Refactor degrade hierarchy with new circuit breaker mechanism and improve strategy

* Add `CircuitBreaker` abstraction (with half-open state) and add circuit breaker state change event observer support.
* Improve circuit breaking strategy (avg RT → slow request ratio) and make statistics of each rule dependent (to support arbitrary statistic interval).
* Add simple "trial" mechanism (aka. half-open).
* Refactor mechanism of metric recording and state change handling for circuit breakers: record RT and error when requests have completed (i.e. `onExit`, based on #1420).

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

sentinel-core/src/main/java/com/alibaba/csp/sentinel/slots/block/degrade/DegradeRule.java

@@ -15,19 +15,12 @@
  */
 package com.alibaba.csp.sentinel.slots.block.degrade;
 
-import com.alibaba.csp.sentinel.concurrent.NamedThreadFactory;
 import com.alibaba.csp.sentinel.context.Context;
-import com.alibaba.csp.sentinel.node.ClusterNode;
 import com.alibaba.csp.sentinel.node.DefaultNode;
 import com.alibaba.csp.sentinel.slots.block.AbstractRule;
 import com.alibaba.csp.sentinel.slots.block.RuleConstant;
-import com.alibaba.csp.sentinel.slots.clusterbuilder.ClusterBuilderSlot;
 
-import java.util.concurrent.Executors;
+import java.util.Objects;
-import java.util.concurrent.ScheduledExecutorService;
-import java.util.concurrent.TimeUnit;
-import java.util.concurrent.atomic.AtomicBoolean;
-import java.util.concurrent.atomic.AtomicLong;
 
 /**
  * <p>
@@ -52,13 +45,10 @@ import java.util.concurrent.atomic.AtomicLong;
  * </ul>
  *
  * @author jialiang.linjl
+ * @author Eric Zhao
  */
 public class DegradeRule extends AbstractRule {
 
-    @SuppressWarnings("PMD.ThreadPoolCreationRule")
-    private static ScheduledExecutorService pool = Executors.newScheduledThreadPool(
-        Runtime.getRuntime().availableProcessors(), new NamedThreadFactory("sentinel-degrade-reset-task", true));
-
     public DegradeRule() {}
 
     public DegradeRule(String resourceName) {
@@ -66,26 +56,20 @@ public class DegradeRule extends AbstractRule {
     }
 
     /**
-     * RT threshold or exception ratio threshold count.
+     * Circuit breaking strategy (0: average RT, 1: exception ratio, 2: exception count).
-     */
-    private double count;
-
-    /**
-     * Degrade recover timeout (in seconds) when degradation occurs.
-     */
-    private int timeWindow;
-
-    /**
-     * Degrade strategy (0: average RT, 1: exception ratio, 2: exception count).
      */
     private int grade = RuleConstant.DEGRADE_GRADE_RT;
 
     /**
-     * Minimum number of consecutive slow requests that can trigger RT circuit breaking.
+     * Threshold count.
-     *
-     * @since 1.7.0
      */
-    private int rtSlowRequestAmount = RuleConstant.DEGRADE_DEFAULT_SLOW_REQUEST_AMOUNT;
+    private double count;
+
+    /**
+     * Recovery timeout (in seconds) when circuit breaker opens. After the timeout, the circuit breaker will
+     * transform to half-open state for trying a few requests.
+     */
+    private int timeWindow;
 
     /**
      * Minimum number of requests (in an active statistic time span) that can trigger circuit breaking.
@@ -94,6 +78,13 @@ public class DegradeRule extends AbstractRule {
      */
     private int minRequestAmount = RuleConstant.DEGRADE_DEFAULT_MIN_REQUEST_AMOUNT;
 
+    /**
+     * The threshold of slow request ratio in RT mode.
+     */
+    private double slowRatioThreshold = 1.0d;
+
+    private int statIntervalMs = 1000;
+
     public int getGrade() {
         return grade;
     }
@@ -121,15 +112,6 @@ public class DegradeRule extends AbstractRule {
         return this;
     }
 
-    public int getRtSlowRequestAmount() {
-        return rtSlowRequestAmount;
-    }
-
-    public DegradeRule setRtSlowRequestAmount(int rtSlowRequestAmount) {
-        this.rtSlowRequestAmount = rtSlowRequestAmount;
-        return this;
-    }
-
     public int getMinRequestAmount() {
         return minRequestAmount;
     }
@@ -139,28 +121,42 @@ public class DegradeRule extends AbstractRule {
         return this;
     }
 
+    public double getSlowRatioThreshold() {
+        return slowRatioThreshold;
+    }
+
+    public DegradeRule setSlowRatioThreshold(double slowRatioThreshold) {
+        this.slowRatioThreshold = slowRatioThreshold;
+        return this;
+    }
+
+    public int getStatIntervalMs() {
+        return statIntervalMs;
+    }
+
+    public DegradeRule setStatIntervalMs(int statIntervalMs) {
+        this.statIntervalMs = statIntervalMs;
+        return this;
+    }
+
     @Override
     public boolean equals(Object o) {
         if (this == o) { return true; }
         if (o == null || getClass() != o.getClass()) { return false; }
         if (!super.equals(o)) { return false; }
-        DegradeRule that = (DegradeRule) o;
+        DegradeRule rule = (DegradeRule)o;
-        return Double.compare(that.count, count) == 0 &&
+        return Double.compare(rule.count, count) == 0 &&
-            timeWindow == that.timeWindow &&
+            timeWindow == rule.timeWindow &&
-            grade == that.grade &&
+            grade == rule.grade &&
-            rtSlowRequestAmount == that.rtSlowRequestAmount &&
+            minRequestAmount == rule.minRequestAmount &&
-            minRequestAmount == that.minRequestAmount;
+            Double.compare(rule.slowRatioThreshold, slowRatioThreshold) == 0 &&
+            statIntervalMs == rule.statIntervalMs;
     }
 
