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，；
4、 如果没有超过，返回false；
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;
}
}
}
}