聊聊sentinel的SystemSlot

序
本文主要研究一下 sentinel 的 SystemSlot
SystemSlot
sentinel-core/src/main/java/com/alibaba/csp/sentinel/slots/system/SystemSlot.java
public class SystemSlot extends AbstractLinkedProcessorSlot<DefaultNode> {

@Override
public void entry(Context context, ResourceWrapper resourceWrapper, DefaultNode node, int count, Object… args)
throws Throwable {
SystemRuleManager.checkSystem(resourceWrapper);
fireEntry(context, resourceWrapper, node, count, args);
}

@Override
public void exit(Context context, ResourceWrapper resourceWrapper, int count, Object… args) {
fireExit(context, resourceWrapper, count, args);
}

}
这里是通过 SystemRuleManager.checkSystem(resourceWrapper) 进行系统限流判断
SystemRuleManager
public static void checkSystem(ResourceWrapper resourceWrapper) throws BlockException {

// 确定开关开了
if (checkSystemStatus.get() == false) {
return;
}

// for inbound traffic only
if (resourceWrapper.getType() != EntryType.IN) {
return;
}

// total qps
double currentQps = Constants.ENTRY_NODE == null ? 0.0 : Constants.ENTRY_NODE.successQps();
if (currentQps > qps) {
throw new SystemBlockException(resourceWrapper.getName(), “qps”);
}

// total thread
int currentThread = Constants.ENTRY_NODE == null ? 0 : Constants.ENTRY_NODE.curThreadNum();
if (currentThread > maxThread) {
throw new SystemBlockException(resourceWrapper.getName(), “thread”);
}

double rt = Constants.ENTRY_NODE == null ? 0 : Constants.ENTRY_NODE.avgRt();
if (rt > maxRt) {
throw new SystemBlockException(resourceWrapper.getName(), “rt”);
}

// 完全按照 RT,BBR 算法来
if (highestSystemLoadIsSet && getCurrentSystemAvgLoad() > highestSystemLoad) {
if (currentThread > 1 &&
currentThread > Constants.ENTRY_NODE.maxSuccessQps() * Constants.ENTRY_NODE.minRt() / 1000) {
throw new SystemBlockException(resourceWrapper.getName(), “load”);
}
}
}

public static double getCurrentSystemAvgLoad() {
return statusListener.getSystemAverageLoad();
}
先判断 qps，在判断总线程数、之后判断 rt，最后判断系统负载有没有超过限制
StatisticNode
sentinel-core/src/main/java/com/alibaba/csp/sentinel/node/StatisticNode.java
@Override
public long successQps() {
return rollingCounterInSecond.success() / IntervalProperty.INTERVAL;
}

@Override
public int curThreadNum() {
return curThreadNum.get();
}

@Override
public long avgRt() {
long successCount = rollingCounterInSecond.success();
if (successCount == 0) {
return 0;
}

return rollingCounterInSecond.rt() / successCount;
}

@Override
public long maxSuccessQps() {
return rollingCounterInSecond.maxSuccess() * SampleCountProperty.sampleCount;
}

@Override
public long minRt() {
return rollingCounterInSecond.minRt();
}
successQps、curThreadNum、avgRt、maxSuccessQps、minRt 指标在 StatisticNode 上进行维护
SystemStatusListener
sentinel-core/src/main/java/com/alibaba/csp/sentinel/slots/system/SystemStatusListener.java
public class SystemStatusListener implements Runnable {

volatile double currentLoad = -1;

volatile String reason = StringUtil.EMPTY;

static final int processor = ManagementFactory.getOperatingSystemMXBean().getAvailableProcessors();

public double getSystemAverageLoad() {
return currentLoad;
}

@Override
public void run() {
try {
if (!SystemRuleManager.getCheckSystemStatus()) {
return;
}

// system average load
OperatingSystemMXBean operatingSystemMXBean = ManagementFactory.getOperatingSystemMXBean();
currentLoad = operatingSystemMXBean.getSystemLoadAverage();

StringBuilder sb = new StringBuilder();
if (currentLoad > SystemRuleManager.getHighestSystemLoad()) {
sb.append(“load:”).append(currentLoad).append(“;”);
sb.append(“qps:”).append(Constants.ENTRY_NODE.passQps()).append(“;”);
sb.append(“rt:”).append(Constants.ENTRY_NODE.avgRt()).append(“;”);
sb.append(“thread:”).append(Constants.ENTRY_NODE.curThreadNum()).append(“;”);
sb.append(“success:”).append(Constants.ENTRY_NODE.successQps()).append(“;”);
sb.append(“minRt:”).append(Constants.ENTRY_NODE.minRt()).append(“;”);
sb.append(“maxSuccess:”).append(Constants.ENTRY_NODE.maxSuccessQps()).append(“;”);
RecordLog.info(sb.toString());
}

} catch (Throwable e) {
RecordLog.info(“could not get system error “, e);
}
}

}
系统负载是通过 SystemRuleManager 定时调度 SystemStatusListener，通过 OperatingSystemMXBean 去获取
static {
checkSystemStatus.set(false);
statusListener = new SystemStatusListener();
scheduler.scheduleAtFixedRate(statusListener, 5, 1, TimeUnit.SECONDS);
currentProperty.addListener(listener);
}

小结
sentinel 的 SystemSlot 是通过判断系统相关指标来进行限流，主要的指标为 qps、总线程数、rt、系统负载。
doc
SystemSlot