     @Override
     public int hashCode() {
-        int result = super.hashCode();
+        return Objects.hash(super.hashCode(), count, timeWindow, grade, minRequestAmount,
-        result = 31 * result + new Double(count).hashCode();
+            slowRatioThreshold, statIntervalMs);
-        result = 31 * result + timeWindow;
-        result = 31 * result + grade;
-        result = 31 * result + rtSlowRequestAmount;
-        result = 31 * result + minRequestAmount;
-        return result;
     }
 
     @Override
@@ -171,84 +167,15 @@ public class DegradeRule extends AbstractRule {
             ", count=" + count +
             ", limitApp=" + getLimitApp() +
             ", timeWindow=" + timeWindow +
-            ", rtSlowRequestAmount=" + rtSlowRequestAmount +
             ", minRequestAmount=" + minRequestAmount +
-            "}";
+            ", slowRatioThreshold=" + slowRatioThreshold +
-    }
+            ", statIntervalMs=" + statIntervalMs +
-
+            '}';
-    // Internal implementation (will be deprecated and moved outside).
-
-    private AtomicLong passCount = new AtomicLong(0);
-    private final AtomicBoolean cut = new AtomicBoolean(false);
-
-    @Override
-    public boolean passCheck(Context context, DefaultNode node, int acquireCount, Object... args) {
-        if (cut.get()) {
-            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() < rtSlowRequestAmount) {
-                return true;
-            }
-        } else if (grade == RuleConstant.DEGRADE_GRADE_EXCEPTION_RATIO) {
-            double exception = clusterNode.exceptionQps();
-            double success = clusterNode.successQps();
-            double total = clusterNode.totalQps();
-            // If total amount is less than minRequestAmount, the request will pass.
-            if (total < minRequestAmount) {
-                return true;
-            }
-
-            // In the same aligned statistic time window,
-            // "success" (aka. completed count) = exception count + non-exception count (realSuccess)
-            double realSuccess = success - exception;
-            if (realSuccess <= 0 && exception < minRequestAmount) {
-                return true;
-            }
-
-            if (exception / success < count) {
-                return true;
-            }
-        } else if (grade == RuleConstant.DEGRADE_GRADE_EXCEPTION_COUNT) {
-            double exception = clusterNode.totalException();
-            if (exception < count) {
-                return true;
-            }
-        }
-
-        if (cut.compareAndSet(false, 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() {
+    @Deprecated
-            rule.passCount.set(0);
+    public boolean passCheck(Context context, DefaultNode node, int count, Object... args) {
-            rule.cut.set(false);
+        return false;
-        }
     }
 }

sentinel-core/src/main/java/com/alibaba/csp/sentinel/slots/block/degrade/DegradeRuleManager.java

@@ -21,29 +21,29 @@ import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
-import java.util.concurrent.ConcurrentHashMap;
 
-import com.alibaba.csp.sentinel.config.SentinelConfig;
-import com.alibaba.csp.sentinel.context.Context;
 import com.alibaba.csp.sentinel.log.RecordLog;
-import com.alibaba.csp.sentinel.node.DefaultNode;
 import com.alibaba.csp.sentinel.property.DynamicSentinelProperty;
 import com.alibaba.csp.sentinel.property.PropertyListener;
 import com.alibaba.csp.sentinel.property.SentinelProperty;
-import com.alibaba.csp.sentinel.slotchain.ResourceWrapper;
-import com.alibaba.csp.sentinel.slots.block.BlockException;
 import com.alibaba.csp.sentinel.slots.block.RuleConstant;
+import com.alibaba.csp.sentinel.slots.block.degrade.circuitbreaker.CircuitBreaker;
+import com.alibaba.csp.sentinel.slots.block.degrade.circuitbreaker.ExceptionCircuitBreaker;
+import com.alibaba.csp.sentinel.slots.block.degrade.circuitbreaker.ResponseTimeCircuitBreaker;
 import com.alibaba.csp.sentinel.util.AssertUtil;
 import com.alibaba.csp.sentinel.util.StringUtil;
 
 /**
+ * The rule manager for circuit breaking rules ({@link DegradeRule}).
+ *
  * @author youji.zj
  * @author jialiang.linjl
  * @author Eric Zhao
  */
 public final class DegradeRuleManager {
 
-    private static final Map<String, Set<DegradeRule>> degradeRules = new ConcurrentHashMap<>();
+    private static volatile Map<String, List<CircuitBreaker>> circuitBreakers = new HashMap<>();
+    private static volatile Map<String, Set<DegradeRule>> ruleMap = new HashMap<>();
 
     private static final RulePropertyListener LISTENER = new RulePropertyListener();
     private static SentinelProperty<List<DegradeRule>> currentProperty
@@ -69,41 +69,37 @@ public final class DegradeRuleManager {
         }
     }
 
-    public static void checkDegrade(ResourceWrapper resource, Context context, DefaultNode node, int count)
+    static List<CircuitBreaker> getCircuitBreakers(String resourceName) {
-        throws BlockException {
+        return circuitBreakers.get(resourceName);
-
-        Set<DegradeRule> rules = degradeRules.get(resource.getName());
-        if (rules == null) {
-            return;
-        }
-
-        for (DegradeRule rule : rules) {
-            if (!rule.passCheck(context, node, count)) {
-                throw new DegradeException(rule.getLimitApp(), rule);
-            }
-        }
     }
 
     public static boolean hasConfig(String resource) {
         if (resource == null) {
             return false;
         }
-        return degradeRules.containsKey(resource);
+        return circuitBreakers.containsKey(resource);
     }
 
