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

TimedOutStatus

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。