服务容错保护断路器Hystrix之九： Hystrix超时实现机制
HystrixCommand在执行的过程中如何探测超时，本篇主要对此进行介绍说明。
1.主入口：executeCommandAndObserve
#com.netflix.hystrix.AbstractCommand#executeCommandAndObserve private Observable<R> executeCommandAndObserve(final AbstractCommand<R> _cmd) { ···省略部分代码··· Observable<R> execution; //判断是否开启超时监测 if (properties.executionTimeoutEnabled().get()) { execution = executeCommandWithSpecifiedIsolation(_cmd) .lift(new HystrixObservableTimeoutOperator<R>(_cmd)); } else { execution = executeCommandWithSpecifiedIsolation(_cmd); } return execution.doOnNext(markEmits) .doOnCompleted(markOnCompleted) .onErrorResumeNext(handleFallback) .doOnEach(setRequestContext); }
executeCommandWithSpecifiedIsolation(_cmd) .lift(new HystrixObservableTimeoutOperator<R>(_cmd));
可以简单的认为lift 里面的对前面的Observable包含，类似装饰者，后面的parent就是指上层的Observable。其中 HystrixObservableTimeoutOperator 就是关键的部分。
2.关键点: HystrixObservableTimeoutOperator
先看下HystrixObservableTimeoutOperator.call()，TimerListener的实现
TimerListener listener = new TimerListener() { @Override public void tick() { if (originalCommand.isCommandTimedOut.compareAndSet(TimedOutStatus.NOT_EXECUTED, TimedOutStatus.TIMED_OUT)) { // 标记事件，可以认为是开的hook，这里暂忽略 originalCommand.eventNotifier.markEvent(HystrixEventType.TIMEOUT, originalCommand.commandKey); //取消原Obserable的订阅 s.unsubscribe(); final HystrixContextRunnable timeoutRunnable = new HystrixContextRunnable(originalCommand.concurrencyStrategy, hystrixRequestContext, new Runnable() { @Override public void run() { child.onError(new HystrixTimeoutException()); } }); timeoutRunnable.run(); } } //获取配置的超时时间配置 @Override public int getIntervalTimeInMilliseconds() { return originalCommand.properties.executionTimeoutInMilliseconds().get(); } };
这段代码的意思就是，给当前command的超时状态置为超时，如果设置成功就抛出HystrixTimeoutException异常，紧接着被command的 doOnErron接收走 fallback逻辑
fallback private Observable<R> executeCommandAndObserve(final AbstractCommand<R> _cmd) { final HystrixRequestContext currentRequestContext = HystrixRequestContext.getContextForCurrentThread(); ................................. final Func1<Throwable, Observable<R>> handleFallback = new Func1<Throwable, Observable<R>>() { @Override public Observable<R> call(Throwable t) { circuitBreaker.markNonSuccess(); Exception e = getExceptionFromThrowable(t); executionResult = executionResult.setExecutionException(e); if (e instanceof RejectedExecutionException) { return handleThreadPoolRejectionViaFallback(e); } else if (t instanceof HystrixTimeoutException) { //此处catch到超时异常 return handleTimeoutViaFallback(); } else if (t instanceof HystrixBadRequestException) { return handleBadRequestByEmittingError(e); } else { /* * Treat HystrixBadRequestException from ExecutionHook like a plain HystrixBadRequestException. */ if (e instanceof HystrixBadRequestException) { eventNotifier.markEvent(HystrixEventType.BAD_REQUEST, commandKey); return Observable.error(e); } return handleFailureViaFallback(e); } } }; ................................. return execution.doOnNext(markEmits) .doOnCompleted(markOnCompleted) .onErrorResumeNext(handleFallback) .doOnEach(setRequestContext); }
同时s.unsubscribe()通知正在执行的线程，终止任务。如何终止呢？
executeCommandWithSpecifiedIsolation.subscribeOn()
subscribeOne的参数就是HystrixContextScheduler, Rxjava里 scheduler具体干活的是 worker，我们先看下Hystrix自定义scheduler的结构示意图