     /**
-     * Get a copy of the rules.
+     * <p>Get existing circuit breaking rules.</p>
+     * <p>Note: DO NOT modify the rules from the returned list directly.
+     * The behavior is <strong>undefined</strong>.</p>
      *
-     * @return a new copy of the rules.
+     * @return list of existing circuit breaking rules, or empty list if no rules were loaded
      */
     public static List<DegradeRule> getRules() {
         List<DegradeRule> rules = new ArrayList<>();
-        for (Map.Entry<String, Set<DegradeRule>> entry : degradeRules.entrySet()) {
+        for (Map.Entry<String, Set<DegradeRule>> entry : ruleMap.entrySet()) {
             rules.addAll(entry.getValue());
         }
         return rules;
     }
 
+    public static Set<DegradeRule> getRulesOfResource(String resource) {
+        AssertUtil.assertNotBlank(resource, "resource name cannot be blank");
+        return ruleMap.get(resource);
+    }
+
     /**
      * Load {@link DegradeRule}s, former rules will be replaced.
      *
@@ -113,7 +109,7 @@ public final class DegradeRuleManager {
         try {
             currentProperty.updateValue(rules);
         } catch (Throwable e) {
-            RecordLog.warn("[DegradeRuleManager] Unexpected error when loading degrade rules", e);
+            RecordLog.error("[DegradeRuleManager] Unexpected error when loading degrade rules", e);
         }
     }
 
@@ -128,7 +124,7 @@ public final class DegradeRuleManager {
     public static boolean setRulesForResource(String resourceName, Set<DegradeRule> rules) {
         AssertUtil.notEmpty(resourceName, "resourceName cannot be empty");
         try {
-            Map<String, Set<DegradeRule>> newRuleMap = new HashMap<>(degradeRules);
+            Map<String, Set<DegradeRule>> newRuleMap = new HashMap<>(ruleMap);
             if (rules == null) {
                 newRuleMap.remove(resourceName);
             } else {
@@ -146,62 +142,41 @@ public final class DegradeRuleManager {
             }
             return currentProperty.updateValue(allRules);
         } catch (Throwable e) {
-            RecordLog.warn(
+            RecordLog.error("[DegradeRuleManager] Unexpected error when setting circuit breaking"
-                "[DegradeRuleManager] Unexpected error when setting degrade rules for resource: " + resourceName, e);
+                + " rules for resource: " + resourceName, e);
             return false;
         }
     }
 
-    private static class RulePropertyListener implements PropertyListener<List<DegradeRule>> {
+    private static CircuitBreaker getExistingSameCbOrNew(/*@Valid*/ DegradeRule rule) {
-
+        List<CircuitBreaker> cbs = getCircuitBreakers(rule.getResource());
-        @Override
+        if (cbs == null || cbs.isEmpty()) {
-        public void configUpdate(List<DegradeRule> conf) {
+            return newCircuitBreakerFrom(rule);
-            Map<String, Set<DegradeRule>> rules = loadDegradeConf(conf);
-            if (rules != null) {
-                degradeRules.clear();
-                degradeRules.putAll(rules);
         }
-            RecordLog.info("[DegradeRuleManager] Degrade rules received: " + degradeRules);
+        for (CircuitBreaker cb : cbs) {
+            if (rule.equals(cb.getRule())) {
+                // Reuse the circuit breaker if the rule remains unchanged.
+                return cb;
+            }
+        }
+        return newCircuitBreakerFrom(rule);
     }
 
-        @Override
+    /**
-        public void configLoad(List<DegradeRule> conf) {
+     * Create a circuit breaker instance from provided circuit breaking rule.
-            Map<String, Set<DegradeRule>> rules = loadDegradeConf(conf);
+     *
-            if (rules != null) {
+     * @param rule a valid circuit breaking rule
-                degradeRules.clear();
+     * @return new circuit breaker based on provided rule; null if rule is invalid or unsupported type
-                degradeRules.putAll(rules);
+     */
-            }
+    private static CircuitBreaker newCircuitBreakerFrom(/*@Valid*/ DegradeRule rule) {
-            RecordLog.info("[DegradeRuleManager] Degrade rules loaded: " + degradeRules);
+        switch (rule.getGrade()) {
-        }
+            case RuleConstant.DEGRADE_GRADE_RT:
-
+                return new ResponseTimeCircuitBreaker(rule);
-        private Map<String, Set<DegradeRule>> loadDegradeConf(List<DegradeRule> list) {
+            case RuleConstant.DEGRADE_GRADE_EXCEPTION_RATIO:
-            Map<String, Set<DegradeRule>> newRuleMap = new ConcurrentHashMap<>();
+            case RuleConstant.DEGRADE_GRADE_EXCEPTION_COUNT:
-
+                return new ExceptionCircuitBreaker(rule);
-            if (list == null || list.isEmpty()) {
+            default:
-                return newRuleMap;
+                return null;
-            }
-
-            for (DegradeRule rule : list) {
-                if (!isValidRule(rule)) {
-                    RecordLog.warn(
-                        "[DegradeRuleManager] Ignoring invalid degrade rule when loading new rules: " + rule);
-                    continue;
-                }
-
-                if (StringUtil.isBlank(rule.getLimitApp())) {
-                    rule.setLimitApp(RuleConstant.LIMIT_APP_DEFAULT);
-                }
-
-                String identity = rule.getResource();
-                Set<DegradeRule> ruleSet = newRuleMap.get(identity);
-                if (ruleSet == null) {
-                    ruleSet = new HashSet<>();
-                    newRuleMap.put(identity, ruleSet);
-                }
-                ruleSet.add(rule);
-            }
-
-            return newRuleMap;
         }
     }
 
