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本文主要研究一下sentinel的DegradeSlot

DegradeSlot

com/alibaba/csp/sentinel/slots/block/degrade/DegradeSlot.java

public class DegradeSlot extends AbstractLinkedProcessorSlot
    
      {    @Override    public void entry(Context context, ResourceWrapper resourceWrapper, DefaultNode node, int count, Object... args)        throws Throwable {        DegradeRuleManager.checkDegrade(resourceWrapper, context, node, count);        fireEntry(context, resourceWrapper, node, count, args);    }    @Override    public void exit(Context context, ResourceWrapper resourceWrapper, int count, Object... args) {        fireExit(context, resourceWrapper, count, args);    }}
    

• 调用DegradeRuleManager.checkDegrade进行降级规则检测

DegradeRuleManager

com/alibaba/csp/sentinel/slots/block/degrade/DegradeRuleManager.java

public class DegradeRuleManager {    private static volatile Map
    
     > degradeRules        = new ConcurrentHashMap
     
      >();    final static RulePropertyListener listener = new RulePropertyListener();    private static SentinelProperty
      
       
        > currentProperty        = new DynamicSentinelProperty
        
         
          >(); static { currentProperty.addListener(listener); } /** * Listen to the {@link SentinelProperty} for {@link DegradeRule}s. The property is the source * of {@link DegradeRule}s. Degrade rules can also be set by {@link #loadRules(List)} directly. * * @param property the property to listen. */ public static void register2Property(SentinelProperty
          
           
            > property) { synchronized (listener) { currentProperty.removeListener(listener); property.addListener(listener); currentProperty = property; } } public static void checkDegrade(ResourceWrapper resource, Context context, DefaultNode node, int count) throws BlockException { if (degradeRules == null) { return; } List
            
              rules = degradeRules.get(resource.getName()); if (rules == null) { return; } for (DegradeRule rule : rules) { if (!rule.passCheck(context, node, count)) { throw new DegradeException(rule.getLimitApp()); } } } //......}
            
           
          
         
        
       
      
     
    

• checkDegrade根据资源名称获取对应的降级规则，然后挨个遍历检查

DegradeRule

com/alibaba/csp/sentinel/slots/block/degrade/DegradeRule.java

/** * 
 * Degrade is used when the resources are in an unstable state, these resources * will be degraded within the next defined time window. There are two ways to * measure whether a resource is stable or not: * 
 * 
    
      * 
     
 * Average response time ({@code DEGRADE_GRADE_RT}): When * the average RT exceeds the threshold ('count' in 'DegradeRule', in milliseconds), the * resource enters a quasi-degraded state. If the RT of next coming 5 * requests still exceed this threshold, this resource will be downgraded, which * means that in the next time window (defined in 'timeWindow', in seconds) all the * access to this resource will be blocked. * 
 * 
     
 * Exception ratio: When the ratio of exception count per second and the * success qps exceeds the threshold, access to the resource will be blocked in * the coming window. * 
 * 
    
 * * @author jialiang.linjl */public class DegradeRule extends AbstractRule {    private static final int RT_MAX_EXCEED_N = 5;    private static ScheduledExecutorService pool = Executors.newScheduledThreadPool(        Runtime.getRuntime().availableProcessors(), new NamedThreadFactory("sentinel-degrade-reset-task", true));    /**     * RT threshold or exception ratio threshold count.     */    private double count;    /**     * Degrade recover timeout (in seconds) when degradation occurs.     */    private int timeWindow;    /**     * Degrade strategy (0: average RT, 1: exception ratio).     */    private int grade = RuleConstant.DEGRADE_GRADE_RT;    private volatile boolean cut = false;    public int getGrade() {        return grade;    }    public void setGrade(int grade) {        this.grade = grade;    }    private AtomicLong passCount = new AtomicLong(0);    private final Object lock = new Object();    //......    @Override    public boolean passCheck(Context context, DefaultNode node, int acquireCount, Object... args) {        if (cut) {            return false;        }        ClusterNode clusterNode = ClusterBuilderSlot.getClusterNode(this.getResource());        if (clusterNode == null) {            return true;        }        if (grade == RuleConstant.DEGRADE_GRADE_RT) {            double rt = clusterNode.avgRt();            if (rt < this.count) {                passCount.set(0);                return true;            }            // Sentinel will degrade the service only if count exceeds.            if (passCount.incrementAndGet() < RT_MAX_EXCEED_N) {                return true;            }        } else {            double exception = clusterNode.exceptionQps();            double success = clusterNode.successQps();            long total = clusterNode.totalQps();            // if total qps less than RT_MAX_EXCEED_N, pass.            if (total < RT_MAX_EXCEED_N) {                return true;            }            if (success == 0) {                return exception < RT_MAX_EXCEED_N;            }            if (exception / (success + exception) < count) {                return true;            }        }        synchronized (lock) {            if (!cut) {                // Automatically degrade.                cut = true;                ResetTask resetTask = new ResetTask(this);                pool.schedule(resetTask, timeWindow, TimeUnit.SECONDS);            }            return false;        }    }    //......    private static final class ResetTask implements Runnable {        private DegradeRule rule;        ResetTask(DegradeRule rule) {            this.rule = rule;        }        @Override        public void run() {            rule.getPassCount().set(0);            rule.setCut(false);        }    }}

• 这个passCheck根据平均响应时间以及异常个数等进行降级判断
• 触发降级时标志cut为true，然后启动一个ResetTask，在指定时间窗口之后重置cut为false并清空passCount计数

小结

sentinel的DegradeSlot主要依据平均响应时间以及异常次数来判断，进入降级模式时启动定时任务在指定时间窗口重置相关计数，恢复到正常模式。

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