本篇假设大家对Hystrix的执行过程及源码有一定的了解，这里介绍Hystrix的熔断器执行机制。
1.Hystrix 熔断器类结构
HystrixCircuitBreaker作为接口定义，具体的实现有NoOpCircuitBreaker和HystrixCircuitBreakerImpl，其中NoOpCircuitBreaker只是个空壳没有具体的实现，相当于不熔断。HystrixCircuitBreakerImpl是主要的熔断逻辑实现。
2.Hystrix 熔断器状态
熔断器有三个状态 CLOSED、OPEN、HALF_OPEN 熔断器默认关闭状态，当触发熔断后状态变更为 OPEN,在等待到指定的时间，Hystrix会放请求检测服务是否开启，这期间熔断器会变为HALF_OPEN 半开启状态，熔断探测服务可用则继续变更为 CLOSED关闭熔断器。
状态变更示意图
3.代码视角
ConcurrentHashMap<String, HystrixCircuitBreaker> circuitBreakersByCommand = new ConcurrentHashMap<String, HystrixCircuitBreaker>();
Hystrix为每个commandKey都维护了一个熔断器，保持着对应的熔断器，所以当new XXXHystrixCommand()的时候依然能够保持着原来熔断器的状态。
3.1 如何判定开启熔断
protected HystrixCircuitBreakerImpl(HystrixCommandKey key, HystrixCommandGroupKey commandGroup, final HystrixCommandProperties properties, HystrixCommandMetrics metrics) {
this.properties = properties;
this.metrics = metrics;
//On a timer, this will set the circuit between OPEN/CLOSED as command executions occur
Subscription s = subscribeToStream();
activeSubscription.set(s);
}
private Subscription subscribeToStream() {
/*
* This stream will recalculate the OPEN/CLOSED status on every onNext from the health stream
*/
return metrics.getHealthCountsStream()
.observe()
.subscribe(new Subscriber<HealthCounts>() {
//.....................省略干扰代码......................
@Override
public void onNext(HealthCounts hc) {
// check if we are past the statisticalWindowVolumeThreshold
if (hc.getTotalRequests() < properties.circuitBreakerRequestVolumeThreshold().get()) {
} else {
if (hc.getErrorPercentage() < properties.circuitBreakerErrorThresholdPercentage().get()) {
} else {
if (status.compareAndSet(Status.CLOSED, Status.OPEN)) {
circuitOpened.set(System.currentTimeMillis());
}
}
}
}
});
}
这里面HystrixBreaker启动的时候会订阅HystrixCommandMetrics的 HealthCountsStream，每当HealthCountsStream搜集到数据，都会触发上面的 onNext方法，然后该方法做下面几个判断
1.当前请求量是否达到设定水位（请求量太小不做阀值控制）
2.当前的请求错误量是否达到阀值，达到后会将熔断器状态置为 OPEN, circuitOpened设置为当前时间戳表示开启的时间。
3.2 attemptExecution
先看下HystrixCommand 的执行Observable
com.netflix.hystrix.AbstractCommand#applyHystrixSemantics
private Observable<R> applyHystrixSemantics(final AbstractCommand<R> _cmd) {
// mark that we're starting execution on the ExecutionHook
// if this hook throws an exception, then a fast-fail occurs with no fallback. No state is left inconsistent
executionHook.onStart(_cmd);
/* determine if we're allowed to execute */
if (circuitBreaker.attemptExecution()) {
··········省略代码··········
这里，每次HystrixCommand执行都会调用 circuitBreaker.attemptExecution()
public boolean attemptExecution() {
if (properties.circuitBreakerForceOpen().get()) {
return false;
}
if (properties.circuitBreakerForceClosed().get()) {
return true;
}
if (circuitOpened.get() == -1) {
return true;
} else {
if (isAfterSleepWindow()) {
if (status.compareAndSet(Status.OPEN, Status.HALF_OPEN)) {
//only the first request after sleep window should execute
return true;
} else {
return false;
}
} else {
return false;
}
}
}
这里代码判断逻辑
1.判断是否强制开启熔断器，是则return false，command不能执行
2.判断是否强制关闭熔断器，是则return true, command可执行
3.判断熔断器是否开启 circuitOpened.get() == -1表示没有开启，则return true，command可执行。
4.到这步证明已经开启了熔断器，那么判断是否可尝试请求，如果可以同时会把熔断器的状态改为HALF_OPEN
3.3 markSuccess&markNonSuccess
com.netflix.hystrix.AbstractCommand#executeCommandAndObserve
private Observable<R> executeCommandAndObserve(final AbstractCommand<R> _cmd) {
......省略干扰代码.......
final Action1<R> markEmits = new Action1<R>() {
@Override
public void call(R r) {
if (shouldOutputOnNextEvents()) {
executionResult = executionResult.addEvent(HystrixEventType.EMIT);
eventNotifier.markEvent(HystrixEventType.EMIT, commandKey);
}
if (commandIsScalar()) {
......省略干扰代码.......
circuitBreaker.markSuccess();
}
}
};
final Action0 markOnCompleted = new Action0() {
@Override
public void call() {
if (!commandIsScalar()) {
......省略干扰代码.......
circuitBreaker.markSuccess();
}
}
};
final Func1<Throwable, Observable<R>> handleFallback = new Func1<Throwable, Observable<R>>() {
@Override
public Observable<R> call(Throwable t) {
circuitBreaker.markNonSuccess();
......省略干扰代码.......
}
};
......省略干扰代码.......
return execution.doOnNext(markEmits)
.doOnCompleted(markOnCompleted)
.onErrorResumeNext(handleFallback)
.doOnEach(setRequestContext);
}
此处表示HystrixCommand执行的过程中对应的熔断器状态变更，上面代码不难看出，当error的时候会触发circuitBreaker.markNonSuccess();，执行成功或者执行完成触发 circuitBreaker.markSuccess();
markNonSuccess
@Override
public void markNonSuccess() {
if (status.compareAndSet(Status.HALF_OPEN, Status.OPEN)) {
//This thread wins the race to re-open the circuit - it resets the start time for the sleep window
circuitOpened.set(System.currentTimeMillis());
}
}
如果能执行到markNonSuccess，说明此时熔断器是关闭状态，或者尝试放流阶段。关闭状态的话不做处理（未触发熔断），尝试放流时，发现依然执行失败，这里讲熔断器状态重新置为开启状态，并把circuitOpened设置为当前的时间戳。
markSuccess
@Override
public void markSuccess() {
if (status.compareAndSet(Status.HALF_OPEN, Status.CLOSED)) {
//This thread wins the race to close the circuit - it resets the stream to start it over from 0
metrics.resetStream();
Subscription previousSubscription = activeSubscription.get();
if (previousSubscription != null) {
previousSubscription.unsubscribe();
}
Subscription newSubscription = subscribeToStream();
activeSubscription.set(newSubscription);
circuitOpened.set(-1L);
}
}
能走到markSuccess说明熔断器此时关闭或者放流阶段，尝试放流阶段则讲熔断器关闭，设置circuitOpened=-1，并重置指标统计。
4.THE END
到这里熔断器的介绍就结束了，回顾下主要有熔断器如何开启、如何关闭、几个状态的变更。一个完整的熔断器就此呈现在大家的面前。
系列文章推荐
Hystrix熔断框架介绍
Hystrix常用功能介绍
Hystrix执行原理
Hystrix熔断器执行机制
Hystrix超时实现机制
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