@@ -211,23 +186,83 @@ public final class DegradeRuleManager {
         if (!baseValid) {
             return false;
         }
-        int maxAllowedRt = SentinelConfig.statisticMaxRt();
+        if (rule.getMinRequestAmount() <= 0 || rule.getStatIntervalMs() <= 0) {
-        if (rule.getGrade() == RuleConstant.DEGRADE_GRADE_RT) {
-            if (rule.getRtSlowRequestAmount() <= 0) {
             return false;
         }
-            // Warn for RT mode that exceeds the {@code TIME_DROP_VALVE}.
+        switch (rule.getGrade()) {
-            if (rule.getCount() > maxAllowedRt) {
+            case RuleConstant.DEGRADE_GRADE_RT:
-                RecordLog.warn(String.format("[DegradeRuleManager] WARN: setting large RT threshold (%.1f ms)"
+                return rule.getSlowRatioThreshold() >= 0 && rule.getSlowRatioThreshold() <= 1;
-                        + " in RT mode will not take effect since it exceeds the max allowed value (%d ms)",
+            case RuleConstant.DEGRADE_GRADE_EXCEPTION_RATIO:
-                    rule.getCount(), maxAllowedRt));
+                return rule.getCount() <= 1;
+            case RuleConstant.DEGRADE_GRADE_EXCEPTION_COUNT:
+                return true;
+            default:
+                return false;
         }
     }
 
-        // Check exception ratio mode.
+    private static class RulePropertyListener implements PropertyListener<List<DegradeRule>> {
-        if (rule.getGrade() == RuleConstant.DEGRADE_GRADE_EXCEPTION_RATIO) {
+
-            return rule.getCount() <= 1 && rule.getMinRequestAmount() > 0;
+        private synchronized void reloadFrom(List<DegradeRule> list) {
+            Map<String, List<CircuitBreaker>> cbs = buildCircuitBreakers(list);
+            Map<String, Set<DegradeRule>> rm = new HashMap<>(cbs.size());
+
+            for (Map.Entry<String, List<CircuitBreaker>> e : cbs.entrySet()) {
+                assert e.getValue() != null && !e.getValue().isEmpty();
+
+                Set<DegradeRule> rules = new HashSet<>(e.getValue().size());
+                for (CircuitBreaker cb : e.getValue()) {
+                    rules.add(cb.getRule());
+                }
+                rm.put(e.getKey(), rules);
+            }
+
+            DegradeRuleManager.circuitBreakers = cbs;
+            DegradeRuleManager.ruleMap = rm;
+        }
+
+        @Override
+        public void configUpdate(List<DegradeRule> conf) {
+            reloadFrom(conf);
+            RecordLog.info("[DegradeRuleManager] Degrade rules has been updated to: " + ruleMap);
+        }
+
+        @Override
+        public void configLoad(List<DegradeRule> conf) {
+            reloadFrom(conf);
+            RecordLog.info("[DegradeRuleManager] Degrade rules loaded: " + ruleMap);
+        }
+
+        private Map<String, List<CircuitBreaker>> buildCircuitBreakers(List<DegradeRule> list) {
+            Map<String, List<CircuitBreaker>> cbMap = new HashMap<>(8);
+            if (list == null || list.isEmpty()) {
+                return cbMap;
+            }
+            for (DegradeRule rule : list) {
+                if (!isValidRule(rule)) {
+                    RecordLog.warn("[DegradeRuleManager] Ignoring invalid rule when loading new rules: " + rule);
+                    continue;
+                }
+
+                if (StringUtil.isBlank(rule.getLimitApp())) {
+                    rule.setLimitApp(RuleConstant.LIMIT_APP_DEFAULT);
+                }
+                CircuitBreaker cb = getExistingSameCbOrNew(rule);
+                if (cb == null) {
+                    RecordLog.warn("[DegradeRuleManager] Unknown circuit breaking strategy, ignoring: " + rule);
+                    continue;
+                }
+
+                String resourceName = rule.getResource();
+
+                List<CircuitBreaker> cbList = cbMap.get(resourceName);
+                if (cbList == null) {
+                    cbList = new ArrayList<>();
+                    cbMap.put(resourceName, cbList);
+                }
+                cbList.add(cb);
+            }
+            return cbMap;
         }
-        return true;
     }
 }

sentinel-core/src/main/java/com/alibaba/csp/sentinel/slots/block/degrade/DegradeSlot.java

@@ -15,30 +15,73 @@
  */
 package com.alibaba.csp.sentinel.slots.block.degrade;
 
+import java.util.List;
+
+import com.alibaba.csp.sentinel.Entry;
 import com.alibaba.csp.sentinel.context.Context;
 import com.alibaba.csp.sentinel.node.DefaultNode;
 import com.alibaba.csp.sentinel.slotchain.AbstractLinkedProcessorSlot;
 import com.alibaba.csp.sentinel.slotchain.ProcessorSlot;
 import com.alibaba.csp.sentinel.slotchain.ResourceWrapper;
+import com.alibaba.csp.sentinel.slots.block.BlockException;
+import com.alibaba.csp.sentinel.slots.block.degrade.circuitbreaker.CircuitBreaker;
 import com.alibaba.csp.sentinel.spi.SpiOrder;
+import com.alibaba.csp.sentinel.util.TimeUtil;
 
 /**
- * A {@link ProcessorSlot} dedicates to {@link DegradeRule} checking.
+ * A {@link ProcessorSlot} dedicates to circuit breaking.
  *
- * @author leyou
+ * @author Carpenter Lee
+ * @author Eric Zhao
  */
 @SpiOrder(-1000)
 public class DegradeSlot extends AbstractLinkedProcessorSlot<DefaultNode> {
 
     @Override
-    public void entry(Context context, ResourceWrapper resourceWrapper, DefaultNode node, int count, boolean prioritized, Object... args)
+    public void entry(Context context, ResourceWrapper resourceWrapper, DefaultNode node, int count,
-        throws Throwable {
+                      boolean prioritized, Object... args) throws Throwable {
-        DegradeRuleManager.checkDegrade(resourceWrapper, context, node, count);
+        performChecking(resourceWrapper);
+
         fireEntry(context, resourceWrapper, node, count, prioritized, args);
     }
 
