Hystrix熔断器技术解析
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<String> {
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<String> 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<String, HystrixCircuitBreaker> circuitBreakersByCommand = new ConcurrentHashMap<String, HystrixCircuitBreaker>();
//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