnumSet.of(RMAppEventType.APP_ACCEPTED,
+            RMAppEventType.APP_REJECTED, RMAppEventType.KILL,
+            RMAppEventType.ATTEMPT_FINISHED, RMAppEventType.ATTEMPT_FAILED,
+            RMAppEventType.NODE_UPDATE, RMAppEventType.START))
+
+     .installTopology();
+```
+
+
+
+Transition定义了“从一个状态转换到另一个状态”的行为，由转换操作、开始状态、事件类型、事件组成：
+
+```java
+public interface StateMachine
+                 <STATE extends Enum<STATE>,
+                  EVENTTYPE extends Enum<EVENTTYPE>, EVENT> {
+  public STATE getCurrentState();
+  public STATE getPreviousState();
+  public STATE doTransition(EVENTTYPE eventType, EVENT event)
+        throws InvalidStateTransitionException;
+}
+```
+
+
+
+
+
+## ResourceManager中状态机
+
+- RMApp：用于维护一个Application的生命周期，实现类 - RMAppImpl
+- RMAppAttempt：用于维护一次试探运行的生命周期，实现类 - RMAppAttemptImpl
+- RMContainer：用于维护一个已分配的资源最小单位Container的生命周期，实现类 - RMContainerImpl
+- RMNode：用于维护一个NodeManager的生命周期，实现类 - RMNodeImpl
+
+NodeManager中状态机：
+
+- Application：用于维护节点上一个Application的生命周期，实现类 - ApplicationImpl
+- Container：用于维护节点上一个容器的生命周期，实现类 - ContainerImpl
+- LocalizedResource：用于维护节点上资源本地化的生命周期，没有使用接口即实现类 - LocalizedResource
+
+
+
+