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：

熔断器开关图.png

说明，上面说的commandKey，就是在初始化的时候设置的andCommandKey(HystrixCommandKey.Factory.asKey("testCommandKey"))

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流程图.png

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() < 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;

}

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<25;i++){

Thread.sleep(500);

HystrixCommand 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

HystrixCircuitBreaker.java.png

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() < properties.circuitBreakerRequestVolumeThreshold().get()) {

return false;

}

//不满足请求总数条件，就再检查错误比率(errorPercentage)是否小于属性中的错误百分比阈值(circuitBreakerErrorThresholdPercentage，默认 50),如果是的话表示断路器可以保持关闭状态，返回 false

if (health.getErrorPercentage() < properties.circuitBreakerErrorThresholdPercentage().get()) {

return false;

} else {

// 如果超过阈值，Hystrix会判定服务的某些地方出现了问题，因此通过CAS操作将断路器设为开启状态，并记录此时的系统时间作为定时器初始时间，最后返回 true

if (circuitOpen.compareAndSet(false, true)) {

circuitOpenedOrLastTestedTime.set(System.currentTimeMillis());

return true;

} else {

return true;

}

}

}

}

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。

熔断器流程交互图.png