+    void performChecking(ResourceWrapper r) throws BlockException {
+        List<CircuitBreaker> circuitBreakers = DegradeRuleManager.getCircuitBreakers(r.getName());
+        if (circuitBreakers == null || circuitBreakers.isEmpty()) {
+            return;
+        }
+        for (CircuitBreaker cb : circuitBreakers) {
+            if (!cb.tryPass()) {
+                throw new DegradeException(cb.getRule().getLimitApp(), cb.getRule());
+            }
+        }
+    }
+
     @Override
-    public void exit(Context context, ResourceWrapper resourceWrapper, int count, Object... args) {
+    public void exit(Context context, ResourceWrapper r, int count, Object... args) {
-        fireExit(context, resourceWrapper, count, args);
+        Entry curEntry = context.getCurEntry();
+        if (curEntry.getBlockError() != null) {
+            fireExit(context, r, count, args);
+            return;
+        }
+        List<CircuitBreaker> circuitBreakers = DegradeRuleManager.getCircuitBreakers(r.getName());
+        if (circuitBreakers == null || circuitBreakers.isEmpty()) {
+            fireExit(context, r, count, args);
+            return;
+        }
+
+        if (curEntry.getBlockError() == null) {
+            long completeTime = curEntry.getCompleteTimestamp();
+            if (completeTime <= 0) {
+                completeTime = TimeUtil.currentTimeMillis();
+            }
+            long rt = completeTime - curEntry.getCreateTimestamp();
+            Throwable error = curEntry.getError();
+            for (CircuitBreaker circuitBreaker : circuitBreakers) {
+                circuitBreaker.onRequestComplete(rt, error);
+            }
+        }
+
+        fireExit(context, r, count, args);
     }
 }

sentinel-core/src/main/java/com/alibaba/csp/sentinel/slots/block/degrade/circuitbreaker/AbstractCircuitBreaker.java

@@ -0,0 +1,147 @@
+/*
+ * Copyright 1999-2019 Alibaba Group Holding Ltd.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.alibaba.csp.sentinel.slots.block.degrade.circuitbreaker;
+
+import java.util.concurrent.atomic.AtomicReference;
+
+import com.alibaba.csp.sentinel.slots.block.degrade.DegradeRule;
+import com.alibaba.csp.sentinel.slots.block.degrade.DegradeRuleManager;
+import com.alibaba.csp.sentinel.util.AssertUtil;
+import com.alibaba.csp.sentinel.util.TimeUtil;
+
+/**
+ * @author Eric Zhao
+ * @since 1.8.0
+ */
+public abstract class AbstractCircuitBreaker implements CircuitBreaker {
+
+    protected final DegradeRule rule;
+    protected final int recoveryTimeoutMs;
+
+    private final EventObserverRegistry observerRegistry;
+
+    protected final AtomicReference<State> currentState = new AtomicReference<>(State.CLOSED);
+    protected volatile long nextRetryTimestamp;
+
+    public AbstractCircuitBreaker(DegradeRule rule) {
+        this(rule, EventObserverRegistry.getInstance());
+    }
+
+    AbstractCircuitBreaker(DegradeRule rule, EventObserverRegistry observerRegistry) {
+        AssertUtil.notNull(observerRegistry, "observerRegistry cannot be null");
+        if (!DegradeRuleManager.isValidRule(rule)) {
+            throw new IllegalArgumentException("Invalid DegradeRule: " + rule);
+        }
+        this.observerRegistry = observerRegistry;
+        this.rule = rule;
+        this.recoveryTimeoutMs = rule.getTimeWindow() * 1000;
+    }
+
+    @Override
+    public DegradeRule getRule() {
+        return rule;
+    }
+
+    @Override
+    public State currentState() {
+        return currentState.get();
+    }
+
+    @Override
+    public boolean tryPass() {
+        // Template implementation.
+        if (currentState.get() == State.CLOSED) {
+            return true;
+        }
+        if (currentState.get() == State.OPEN) {
+            // For half-open state we allow a request for trial.
+            return retryTimeoutArrived() && fromOpenToHalfOpen();
+        }
+        return false;
+    }
+
+    /**
+     * Reset the statistic data.
+     */
+    abstract void resetStat();
+
+    protected boolean retryTimeoutArrived() {
+        return TimeUtil.currentTimeMillis() >= nextRetryTimestamp;
+    }
+
+    protected void updateNextRetryTimestamp() {
+        this.nextRetryTimestamp = TimeUtil.currentTimeMillis() + recoveryTimeoutMs;
+    }
+
+    protected boolean fromCloseToOpen(double snapshotValue) {
+        State prev = State.CLOSED;
+        if (currentState.compareAndSet(prev, State.OPEN)) {
+            updateNextRetryTimestamp();
+
+            for (CircuitBreakerStateChangeObserver observer : observerRegistry.getStateChangeObservers()) {
+                observer.onStateChange(prev, State.OPEN, rule, snapshotValue);
+            }
+            return true;
+        }
+        return false;
+    }
+
+    protected boolean fromOpenToHalfOpen() {
+        if (currentState.compareAndSet(State.OPEN, State.HALF_OPEN)) {
+            for (CircuitBreakerStateChangeObserver observer : observerRegistry.getStateChangeObservers()) {
+                observer.onStateChange(State.OPEN, State.HALF_OPEN, rule, null);
+            }
+            return true;
+        }
+        return false;
+    }
+
+    protected boolean fromHalfOpenToOpen(double snapshotValue) {
+        if (currentState.compareAndSet(State.HALF_OPEN, State.OPEN)) {
+            updateNextRetryTimestamp();
+            for (CircuitBreakerStateChangeObserver observer : observerRegistry.getStateChangeObservers()) {
+                observer.onStateChange(State.HALF_OPEN, State.OPEN, rule, snapshotValue);
+            }
+            return true;
+        }
+        return false;
+    }
+
+    protected boolean fromHalfOpenToClose() {
+        if (currentState.compareAndSet(State.HALF_OPEN, State.CLOSED)) {
+            resetStat();
+            for (CircuitBreakerStateChangeObserver observer : observerRegistry.getStateChangeObservers()) {
+                observer.onStateChange(State.HALF_OPEN, State.CLOSED, rule, null);
+            }
+            return true;
+        }
+        return false;
+    }
+
+    protected void transformToOpen(double triggerValue) {
+        State cs = currentState.get();
+        switch (cs) {
+            case CLOSED:
+                fromCloseToOpen(triggerValue);
+                break;
+            case HALF_OPEN:
+                fromHalfOpenToOpen(triggerValue);
+                break;
+            default:
+                break;
+        }
+    }
+}

sentinel-core/src/main/java/com/alibaba/csp/sentinel/slots/block/degrade/circuitbreaker/CircuitBreaker.java

