hystrix(3) 熔断器

　　讲完metrics我们就来了解一下熔断器的执行情况，熔断器的判断取决metrics数据。

　　hystrix在执行命令前需要经过熔断器判断，如果服务被熔断，则执行fallback流程，熔断判断逻辑如下：

1. 如果强制未开启，返回true（未熔断）。
2. 如果强制开启，返回false（熔断）。
3. 判断熔断标识
   1. 如果未熔断则返回true。
   2. 如果half_open，返回false（熔断）。
   3. 如果熔断，判断当前时间是否超过短路窗口期，
   1. 如果没有超过，返回false。
   2. 如果超过则返回true。 并设置熔断状态为half_open。

　　命令执行失败后逻辑如下：

　　　　如果熔断标识为half_open，并重新计算短路窗口期（记录当前时间）。

　　　　如果熔断标识为close，通过命令metric组件，获取命令指定窗口时间内执行总错误数和错误率。如果实际错误率或错误数高于配置错误率或错误数，则设置熔断标识为熔断。

　　命令执行成功后逻辑如下：

　　　　只有在熔断状态为half_open状态下，才能解除熔断。

　　　　如果请求执行成功，解除熔断。

　　熔断器还会监听metrics数据流，当错误比率或者请求量大于配置的值时，就会设置熔断标识为熔断。每个commandkey都会对应一个熔断器。

熔断器判断

private Observable<R> applyHystrixSemantics(final AbstractCommand<R> _cmd) {
        ...
        if (circuitBreaker.allowRequest()) {
            ...
           return executeCommandAndObserve(_cmd)
                            .doOnError(markExceptionThrown)
                            .doOnTerminate(singleSemaphoreRelease)
                            .doOnUnsubscribe(singleSemaphoreRelease);
            ...
        } else {
            return handleShortCircuitViaFallback();
        }
    }

熔断器逻辑

static class HystrixCircuitBreakerImpl implements HystrixCircuitBreaker {
        private final HystrixCommandProperties properties;
        private final HystrixCommandMetrics metrics;
        /* track whether this circuit is open/closed at any given point in time (default to false==closed) */
        private AtomicBoolean circuitOpen = new AtomicBoolean(false);
        /* when the circuit was marked open or was last allowed to try a 'singleTest' */
        private AtomicLong circuitOpenedOrLastTestedTime = new AtomicLong();
        protected HystrixCircuitBreakerImpl(HystrixCommandKey key, HystrixCommandGroupKey commandGroup, HystrixCommandProperties properties, HystrixCommandMetrics metrics) {
            this.properties = properties;
            this.metrics = metrics;
        }
        public void markSuccess() {
            if (circuitOpen.get()) {
                if (circuitOpen.compareAndSet(true, false)) {
                    //win the thread race to reset metrics
                    //Unsubscribe from the current stream to reset the health counts stream.  This only affects the health counts view,
                    //and all other metric consumers are unaffected by the reset
                    metrics.resetStream();
                }
            }
        }

        @Override
        public boolean allowRequest() {
            if (properties.circuitBreakerForceOpen().get()) {
                // properties have asked us to force the circuit open so we will allow NO requests
                return false;
            }
            if (properties.circuitBreakerForceClosed().get()) {
                // we still want to allow isOpen() to perform it's calculations so we simulate normal behavior
                isOpen();
                // properties have asked us to ignore errors so we will ignore the results of isOpen and just allow all traffic through
                return true;
            }
            return !isOpen() || allowSingleTest();
        }

        public boolean allowSingleTest() {
            long timeCircuitOpenedOrWasLastTested = circuitOpenedOrLastTestedTime.get();
            // 1) if the circuit is open
            // 2) and it's been longer than 'sleepWindow' since we opened the circuit
            if (circuitOpen.get() && System.currentTimeMillis() > timeCircuitOpenedOrWasLastTested + properties.circuitBreakerSleepWindowInMilliseconds().get()) {
                // We push the 'circuitOpenedTime' ahead by 'sleepWindow' since we have allowed one request to try.
                // If it succeeds the circuit will be closed, otherwise another singleTest will be allowed at the end of the 'sleepWindow'.
                if (circuitOpenedOrLastTestedTime.compareAndSet(timeCircuitOpenedOrWasLastTested, System.currentTimeMillis())) {
                    // if this returns true that means we set the time so we'll return true to allow the singleTest
                    // if it returned false it means another thread raced us and allowed the singleTest before we did
                    return true;
                }
            }
            return false;
        }

        @Override
        public boolean isOpen() {
            if (circuitOpen.get()) {
                // if we're open we immediately return true and don't bother attempting to 'close' ourself as that is left to allowSingleTest and a subsequent successful test to close
                return true;
            }
            // we're closed, so let's see if errors have made us so we should trip the circuit open
            HealthCounts health = metrics.getHealthCounts();
            // check if we are past the statisticalWindowVolumeThreshold
            if (health.getTotalRequests() < properties.circuitBreakerRequestVolumeThreshold().get()) {
                // we are not past the minimum volume threshold for the statisticalWindow so we'll return false immediately and not calculate anything
                return false;
            }
            if (health.getErrorPercentage() < properties.circuitBreakerErrorThresholdPercentage().get()) {
                return false;
            } else {
                // our failure rate is too high, trip the circuit
                if (circuitOpen.compareAndSet(false, true)) {
                    // if the previousValue was false then we want to set the currentTime
                    circuitOpenedOrLastTestedTime.set(System.currentTimeMillis());
                    return true;
                } else {
                    // How could previousValue be true? If another thread was going through this code at the same time a race-condition could have
                    // caused another thread to set it to true already even though we were in the process of doing the same
                    // In this case, we know the circuit is open, so let the other thread set the currentTime and report back that the circuit is open
                    return true;
                }
            }
        }

    }