Hystrix 熔断机制原理
相关配置
circuitBreaker.enabled 是否开启熔断
circuitBreaker.requestVolumeThreshold 熔断最低触发请求数阈值
circuitBreaker.sleepWindowInMilliseconds 产生熔断后恢复窗口
circuitBreaker.errorThresholdPercentage 错误率阈值
circuitBreaker.forceOpen 强制打开熔断
circuitBreaker.forceClosed 强制关闭熔断
状态图
执行流程
命令执行前调用circuitBreaker.attemptExecution()，正常情况下会执行返回true，但是如果发生熔断，则需要通过sleepWindows来进行恢复
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;
}
}
}
发生熔断流程
在新版本1.5.12中，会有一个后台线程订阅metrics流实时计算：
- 如果没有达到RequestVolume，则直接返回，不计算是否需要熔断
- 如果当前错误率大于设置的阈值，则触发熔断，状态由CLOSED切换到OPEN，并设置当前熔断的时间（用于后续SleepWindows判断使用）
if (hc.getTotalRequests() < properties.circuitBreakerRequestVolumeThreshold().get()) {
// we are not past the minimum volume threshold for the stat window,
// so no change to circuit status.
// if it was CLOSED, it stays CLOSED
// if it was half-open, we need to wait for a successful command execution
// if it was open, we need to wait for sleep window to elapse
} else {
if (hc.getErrorPercentage() < properties.circuitBreakerErrorThresholdPercentage().get()) {
//we are not past the minimum error threshold for the stat window,
// so no change to circuit status.
// if it was CLOSED, it stays CLOSED
// if it was half-open, we need to wait for a successful command execution
// if it was open, we need to wait for sleep window to elapse
} else {
// our failure rate is too high, we need to set the state to OPEN
if (status.compareAndSet(Status.CLOSED, Status.OPEN)) {
circuitOpened.set(System.currentTimeMillis());
}
}
}
熔断恢复流程
当发生熔断，达到SleepWindows指定时间后，则状态会由OPEN转换为HALF_OPEN，此时只会让一个请求通过，如果该请求执行成功，状态则会转换为CLOSED，否则会回到OPEN状态，并且熔断时间设置为当前时间，重新等待SleepWindows的时间
// 执行成功后调用
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);
}
}
// 执行失败后调用
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());
}
}