@@ -0,0 +1,79 @@
+/*
+ * Copyright 1999-2019 Alibaba Group Holding Ltd.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.alibaba.csp.sentinel.slots.block.degrade.circuitbreaker;
+
+import com.alibaba.csp.sentinel.slots.block.degrade.DegradeRule;
+
+/**
+ * <p>Basic <a href="https://martinfowler.com/bliki/CircuitBreaker.html">circuit breaker</a> interface.</p>
+ *
+ * @author Eric Zhao
+ */
+public interface CircuitBreaker {
+
+    /**
+     * Get the associated circuit breaking rule.
+     *
+     * @return associated circuit breaking rule
+     */
+    DegradeRule getRule();
+
+    /**
+     * Acquires permission of an invocation only if it is available at the time of invocation.
+     *
+     * @return {@code true} if permission was acquired and {@code false} otherwise
+     */
+    boolean tryPass();
+
+    /**
+     * Get current state of the circuit breaker.
+     *
+     * @return current state of the circuit breaker
+     */
+    State currentState();
+
+    /**
+     * Record a completed request with the given response time and error (if present) and
+     * handle state transformation of the circuit breaker.
+     *
+     * @param rt the response time of this entry
+     * @param error the error of this entry (if present)
+     */
+    void onRequestComplete(long rt, Throwable error);
+
+    /**
+     * Circuit breaker state.
+     */
+    enum State {
+        /**
+         * In {@code OPEN} state, all requests will be rejected until the next recovery time point.
+         */
+        OPEN,
+        /**
+         * In {@code HALF_OPEN} state, the circuit breaker will allow a "probe" invocation.
+         * If the invocation is abnormal according to the strategy (e.g. it's slow), the circuit breaker
+         * will re-transform to the {@code OPEN} state and wait for the next recovery time point;
+         * otherwise the resource will be regarded as "recovered" and the circuit breaker
+         * will cease cutting off requests and transform to {@code CLOSED} state.
+         */
+        HALF_OPEN,
+        /**
+         * In {@code CLOSED} state, all requests are permitted. When current metric value exceeds the threshold,
+         * the circuit breaker will transform to {@code OPEN} state.
+         */
+        CLOSED
+    }
+}

sentinel-core/src/main/java/com/alibaba/csp/sentinel/slots/block/degrade/circuitbreaker/CircuitBreakerStateChangeObserver.java

@@ -0,0 +1,42 @@
+/*
+ * Copyright 1999-2019 Alibaba Group Holding Ltd.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.alibaba.csp.sentinel.slots.block.degrade.circuitbreaker;
+
+import com.alibaba.csp.sentinel.slots.block.degrade.DegradeRule;
+
+/**
+ * @author Eric Zhao
+ * @since 1.8.0
+ */
+public interface CircuitBreakerStateChangeObserver {
+
+    /**
+     * <p>Observer method triggered when circuit breaker state changed. The transformation could be:</p>
+     * <ul>
+     * <li>From {@code CLOSED} to {@code OPEN} (with the triggered metric)</li>
+     * <li>From {@code OPEN} to {@code HALF_OPEN}</li>
+     * <li>From {@code OPEN} to {@code CLOSED}</li>
+     * <li>From {@code HALF_OPEN} to {@code OPEN} (with the triggered metric)</li>
+     * </ul>
+     *
+     * @param prevState     previous state of the circuit breaker
+     * @param newState      new state of the circuit breaker
+     * @param rule          associated rule
+     * @param snapshotValue triggered value on circuit breaker opens (null if the new state is CLOSED or HALF_OPEN)
+     */
+    void onStateChange(CircuitBreaker.State prevState, CircuitBreaker.State newState, DegradeRule rule,
+                       Double snapshotValue);
+}

sentinel-core/src/main/java/com/alibaba/csp/sentinel/slots/block/degrade/circuitbreaker/CircuitBreakerStrategy.java

@@ -0,0 +1,46 @@
+/*
+ * Copyright 1999-2020 Alibaba Group Holding Ltd.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.alibaba.csp.sentinel.slots.block.degrade.circuitbreaker;
+
+/**
+ * @author Eric Zhao
+ * @since 1.8.0
+ */
+public enum CircuitBreakerStrategy {
+
+    /**
+     * Circuit breaker opens (cuts off) when slow request ratio exceeds the threshold.
+     */
+    SLOW_REQUEST_RATIO(0),
+    /**
+     * Circuit breaker opens (cuts off) when error ratio exceeds the threshold.
+     */
+    ERROR_RATIO(1),
+    /**
+     * Circuit breaker opens (cuts off) when error count exceeds the threshold.
+     */
+    ERROR_COUNT(2);
+
+    private int type;
+
+    CircuitBreakerStrategy(int type) {
+        this.type = type;
+    }
+
+    public int getType() {
+        return type;
+    }
+}

sentinel-core/src/main/java/com/alibaba/csp/sentinel/slots/block/degrade/circuitbreaker/EventObserverRegistry.java

