Spring Cloud Hystrix 熔断指标配置项

hystrix执行请求的代码逻辑如下图

以下为比较重要的hystrix熔断参数。

hystrix:
  command:
    default:
      execution:
        isolation:
          strategy: THREAD
          thread:
            timeoutInMilliseconds: 10000
      circuitBreaker:
        requestVolumeThreshold: 30 
        sleepWindowInMilliseconds: 5000 
        errorThresholdPercentage: 90 

• strategy指定策略为thread形式，有2种选择分别是SEMAPHORE, THREAD，后者请求执行逻辑在单独线程内执行，前者在当前请求线程内执行。
• timeoutInMilliseconds线程超市时间，我在图里没有画出来。
• errorThresholdPercentage 窗口内请求失败数占比。例如超过90%请求触发失败，才允许打开熔断器。
• requestVolumeThreshold 如果熔断器要触发熔断，当前时间窗口内必须至少有requestVolumeThreshold配置的请求个数。如果errorThresholdPercentage配置了50%，requestVolumeThreshold为1，如果只有进来一个请求，如果请求失败失败，就会立即触发熔断，不合理。
• sleepWindowInMilliseconds熔断触发后，熔断时间。这段时间内都返回fallback结果。

requestVolumeThreshold，sleepWindowInMilliseconds和errorThresholdPercentage在circuitBreaker.allowRequest()里被用到。hystrix根据实际请求的状态决定是否允许执行请求。

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;
    }
    // 熔断器未打开或者熔断时间已经超过熔断时间间隔，则允许请求执行
    return !isOpen() || allowSingleTest();
}


public boolean allowSingleTest() {
    long timeCircuitOpenedOrWasLastTested = circuitOpenedOrLastTestedTime.get();
    // 如果当前熔断器已经打开，并且当前的开启时间已经超过 `sleepWindowInMilliseconds`，则允许请求，并更新`circuitOpenedOrLastTestedTime.compareAndSet`。
    // 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
    // 当前窗口请求数未达到 requestVolumeThreshold 数，不打开熔断器
    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;
    }

    // 当前窗口请求错误占比未达到 errorThresholdPercentage 值，不打开熔断器
    if (health.getErrorPercentage() < properties.circuitBreakerErrorThresholdPercentage().get()) {
        return false;
    } else {
        // 打开熔断器
        // 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;
        }
    }
}

除了以上的几个参数外，还有以下几个比较重要的参数。

hystrix:
  threadpool:
    default:
      metrics:
        rollingStats:
          timeInMilliseconds: 60000
          numBuckets: 1

timeInMilliseconds 表示窗口的时间范围，numBuckets表示桶数量，每个桶的时间为timeInMilliseconds/numBuckets,hystrix统计桶时间内错误数和错误比例来决定是否熔断。