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# 简介
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Yarn采用了基于事件驱动的并发模型：
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- 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。
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- 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。
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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。
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# 事件处理器定义
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事件处理器定义如下：
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```java
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@SuppressWarnings("rawtypes")
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@Public
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@Evolving
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public interface EventHandler<T extends Event> {
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void handle(T event);
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}
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```
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只有一个handler函数，如参是事件：
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# 中央处理器AsyncDispatcher
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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：
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- 注册不同类型的事件，主要包含事件类型和事件处理器。
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- 获取事件处理器，用来派发事件，等待异步执行真正的EventHandler。
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```java
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@Public
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@Evolving
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public interface Dispatcher {
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EventHandler<Event> getEventHandler();
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void register(Class<? extends Enum> eventType, EventHandler handler);
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}
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```
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AsyncDispatcher实现了Dispatcher接口，也扩展了AbstractService，表明AsyncDispatcher也是一个服务，
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是一个典型的生产者消费这模型。
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```java
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public class AsyncDispatcher extends AbstractService implements Dispatcher {
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...
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}
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```
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# 事件处理器的注册
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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。
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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。
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```java
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public void register(Class<? extends Enum> eventType,
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EventHandler handler) {
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/* check to see if we have a listener registered */
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EventHandler<Event> registeredHandler = (EventHandler<Event>) eventDispatchers.get(eventType);
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LOG.info("Registering " + eventType + " for " + handler.getClass());
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if (registeredHandler == null) {
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eventDispatchers.put(eventType, handler);
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} else if (!(registeredHandler instanceof MultiListenerHandler)){
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/* for multiple listeners of an event add the multiple listener handler */
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MultiListenerHandler multiHandler = new MultiListenerHandler();
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multiHandler.addHandler(registeredHandler);
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multiHandler.addHandler(handler);
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eventDispatchers.put(eventType, multiHandler);
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} else {
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/* already a multilistener, just add to it */
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MultiListenerHandler multiHandler
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= (MultiListenerHandler) registeredHandler;
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multiHandler.addHandler(handler);
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}
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}
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```
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# 事件处理
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AsyncDispatcher#getEventHandler()是异步派发的关键：
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```java
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private final EventHandler<Event> handlerInstance = new GenericEventHandler();
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// 省略.....
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@Override
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public EventHandler<Event> getEventHandler() {
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return handlerInstance;
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}
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```
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## GenericEventHandler：一个特殊的事件处理器
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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。
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```java
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public void handle(Event event) {
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if (blockNewEvents) {
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return;
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}
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drained = false;
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/* all this method does is enqueue all the events onto the queue */
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int qSize = eventQueue.size();
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if (qSize != 0 && qSize % 1000 == 0
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&& lastEventQueueSizeLogged != qSize) {
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lastEventQueueSizeLogged = qSize;
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LOG.info("Size of event-queue is " + qSize);
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}
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if (qSize != 0 && qSize % detailsInterval == 0
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&& lastEventDetailsQueueSizeLogged != qSize) {
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lastEventDetailsQueueSizeLogged = qSize;
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printEventQueueDetails();
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printTrigger = true;
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}
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int remCapacity = eventQueue.remainingCapacity();
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if (remCapacity < 1000) {
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LOG.warn("Very low remaining capacity in the event-queue: "
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+ remCapacity);
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}
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try {
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eventQueue.put(event);
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} catch (InterruptedException e) {
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if (!stopped) {
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LOG.warn("AsyncDispatcher thread interrupted", e);
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}
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// Need to reset drained flag to true if event queue is empty,
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// otherwise dispatcher will hang on stop.