@@ -0,0 +1,70 @@
+/*
+ * Copyright 1999-2020 Alibaba Group Holding Ltd.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.alibaba.csp.sentinel.slots.block.degrade.circuitbreaker;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+import com.alibaba.csp.sentinel.util.AssertUtil;
+
+/**
+ * <p>Registry for circuit breaker event observers.</p>
+ *
+ * @author Eric Zhao
+ * @since 1.8.0
+ */
+public class EventObserverRegistry {
+
+    private final Map<String, CircuitBreakerStateChangeObserver> stateChangeObserverMap = new HashMap<>();
+
+    /**
+     * Register a circuit breaker state change observer.
+     *
+     * @param name observer name
+     * @param observer a valid observer
+     */
+    public void addStateChangeObserver(String name, CircuitBreakerStateChangeObserver observer) {
+        AssertUtil.notNull(name, "name cannot be null");
+        AssertUtil.notNull(observer, "observer cannot be null");
+        stateChangeObserverMap.put(name, observer);
+    }
+
+    public boolean removeStateChangeObserver(String name) {
+        AssertUtil.notNull(name, "name cannot be null");
+        return stateChangeObserverMap.remove(name) != null;
+    }
+
+    /**
+     * Get all registered state chane observers.
+     *
+     * @return all registered state chane observers
+     */
+    public List<CircuitBreakerStateChangeObserver> getStateChangeObservers() {
+        return new ArrayList<>(stateChangeObserverMap.values());
+    }
+
+    public static EventObserverRegistry getInstance() {
+        return InstanceHolder.instance;
+    }
+
+    private static class InstanceHolder {
+        private static EventObserverRegistry instance = new EventObserverRegistry();
+    }
+
+    EventObserverRegistry() {}
+}

sentinel-core/src/main/java/com/alibaba/csp/sentinel/slots/block/degrade/circuitbreaker/ExceptionCircuitBreaker.java

@@ -0,0 +1,156 @@
+/*
+ * Copyright 1999-2019 Alibaba Group Holding Ltd.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.alibaba.csp.sentinel.slots.block.degrade.circuitbreaker;
+
+import java.util.List;
+
+import com.alibaba.csp.sentinel.slots.block.degrade.DegradeRule;
+import com.alibaba.csp.sentinel.slots.statistic.base.LeapArray;
+import com.alibaba.csp.sentinel.slots.statistic.base.LongAdder;
+import com.alibaba.csp.sentinel.slots.statistic.base.WindowWrap;
+import com.alibaba.csp.sentinel.util.AssertUtil;
+
+import static com.alibaba.csp.sentinel.slots.block.RuleConstant.DEGRADE_GRADE_EXCEPTION_RATIO;
+import static com.alibaba.csp.sentinel.slots.block.RuleConstant.DEGRADE_GRADE_EXCEPTION_COUNT;
+
+/**
+ * @author Eric Zhao
+ * @since 1.8.0
+ */
+public class ExceptionCircuitBreaker extends AbstractCircuitBreaker {
+
+    private final int strategy;
+    private final int minRequestAmount;
+    private final double threshold;
+
+    private final LeapArray<SimpleErrorCounter> stat;
+
+    public ExceptionCircuitBreaker(DegradeRule rule) {
+        this(rule, new SimpleErrorCounterLeapArray(1, rule.getStatIntervalMs()));
+    }
+
+    ExceptionCircuitBreaker(DegradeRule rule, LeapArray<SimpleErrorCounter> stat) {
+        super(rule);
+        this.strategy = rule.getGrade();
+        boolean modeOk = strategy == DEGRADE_GRADE_EXCEPTION_RATIO || strategy == DEGRADE_GRADE_EXCEPTION_COUNT;
+        AssertUtil.isTrue(modeOk, "rule strategy should be error-ratio or error-count");
+        AssertUtil.notNull(stat, "stat cannot be null");
+        this.minRequestAmount = rule.getMinRequestAmount();
+        this.threshold = rule.getCount();
+        this.stat = stat;
+    }
+
+    @Override
+    protected void resetStat() {
+        // Reset current bucket (bucket count = 1).
+        stat.currentWindow().value().reset();
+    }
+
+    @Override
+    public void onRequestComplete(long rt, Throwable error) {
+        SimpleErrorCounter counter = stat.currentWindow().value();
+        if (error != null) {
+            counter.getErrorCount().add(1);
+        }
+        counter.getTotalCount().add(1);
+
+        handleStateChangeWhenThresholdExceeded(error);
+    }
+
+    private void handleStateChangeWhenThresholdExceeded(Throwable error) {
+        if (currentState.get() == State.OPEN) {
+            return;
+        }
+        if (currentState.get() == State.HALF_OPEN) {
+            if (error == null) {
+                fromHalfOpenToClose();
+            } else {
+                fromHalfOpenToOpen(1.0d);
+            }
+            return;
+        }
+        List<SimpleErrorCounter> counters = stat.values();
+        long errCount = 0;
+        long totalCount = 0;
+        for (SimpleErrorCounter counter : counters) {
+            errCount += counter.errorCount.sum();
+            totalCount += counter.totalCount.sum();
+        }
+        if (totalCount < minRequestAmount) {
+            return;
+        }
+        double curCount = errCount;
+        if (strategy == DEGRADE_GRADE_EXCEPTION_RATIO) {
+            // Use errorRatio
+            curCount = errCount * 1.0d / totalCount;
+        }
+        if (curCount > threshold) {
+            transformToOpen(curCount);
+        }
+    }
+
+    static class SimpleErrorCounter {
+        private LongAdder errorCount;
+        private LongAdder totalCount;
+
+        public SimpleErrorCounter() {
+            this.errorCount = new LongAdder();
+            this.totalCount = new LongAdder();
+        }
+
+        public LongAdder getErrorCount() {
+            return errorCount;
+        }
+
+        public LongAdder getTotalCount() {
+            return totalCount;
+        }
+
+        public SimpleErrorCounter reset() {
+            errorCount.reset();
+            totalCount.reset();
+            return this;
+        }
+
+        @Override
+        public String toString() {
+            return "SimpleErrorCounter{" +
+                "errorCount=" + errorCount +
+                ", totalCount=" + totalCount +
+                '}';
+        }
+    }
+
+    static class SimpleErrorCounterLeapArray extends LeapArray<SimpleErrorCounter> {
+
+        public SimpleErrorCounterLeapArray(int sampleCount, int intervalInMs) {
+            super(sampleCount, intervalInMs);
+        }
+
+        @Override
+        public SimpleErrorCounter newEmptyBucket(long timeMillis) {
+            return new SimpleErrorCounter();
+        }
+
+        @Override
+        protected WindowWrap<SimpleErrorCounter> resetWindowTo(WindowWrap<SimpleErrorCounter> w, long startTime) {
+            // Update the start time and reset value.
+            w.resetTo(startTime);
+            w.value().reset();
+            return w;
+        }
+    }
+}

