一、熔断器(Circuit Breaker)介绍

熔断器，现实生活中有一个很好的类比，就是家庭电路中都会安装一个保险盒，当电流过大的时候保险盒里面的保险丝会自动断掉，来保护家里的各种电器及电路。Hystrix中的熔断器(Circuit Breaker)也是起到这样的作用，Hystrix在运行过程中会向每个commandKey对应的熔断器报告 成功、失败、超时和拒绝的状态，熔断器维护计算统计的数据，根据这些统计的信息来确定熔断器是否打开。如果打开，后续的请求都会被截断。然后会隔一段时间默认是5s，尝试半开，放入一部分流量请求进来，相当于对依赖服务进行一次健康检查，如果恢复，熔断器关闭，随后完全恢复调用。如下图：

再来看下熔断器在整个Hystrix流程图中的位置，从 步骤4开始，如下图：

Hystrix会检查Circuit Breaker的状态。如果Circuit Breaker的状态为开启状态，Hystrix将不会执行对应指令，而是直接进入失败处理状态（图中8 Fallback）。如果Circuit Breaker的状态为关闭状态，Hystrix会继续进行线程池、任务队列、信号量的检查（图中5）

二、如何使用熔断器(Circuit Breaker)

由于Hystrix是一个容错框架，因此我们在使用的时候，要达到熔断的目的只需配置一些参数就可以了。但我们要达到真正的效果，就必须要了解这些参数。Circuit Breaker一共包括如下6个参数。
1、circuitBreaker.enabled
是否启用熔断器，默认是TURE。
2、circuitBreaker.forceOpen
熔断器强制打开，始终保持打开状态。默认值FLASE。
3、circuitBreaker.forceClosed
熔断器强制关闭，始终保持关闭状态。默认值FLASE。
4、circuitBreaker.errorThresholdPercentage
设定错误百分比，默认值50%，例如一段时间（10s）内有100个请求，其中有55个超时或者异常返回了，那么这段时间内的错误百分比是55%，大于了默认值50%，这种情况下触发熔断器-打开。
5、circuitBreaker.requestVolumeThreshold
默认值20.意思是至少有20个请求才进行errorThresholdPercentage错误百分比计算。比如一段时间（10s）内有19个请求全部失败了。错误百分比是100%，但熔断器不会打开，因为requestVolumeThreshold的值是20. 这个参数非常重要，熔断器是否打开首先要满足这个条件，源代码如下

      // check if we are past the statisticalWindowVolumeThreshold
if (health.getTotalRequests() 

6、circuitBreaker.sleepWindowInMilliseconds
半开试探休眠时间，默认值5000ms。当熔断器开启一段时间之后比如5000ms，会尝试放过去一部分流量进行试探，确定依赖服务是否恢复。

测试代码（模拟10次调用，错误百分比为5%的情况下，打开熔断器开关。）：

      package myHystrix.threadpool;

import com.netflix.hystrix.*;
import org.junit.Test;

import java.util.Random;

/**
 * Created by wangxindong on 2017/8/15.
 */
public class GetOrderCircuitBreakerCommand extends HystrixCommand {

    public GetOrderCircuitBreakerCommand(String name){
        super(Setter.withGroupKey(HystrixCommandGroupKey.Factory.asKey("ThreadPoolTestGroup"))
                .andCommandKey(HystrixCommandKey.Factory.asKey("testCommandKey"))
                .andThreadPoolKey(HystrixThreadPoolKey.Factory.asKey(name))
                .andCommandPropertiesDefaults(
                        HystrixCommandProperties.Setter()
                                .withCircuitBreakerEnabled(true)//默认是true，本例中为了展现该参数
                                .withCircuitBreakerForceOpen(false)//默认是false，本例中为了展现该参数
                                .withCircuitBreakerForceClosed(false)//默认是false，本例中为了展现该参数
                                .withCircuitBreakerErrorThresholdPercentage(5)//(1)错误百分比超过5%
                                .withCircuitBreakerRequestVolumeThreshold(10)//(2)10s以内调用次数10次，同时满足(1)(2)熔断器打开
                                .withCircuitBreakerSleepWindowInMilliseconds(5000)//隔5s之后，熔断器会尝试半开(关闭)，重新放进来请求
//                                .withExecutionTimeoutInMilliseconds(1000)
                )
                .andThreadPoolPropertiesDefaults(
                        HystrixThreadPoolProperties.Setter()
                                .withMaxQueueSize(10)   //配置队列大小
                                .withCoreSize(2)    // 配置线程池里的线程数
                )
        );
    }

    @Override
    protected String run() throws Exception {
        Random rand = new Random();
        //模拟错误百分比(方式比较粗鲁但可以证明问题)
        if(1==rand.nextInt(2)){
//            System.out.println("make exception");
            throw new Exception("make exception");
        }
        return "running:  ";
    }

    @Override
    protected String getFallback() {
//        System.out.println("FAILBACK");
        return "fallback: ";
    }

    public static class UnitTest{

        @Test
        public void testCircuitBreaker() throws Exception{
            for(int i=0;i command = new GetOrderCircuitBreakerCommand("testCircuitBreaker");
                String result = command.execute();
                //本例子中从第11次，熔断器开始打开
                System.out.println("call times:"+(i+1)+"   result:"+result +" isCircuitBreakerOpen: "+command.isCircuitBreakerOpen());
                //本例子中5s以后，熔断器尝试关闭，放开新的请求进来
            }
        }
    }
}

