序
本文主要研究一下apache gossip的FailureDetector
FailureDetector
incubator-retired-gossip/gossip-base/src/main/java/org/apache/gossip/accrual/FailureDetector.java
public class FailureDetector {
public static final Logger LOGGER = Logger.getLogger(FailureDetector.class);
private final DescriptiveStatistics descriptiveStatistics;
private final long minimumSamples;
private volatile long latestHeartbeatMs = -1;
private final String distribution;
public FailureDetector(long minimumSamples, int windowSize, String distribution) {
descriptiveStatistics = new DescriptiveStatistics(windowSize);
this.minimumSamples = minimumSamples;
this.distribution = distribution;
}
/**
* Updates the statistics based on the delta between the last
* heartbeat and supplied time
*
* @param now the time of the heartbeat in milliseconds
*/
public synchronized void recordHeartbeat(long now) {
if (now <= latestHeartbeatMs) {
return;
}
if (latestHeartbeatMs != -1) {
descriptiveStatistics.addValue(now - latestHeartbeatMs);
}
latestHeartbeatMs = now;
}
public synchronized Double computePhiMeasure(long now) {
if (latestHeartbeatMs == -1 || descriptiveStatistics.getN() < minimumSamples) {
return null;
}
long delta = now - latestHeartbeatMs;
try {
double probability;
if (distribution.equals("normal")) {
double standardDeviation = descriptiveStatistics.getStandardDeviation();
standardDeviation = standardDeviation < 0.1 ? 0.1 : standardDeviation;
probability = new NormalDistributionImpl(descriptiveStatistics.getMean(), standardDeviation).cumulativeProbability(delta);
} else {
probability = new ExponentialDistributionImpl(descriptiveStatistics.getMean()).cumulativeProbability(delta);
}
final double eps = 1e-12;
if (1 - probability < eps) {
probability = 1.0;
}
return -1.0d * Math.log10(1.0d - probability);
} catch (MathException | IllegalArgumentException e) {
LOGGER.debug(e);
return null;
}
}
}
- FailureDetector的构造器接收三个参数，分别是minimumSamples, windowSize, distribution
- 其中minimumSamples表示最少需要多少统计值的时候才真正计算phi值，windowSize表示统计窗口的大小，distribution表示使用哪种分布，normal表示NormalDistribution，其他表示ExponentialDistribution
- FailureDetector使用了apache commons math的DescriptiveStatistics来作为Heartbeat Interval的时间窗口统计；使用了NormalDistribution、ExponentialDistribution来完成正态分布、指数分布的累积分布probability，最后使用公式-1.0d * Math.log10(1.0d - probability)来计算phi值
小结
- The Phi Accrual Failure Detector by Hayashibara et al论文提出了基于phi值的Accrual Failure Detector方法
- 业界关于Failure Detector的实现大致有两种，一种是以akka为代表的按照论文基于NormalDistribution来计算；一种是以cassandra为代表的基于ExponentialDistribution来计算
- apache gossip的FailureDetector则集大成地同时支持了NormalDistribution及ExponentialDistribution两种实现方式