那么我们直奔主题，直接看
ThreadPoolWorker
//ThreadPoolWorker.schedule @Override public Subscription schedule(final Action0 action) { if (subscription.isUnsubscribed()) { return Subscriptions.unsubscribed(); } ScheduledAction sa = new ScheduledAction(action); subscription.add(sa); sa.addParent(subscription); ThreadPoolExecutor executor = (ThreadPoolExecutor) threadPool.getExecutor(); FutureTask<?> f = (FutureTask<?>) executor.submit(sa); sa.add(new FutureCompleterWithConfigurableInterrupt(f, shouldInterruptThread, executor)); return sa; }
1.开始的时候判断observable是否被订阅
2.被订阅后，将任务 submit到线程池
3.FutureCompleterWithConfigurableInterrupt scheduler在执行的时候，增加了observable的中断探测
private static class FutureCompleterWithConfigurableInterrupt implements Subscription { private final FutureTask<?> f; private final Func0<Boolean> shouldInterruptThread; private final ThreadPoolExecutor executor; private FutureCompleterWithConfigurableInterrupt(FutureTask<?> f, Func0<Boolean> shouldInterruptThread, ThreadPoolExecutor executor) { this.f = f; this.shouldInterruptThread = shouldInterruptThread; this.executor = executor; } @Override public void unsubscribe() { executor.remove(f); if (shouldInterruptThread.call()) { f.cancel(true); } else { f.cancel(false); } } .....省略代码....... }
当observable 取消订阅时，就会把当前任务移除，并中断任务
到这里只是讲说了超时后的处理，如何认定执行超时呢？
3.匠心之巧
这里有个很巧妙的设计，再探HystrixObservableTimeoutOperator
final Reference<TimerListener> tl = HystrixTimer.getInstance().addTimerListener(listener); #com.netflix.hystrix.util.HystrixTimer#addTimerListener public Reference<TimerListener> addTimerListener(final TimerListener listener) { startThreadIfNeeded(); // add the listener Runnable r = new Runnable() { @Override 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); }
利用了ScheduledThreadPoolExecutor，延迟执行，延迟时间就是我们设定的超时时间，我们再看下
#HystrixObservableTimeoutOperator Subscriber<R> parent = new Subscriber<R>() { @Override public void onCompleted() { if (isNotTimedOut()) { // stop timer and pass notification through tl.clear(); child.onCompleted(); } } @Override public void onError(Throwable e) { if (isNotTimedOut()) { // stop timer and pass notification through tl.clear(); child.onError(e); } } ..... ..... ..... ..... ..... ..... ..... ..... ..... private boolean isNotTimedOut() { // if already marked COMPLETED (by onNext) or succeeds in setting to COMPLETED return originalCommand.isCommandTimedOut.get() == TimedOutStatus.COMPLETED || originalCommand.isCommandTimedOut.compareAndSet(TimedOutStatus.NOT_EXECUTED, TimedOutStatus.COMPLETED); } };
这里parent就是指上层的obserable，这里可以抽象的认为是我们的HystrixCommand执行线程, 当command执行线程执行完成的时候或异常的时候，会执行 tl.clear()， 也就是Future.cancel()会中断 TimerListener 的ScheduledFuture 线程，迫使超时机制失效。
// tl.clear() private static class TimerReference extends SoftReference<TimerListener> { private final ScheduledFuture<?> f; .... .... .... .... .... @Override public void clear() { super.clear(); // stop this ScheduledFuture from any further executions f.cancel(false); } }
4.回归文字
HystrixCommand里有个 TimedOutStatus 超时状态
现在可以认为有两个线程，一个是hystrixCommand任务执行线程，一个是等着给hystrixCommand判定超时的线程，现在两个线程看谁能先把hystrixCommand的状态置换，只要任何一个线程对hystrixCommand打上标就意味着超时判定结束。
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