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承接上一篇文章🏹「Hystrix」（2）参数配置的详细介绍

• 在这里推荐给大家martinfowler的熔断器介绍和权威指南,有兴趣的小伙伴们可以研究一下哈。

• 主要介绍相关：官网说明

• 关于 【Hystrix如何运行的介绍】的介绍

[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<R>(_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：

1. 在超时后执行listener.tick()方法后执行类TimerListener的tick方法

2. 在TimerListener类的tick方法中执行timeoutRunnable.run()后执行HystrixContextRunnable的run方法

3. 在HystrixContextRunnable类run方法中执行child.onError(new HystrixTimeoutException())实现超时。

4. executeCommandWithSpecifiedIsolation(_cmd).lift(new HystrixObservableTimeoutOperator<R>(_cmd));

private static class HystrixObservableTimeoutOperator<R> implements Operator<R, R> {

        final AbstractCommand<R> originalCommand;

        public HystrixObservableTimeoutOperator(final AbstractCommand<R> originalCommand) {
            this.originalCommand = originalCommand;
        }

        @Override
        public Subscriber<? super R> call(final Subscriber<? super R> child) {
            final CompositeSubscription s = new CompositeSubscription();
            // if the child unsubscribes we unsubscribe our parent as well
            child.add(s);
            //capture the HystrixRequestContext upfront so that we can use it in the timeout thread later
            final HystrixRequestContext hystrixRequestContext = 
							HystrixRequestContext.getContextForCurrentThread();
            TimerListener listener = new TimerListener() {
                @Override
                public void tick() {
                  if(originalCommand.isCommandTimedOut
						.compareAndSet(TimedOutStatus.NOT_EXECUTED, TimedOutStatus.TIMED_OUT)) {
                        // report timeout failure
                        originalCommand.eventNotifier.markEvent(HystrixEventType.TIMEOUT, 
							originalCommand.commandKey);
                        // shut down the original request
                        s.unsubscribe();
                        final HystrixContextRunnable timeoutRunnable = new HystrixContextRunnable(
                                originalCommand.concurrencyStrategy, hystrixRequestContext, new Runnable() {
                            @Override
                            public void run() {
                                child.onError(new HystrixTimeoutException());
                            }
                        });
                        timeoutRunnable.run();
                    }
                }
                @Override
                public int getIntervalTimeInMilliseconds() {
                    return originalCommand.properties.executionTimeoutInMilliseconds().get();
                }
            };
            final Reference<TimerListener> tl = HystrixTimer.getInstance().addTimerListener(listener);
            // set externally so execute/queue can see this
            originalCommand.timeoutTimer.set(tl);
            /**
             * If this subscriber receives values it means the parent succeeded/completed
             */
            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);
                    }
                }
                @Override
                public void onNext(R v) {
                    if (isNotTimedOut()) {
                        child.onNext(v);
                    }
                }
                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);
                }
            };

            // if s is unsubscribed we want to unsubscribe the parent
            s.add(parent);
            return parent;
        }
    }
    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);
                }
            }
        };
        //这里直接简单粗暴的scheduleAtFixedRate以超时时间作为周期去判断是否执行完成
        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<Void> actual;
    	private final HystrixRequestContext parentThreadState;
	    public HystrixContextRunnable(Runnable actual) {
    	    this(HystrixPlugins.getInstance().getConcurrencyStrategy(), actual);
    	}
	    public HystrixContextRunnable(HystrixConcurrencyStrategy concurrencyStrategy, final Runnable 
			actual) {
        	this(concurrencyStrategy, HystrixRequestContext.getContextForCurrentThread(), actual);
    	}
	    public HystrixContextRunnable(final HystrixConcurrencyStrategy concurrencyStrategy, 
                                  final HystrixRequestContext hystrixRequestContext, final Runnable actual) {
    	    this.actual = concurrencyStrategy.wrapCallable(new Callable<Void>() {
            @Override
            public Void call() throws Exception {
                actual.run();
                return null;
            }

        });
        this.parentThreadState = hystrixRequestContext;
    }

    @Override
    public void run() {
        HystrixRequestContext existingState = HystrixRequestContext.getContextForCurrentThread();
        try {
            // set the state of this thread to that of its parent
            HystrixRequestContext.setContextOnCurrentThread(parentThreadState);
            // execute actual Callable with the state of the parent
            try {
                actual.call();
            } catch (Exception e) {
                throw new RuntimeException(e);
            }
        } finally {
            // restore this thread back to its original state
            HystrixRequestContext.setContextOnCurrentThread(existingState);
        }
    }
}