测试结果：

call times:1 result:fallback: isCircuitBreakerOpen: False
call times:2 result:running: isCircuitBreakerOpen: false
call times:3 result:running: isCircuitBreakerOpen: false
call times:4 result:fallback: isCircuitBreakerOpen: false
call times:5 result:running: isCircuitBreakerOpen: false
call times:6 result:fallback: isCircuitBreakerOpen: false
call times:7 result:fallback: isCircuitBreakerOpen: false
call times:8 result:fallback: isCircuitBreakerOpen: false
call times:9 result:fallback: isCircuitBreakerOpen: false
call times:10 result:fallback: isCircuitBreakerOpen: false
熔断器打开
call times:11 result:fallback: isCircuitBreakerOpen: true
call times:12 result:fallback: isCircuitBreakerOpen: true
call times:13 result:fallback: isCircuitBreakerOpen: true
call times:14 result:fallback: isCircuitBreakerOpen: true
call times:15 result:fallback: isCircuitBreakerOpen: true
call times:16 result:fallback: isCircuitBreakerOpen: true
call times:17 result:fallback: isCircuitBreakerOpen: true
call times:18 result:fallback: isCircuitBreakerOpen: true
call times:19 result:fallback: isCircuitBreakerOpen: true
call times:20 result:fallback: isCircuitBreakerOpen: true
5s后熔断器关闭
call times:21 result:running: isCircuitBreakerOpen: false
call times:22 result:running: isCircuitBreakerOpen: false
call times:23 result:fallback: isCircuitBreakerOpen: false
call times:24 result:running: isCircuitBreakerOpen: false
call times:25 result:running: isCircuitBreakerOpen: false

三、熔断器(Circuit Breaker)源代码HystrixCircuitBreaker.java分析

Factory 是一个工厂类，提供HystrixCircuitBreaker实例

      public static class Factory {
        //用一个ConcurrentHashMap来保存HystrixCircuitBreaker对象
        private static ConcurrentHashMap circuitBreakersByCommand = new ConcurrentHashMap();
        
//Hystrix首先会检查ConcurrentHashMap中有没有对应的缓存的断路器，如果有的话直接返回。如果没有的话就会新创建一个HystrixCircuitBreaker实例，将其添加到缓存中并且返回
        public static HystrixCircuitBreaker getInstance(HystrixCommandKey key, HystrixCommandGroupKey group, HystrixCommandProperties properties, HystrixCommandMetrics metrics) {
            
            HystrixCircuitBreaker previouslyCached = circuitBreakersByCommand.get(key.name());
            if (previouslyCached != null) {
                return previouslyCached;
            }

            
            HystrixCircuitBreaker cbForCommand = circuitBreakersByCommand.putIfAbsent(key.name(), new HystrixCircuitBreakerImpl(key, group, properties, metrics));
            if (cbForCommand == null) {
                return circuitBreakersByCommand.get(key.name());
            } else {
                return cbForCommand;
            }
        }

        
        public static HystrixCircuitBreaker getInstance(HystrixCommandKey key) {
            return circuitBreakersByCommand.get(key.name());
        }

        static void reset() {
            circuitBreakersByCommand.clear();
        }
}

HystrixCircuitBreakerImpl是HystrixCircuitBreaker的实现，allowRequest()、isOpen()、markSuccess()都会在HystrixCircuitBreakerImpl有默认的实现。

      static class HystrixCircuitBreakerImpl implements HystrixCircuitBreaker {
        private final HystrixCommandProperties properties;
        private final HystrixCommandMetrics metrics;

        /* 变量circuitOpen来代表断路器的状态，默认是关闭 */
        private AtomicBoolean circuitOpen = new AtomicBoolean(false);

        /* 变量circuitOpenedOrLastTestedTime记录着断路恢复计时器的初始时间，用于Open状态向Close状态的转换 */
        private AtomicLong circuitOpenedOrLastTestedTime = new AtomicLong();

        protected HystrixCircuitBreakerImpl(HystrixCommandKey key, HystrixCommandGroupKey commandGroup, HystrixCommandProperties properties, HystrixCommandMetrics metrics) {
            this.properties = properties;
            this.metrics = metrics;
        }

        /*用于关闭熔断器并重置统计数据*/
        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() {
            //是否设置强制开启
            if (properties.circuitBreakerForceOpen().get()) {
                return false;
            }
            if (properties.circuitBreakerForceClosed().get()) {//是否设置强制关闭
                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();
            //获取熔断恢复计时器记录的初始时间circuitOpenedOrLastTestedTime，然后判断以下两个条件是否同时满足：
            // 1) 熔断器的状态为开启状态(circuitOpen.get() == true)
            // 2) 当前时间与计时器初始时间之差大于计时器阈值circuitBreakerSleepWindowInMilliseconds(默认为 5 秒)
            //如果同时满足的话，表示可以从Open状态向Close状态转换。Hystrix会通过CAS操作将circuitOpenedOrLastTestedTime设为当前时间，并返回true。如果不同时满足，返回false，代表熔断器关闭或者计时器时间未到。
            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()) {//获取断路器的状态
                // 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;
            }

            // Metrics数据中获取HealthCounts对象
            HealthCounts health = metrics.getHealthCounts();

            // 检查对应的请求总数(totalCount)是否小于属性中的请求容量阈值circuitBreakerRequestVolumeThreshold,默认20，如果是的话表示熔断器可以保持关闭状态，返回false
            if (health.getTotalRequests() 

四、总结

每个熔断器默认维护10个bucket,每秒一个bucket,每个blucket记录成功,失败,超时,拒绝的状态，默认错误超过50%且10秒内超过20个请求进行中断拦截。下图显示HystrixCommand或HystrixObservableCommand如何与HystrixCircuitBreaker及其逻辑和决策流程进行交互，包括计数器在断路器中的行为。

转载请注明出处，并附上链接 http://www.jianshu.com/p/14958039fd15
参考资料： https://github.com/Netflix/Hystrix/wiki