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drained = eventQueue.isEmpty();
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throw new YarnRuntimeException(e);
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}
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};
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```
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- 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。
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- 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。
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## 事件处理线程
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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。
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```java
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Runnable createThread() {
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return new Runnable() {
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@Override
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public void run() {
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while (!stopped && !Thread.currentThread().isInterrupted()) {
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drained = eventQueue.isEmpty();
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// 省略。。。
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Event event;
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try {
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event = eventQueue.take();
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} catch(InterruptedException ie) {
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if (!stopped) {
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LOG.warn("AsyncDispatcher thread interrupted", ie);
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}
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return;
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}
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if (event != null) {
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// 省略。。。
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dispatch(event);
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// 省略。。。
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}
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}
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}
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};
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}
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```
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### dispatch详解
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- 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。
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- 如果当前handler处理出现异常时，默认会退出RM。
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```java
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protected void dispatch(Event event) {
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//all events go thru this loop
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LOG.debug("Dispatching the event {}.{}", event.getClass().getName(),
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event);
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Class<? extends Enum> type = event.getType().getDeclaringClass();
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try{
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EventHandler handler = eventDispatchers.get(type);
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if(handler != null) {
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handler.handle(event);
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} else {
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throw new Exception("No handler for registered for " + type);
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}
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} catch (Throwable t) {
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//TODO Maybe log the state of the queue
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LOG.error(FATAL, "Error in dispatcher thread", t);
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// If serviceStop is called, we should exit this thread gracefully.
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if (exitOnDispatchException
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&& (ShutdownHookManager.get().isShutdownInProgress()) == false
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&& stopped == false) {
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stopped = true;
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Thread shutDownThread = new Thread(createShutDownThread());
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shutDownThread.setName("AsyncDispatcher ShutDown handler");
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shutDownThread.start();
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}
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}
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}
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```
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# 状态机
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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。
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```java
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private static final StateMachineFactory<RMAppImpl,
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RMAppState,
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RMAppEventType,
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RMAppEvent> stateMachineFactory
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= new StateMachineFactory<RMAppImpl,
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RMAppState,
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RMAppEventType,
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RMAppEvent>(RMAppState.NEW)
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// Transitions from NEW state
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.addTransition(RMAppState.NEW, RMAppState.NEW,
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RMAppEventType.NODE_UPDATE, new RMAppNodeUpdateTransition())
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.addTransition(RMAppState.NEW, RMAppState.NEW_SAVING,
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RMAppEventType.START, new RMAppNewlySavingTransition())
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.addTransition(RMAppState.NEW, EnumSet.of(RMAppState.SUBMITTED,
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RMAppState.ACCEPTED, RMAppState.FINISHED, RMAppState.FAILED,
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RMAppState.KILLED, RMAppState.FINAL_SAVING),
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RMAppEventType.RECOVER, new RMAppRecoveredTransition())
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.addTransition(RMAppState.NEW, RMAppState.KILLED, RMAppEventType.KILL,
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new AppKilledTransition())
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.addTransition(RMAppState.NEW, RMAppState.FINAL_SAVING,
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RMAppEventType.APP_REJECTED,
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new FinalSavingTransition(new AppRejectedTransition(),
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RMAppState.FAILED))
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.addTransition(
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RMAppState.KILLED,
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RMAppState.KILLED,
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EnumSet.of(RMAppEventType.APP_ACCEPTED,
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RMAppEventType.APP_REJECTED, RMAppEventType.KILL,
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RMAppEventType.ATTEMPT_FINISHED, RMAppEventType.ATTEMPT_FAILED,
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RMAppEventType.NODE_UPDATE, RMAppEventType.START))
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.installTopology();
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```
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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：
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```java
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public interface StateMachine
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<STATE extends Enum<STATE>,
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EVENTTYPE extends Enum<EVENTTYPE>, EVENT> {
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public STATE getCurrentState();
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public STATE getPreviousState();
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public STATE doTransition(EVENTTYPE eventType, EVENT event)
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throws InvalidStateTransitionException;
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}
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```
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## ResourceManager中状态机
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- RMApp：用于维护一个Application的生命周期，实现类 - RMAppImpl
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- RMAppAttempt：用于维护一次试探运行的生命周期，实现类 - RMAppAttemptImpl
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- 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
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- RMNode：用于维护一个NodeManager的生命周期，实现类 - RMNodeImpl
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NodeManager中状态机：
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- Application：用于维护节点上一个Application的生命周期，实现类 - ApplicationImpl
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- Container：用于维护节点上一个容器的生命周期，实现类 - ContainerImpl
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- 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
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