【Hystrix技术指南】（3）超时机制的原理和实现

[每日一句]

也许你度过了很糟糕的一天，但这并不代表你会因此度过糟糕的一生。

[背景介绍]

分布式系统的规模和复杂度不断增加，随着而来的是对分布式系统可用性的要求越来越高。在各种高可用设计模式中，【熔断、隔离、降级、限流】是经常被使用的。而相关的技术，Hystrix本身早已算不上什么新技术，但它却是最经典的技术体系！。
Hystrix以实现熔断降级的设计，从而提高了系统的可用性。
Hystrix是一个在调用端上，实现断路器模式，以及隔舱模式，通过避免级联故障，提高系统容错能力，从而实现高可用设计的一个Java服务组件库。
*Hystrix实现了资源隔离机制

前提介绍

Hystrix的超时检测本质上通过启动单独线程去检测的，线程的执行的时间刚好就是任务超时的时间，本质上就是这么个简单的逻辑。

Hystrix超时后会抛出一个 HystrixTimeoutException的异常。

超时检测逻辑

Hystrix的超时包括注册过程和执行过程两个，注册过程如下：

执行lift(new HystrixObservableTimeoutOperator(_cmd))关联超时检测任务。
在HystrixObservableTimeoutOperator类中，new TimerListener()负责创建检测任务，HystrixTimer.getInstance().addTimerListener(listener)负责关联定时任务。
- 在HystrixObservableTimeoutOperator类中，addTimerListener通过java的定时任务服务scheduleAtFixedRate在延迟超时时间后执行。

Hystrix的超时执行过程如下：

在超时后执行listener.tick()方法后执行类TimerListener的tick方法
在TimerListener类的tick方法中执行timeoutRunnable.run()后执行HystrixContextRunnable的run方法
在HystrixContextRunnable类run方法中执行child.onError(new HystrixTimeoutException())实现超时。
executeCommandWithSpecifiedIsolation(_cmd).lift(new HystrixObservableTimeoutOperator(_cmd));

private static class HystrixObservableTimeoutOperator implements Operator {final AbstractCommand originalCommand;public HystrixObservableTimeoutOperator(final AbstractCommand originalCommand) {this.originalCommand = originalCommand;}public Subscribersuper R> call(final Subscribersuper R> child) {final CompositeSubscription s = new CompositeSubscription();child.add(s);final HystrixRequestContext hystrixRequestContext =HystrixRequestContext.getContextForCurrentThread();TimerListener listener = new TimerListener() {public void tick() {if(originalCommand.isCommandTimedOut.compareAndSet(TimedOutStatus.NOT_EXECUTED, TimedOutStatus.TIMED_OUT)) {originalCommand.eventNotifier.markEvent(HystrixEventType.TIMEOUT,originalCommand.commandKey);s.unsubscribe();final HystrixContextRunnable timeoutRunnable = new HystrixContextRunnable(originalCommand.concurrencyStrategy, hystrixRequestContext, new Runnable() {public void run() {child.onError(new HystrixTimeoutException());}});timeoutRunnable.run();}}public int getIntervalTimeInMilliseconds() {return originalCommand.properties.executionTimeoutInMilliseconds().get();}};final Reference tl = HystrixTimer.getInstance().addTimerListener(listener);originalCommand.timeoutTimer.set(tl);Subscriber parent = new Subscriber() {public void onCompleted() {if (isNotTimedOut()) {tl.clear();child.onCompleted();}}public void onError(Throwable e) {if (isNotTimedOut()) {tl.clear();child.onError(e);}}public void onNext(R v) {if (isNotTimedOut()) {child.onNext(v);}}private boolean isNotTimedOut() {return originalCommand.isCommandTimedOut.get() == TimedOutStatus.COMPLETED ||originalCommand.isCommandTimedOut.compareAndSet(TimedOutStatus.NOT_EXECUTED,TimedOutStatus.COMPLETED);}};s.add(parent);return parent;}}public Reference addTimerListener(final TimerListener listener) {startThreadIfNeeded();Runnable r = new Runnable() {public void run() {try {listener.tick();} catch (Exception e) {logger.error("Failed while ticking TimerListener", e);}}};ScheduledFuture f = executor.get().getThreadPool().scheduleAtFixedRate(r,listener.getIntervalTimeInMilliseconds(), listener.getIntervalTimeInMilliseconds(),TimeUnit.MILLISECONDS);return new TimerReference(listener, f);}public class HystrixContextRunnable implements Runnable {private final Callable actual;private final HystrixRequestContext parentThreadState;public HystrixContextRunnable(Runnable actual) {this(HystrixPlugins.getInstance().getConcurrencyStrategy(), actual);}public HystrixContextRunnable(HystrixConcurrencyStrategy concurrencyStrategy, final Runnableactual) {this(concurrencyStrategy, HystrixRequestContext.getContextForCurrentThread(), actual);}public HystrixContextRunnable(final HystrixConcurrencyStrategy concurrencyStrategy,final HystrixRequestContext hystrixRequestContext, final Runnable actual) {this.actual = concurrencyStrategy.wrapCallable(new Callable() {public Void call() throws Exception {actual.run();return null;}});this.parentThreadState = hystrixRequestContext;}public void run() {HystrixRequestContext existingState = HystrixRequestContext.getContextForCurrentThread();try {HystrixRequestContext.setContextOnCurrentThread(parentThreadState);try {actual.call();} catch (Exception e) {throw new RuntimeException(e);}} finally {HystrixRequestContext.setContextOnCurrentThread(existingState);}}
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