sentinel-core/src/main/java/com/alibaba/csp/sentinel/slots/block/degrade/circuitbreaker/ResponseTimeCircuitBreaker.java

@@ -0,0 +1,150 @@
+/*
+ * Copyright 1999-2019 Alibaba Group Holding Ltd.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.alibaba.csp.sentinel.slots.block.degrade.circuitbreaker;
+
+import java.util.List;
+
+import com.alibaba.csp.sentinel.slots.block.RuleConstant;
+import com.alibaba.csp.sentinel.slots.block.degrade.DegradeRule;
+import com.alibaba.csp.sentinel.slots.statistic.base.LeapArray;
+import com.alibaba.csp.sentinel.slots.statistic.base.LongAdder;
+import com.alibaba.csp.sentinel.slots.statistic.base.WindowWrap;
+import com.alibaba.csp.sentinel.util.AssertUtil;
+
+/**
+ * @author Eric Zhao
+ * @since 1.8.0
+ */
+public class ResponseTimeCircuitBreaker extends AbstractCircuitBreaker {
+
+    private final long maxAllowedRt;
+    private final double maxSlowRequestRatio;
+    private final int minRequestAmount;
+
+    private final LeapArray<SlowRequestCounter> slidingCounter;
+
+    public ResponseTimeCircuitBreaker(DegradeRule rule) {
+        this(rule, new SlowRequestLeapArray(1, rule.getStatIntervalMs()));
+    }
+
+    ResponseTimeCircuitBreaker(DegradeRule rule, LeapArray<SlowRequestCounter> stat) {
+        super(rule);
+        AssertUtil.isTrue(rule.getGrade() == RuleConstant.DEGRADE_GRADE_RT, "rule metric type should be RT");
+        AssertUtil.notNull(stat, "stat cannot be null");
+        this.maxAllowedRt = Math.round(rule.getCount());
+        this.maxSlowRequestRatio = rule.getSlowRatioThreshold();
+        this.minRequestAmount = rule.getMinRequestAmount();
+        this.slidingCounter = stat;
+    }
+
+    @Override
+    public void resetStat() {
+        // Reset current bucket (bucket count = 1).
+        slidingCounter.currentWindow().value().reset();
+    }
+
+    @Override
+    public void onRequestComplete(long rt, Throwable error) {
+        SlowRequestCounter counter = slidingCounter.currentWindow().value();
+        if (rt > maxAllowedRt) {
+            counter.slowCount.add(1);
+        }
+        counter.totalCount.add(1);
+
+        handleStateChangeWhenThresholdExceeded(rt);
+    }
+
+    private void handleStateChangeWhenThresholdExceeded(long rt) {
+        if (currentState.get() == State.OPEN) {
+            return;
+        }
+        if (currentState.get() == State.HALF_OPEN) {
+            // TODO: improve logic for half-open recovery
+            if (rt > maxAllowedRt) {
+                fromHalfOpenToOpen(1.0d);
+            } else {
+                fromHalfOpenToClose();
+            }
+            return;
+        }
+
+        List<SlowRequestCounter> counters = slidingCounter.values();
+        long slowCount = 0;
+        long totalCount = 0;
+        for (SlowRequestCounter counter : counters) {
+            slowCount += counter.slowCount.sum();
+            totalCount += counter.totalCount.sum();
+        }
+        if (totalCount < minRequestAmount) {
+            return;
+        }
+        double currentRatio = slowCount * 1.0d / totalCount;
+        if (currentRatio > maxSlowRequestRatio) {
+            transformToOpen(currentRatio);
+        }
+    }
+
+    static class SlowRequestCounter {
+        private LongAdder slowCount;
+        private LongAdder totalCount;
+
+        public SlowRequestCounter() {
+            this.slowCount = new LongAdder();
+            this.totalCount = new LongAdder();
+        }
+
+        public LongAdder getSlowCount() {
+            return slowCount;
+        }
+
+        public LongAdder getTotalCount() {
+            return totalCount;
+        }
+
+        public SlowRequestCounter reset() {
+            slowCount.reset();
+            totalCount.reset();
+            return this;
+        }
+
+        @Override
+        public String toString() {
+            return "SlowRequestCounter{" +
+                "slowCount=" + slowCount +
+                ", totalCount=" + totalCount +
+                '}';
+        }
+    }
+
+    static class SlowRequestLeapArray extends LeapArray<SlowRequestCounter> {
+
+        public SlowRequestLeapArray(int sampleCount, int intervalInMs) {
+            super(sampleCount, intervalInMs);
+        }
+
+        @Override
+        public SlowRequestCounter newEmptyBucket(long timeMillis) {
+            return new SlowRequestCounter();
+        }
+
+        @Override
+        protected WindowWrap<SlowRequestCounter> resetWindowTo(WindowWrap<SlowRequestCounter> w, long startTime) {
+            w.resetTo(startTime);
+            w.value().reset();
+            return w;
+        }
+    }
+}