0. 像喝点东西，但不知道喝什么

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先来段源码，看一下 我们在dashboard 录入的降级规则，都映射到哪些字段上

package com.alibaba.csp.sentinel.slots.block.degrade;

public class DegradeRule extends AbstractRule {

    public DegradeRule(String resourceName) {
        setResource(resourceName);
    }

    /**
     * Circuit breaking strategy (0: average RT, 1: exception ratio, 2: exception count).
     */
    private int grade = RuleConstant.DEGRADE_GRADE_RT;

    /**
     * Threshold count.
     */
    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.
     *
     * @since 1.7.0
     */
    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;
}

1. sentinel 的断路器实现

• 效果跟 netflix.hystrix 差不离

package com.alibaba.csp.sentinel.slots.block.degrade.circuitbreaker;

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;
    }

	// true, 成功获得令牌
    @Override
    public boolean tryPass(Context context) {
		// 断路器关闭
        // Template implementation.
        if (currentState.get() == State.CLOSED) {
            return true;
        }
		// 断路器开启
        if (currentState.get() == State.OPEN) {
			// 半开状态，允许通过1个请求来尝试，可行的话，即 true
            // For half-open state we allow a request for probing.
            return retryTimeoutArrived() && fromOpenToHalfOpen(context);
        }
        return false;
    }

	// 当前系统时间 >= 下一次重试时间
    protected boolean retryTimeoutArrived() {
        return TimeUtil.currentTimeMillis() >= nextRetryTimestamp;
    }

    protected boolean fromOpenToHalfOpen(Context context) {
        if (currentState.compareAndSet(State.OPEN, State.HALF_OPEN)) {
			// 通知订阅者: 状态的变化 开 -> 半开
            notifyObservers(State.OPEN, State.HALF_OPEN, null);
            Entry entry = context.getCurEntry();
			
			// 过程中断时的回调，回滚状态
            entry.whenTerminate(new BiConsumer<Context, Entry>() {
                @Override
                public void accept(Context context, Entry entry) {
                    // Note: This works as a temporary workaround for https://github.com/alibaba/Sentinel/issues/1638
                    // Without the hook, the circuit breaker won't recover from half-open state in some circumstances
                    // when the request is actually blocked by upcoming rules (not only degrade rules).
                    if (entry.getBlockError() != null) {
                        // Fallback to OPEN due to detecting request is blocked
                        currentState.compareAndSet(State.HALF_OPEN, State.OPEN);
                        notifyObservers(State.HALF_OPEN, State.OPEN, 1.0d);
                    }
                }
            });
            return true;
        }
        return false;
    }
    
    private void notifyObservers(CircuitBreaker.State prevState, CircuitBreaker.State newState, Double snapshotValue) {
        for (CircuitBreakerStateChangeObserver observer : observerRegistry.getStateChangeObservers()) {
            observer.onStateChange(prevState, newState, rule, snapshotValue);
        }
    }
}

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从 DegradeRule.grade 可知道:

• 默认的降级策略，即 响应时长
• 除此之外，还支持 异常 的触发方式

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下面分别借助两个 AbstractCircuitBreaker 的实现类来说明实现细节

1.1 ResponseTimeCircuitBreaker（RT）

package com.alibaba.csp.sentinel.slots.block.degrade.circuitbreaker;

// 根据请求的响应时间(慢调用比例)
public class ResponseTimeCircuitBreaker extends AbstractCircuitBreaker {

    private static final double SLOW_REQUEST_RATIO_MAX_VALUE = 1.0d;

    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 onRequestComplete(Context context) {
        SlowRequestCounter counter = slidingCounter.currentWindow().value();
        Entry entry = context.getCurEntry();
        if (entry == null) {
            return;
        }
        long completeTime = entry.getCompleteTimestamp();
        if (completeTime <= 0) {
            completeTime = TimeUtil.currentTimeMillis();
        }
        long rt = completeTime - entry.getCreateTimestamp();
        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) {
            // In detecting request
            // 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);
        }
        if (Double.compare(currentRatio, maxSlowRequestRatio) == 0 &&
                Double.compare(maxSlowRequestRatio, SLOW_REQUEST_RATIO_MAX_VALUE) == 0) {
            transformToOpen(currentRatio);
        }
    }

    static class SlowRequestCounter {
        private LongAdder slowCount;
        private LongAdder totalCount;
    }
}

1.2 ExceptionCircuitBreaker

package com.alibaba.csp.sentinel.slots.block.degrade.circuitbreaker;

// 策略:异常比例、异常数
public class ExceptionCircuitBreaker extends AbstractCircuitBreaker {

    private final int strategy;	// 策略的枚举值
    private final int minRequestAmount;	// 最小请求数
    private final double threshold;	// 设置的阈值

    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
    public void onRequestComplete(Context context) {
        Entry entry = context.getCurEntry();
        if (entry == null) {
            return;
        }
        Throwable error = entry.getError();
        SimpleErrorCounter counter = stat.currentWindow().value();
		// 异常发生了，累加
        if (error != null) {
            counter.getErrorCount().add(1);
        }
		// 总数(异常+非异常),同样累加
        counter.getTotalCount().add(1);

		// step into ...
        handleStateChangeWhenThresholdExceeded(error);
    }

    private void handleStateChangeWhenThresholdExceeded(Throwable error) {
		// 断路器早已启动? 好吧，后面不用看了
        if (currentState.get() == State.OPEN) {
            return;
        }
        
		// 半开状态，试探一下
        if (currentState.get() == State.HALF_OPEN) {
            // In detecting request
            if (error == null) {
                fromHalfOpenToClose();
            } else {
                fromHalfOpenToOpen(1.0d);
            }
            return;
        }
        
		// 把这个异常计数传播到整个时间窗(LeapArray)的计数器中
        List<SimpleErrorCounter> counters = stat.values();
        long errCount = 0;
        long totalCount = 0;
        for (SimpleErrorCounter counter : counters) {
            errCount += counter.errorCount.sum();
            totalCount += counter.totalCount.sum();
        }
		// 虽然有异常，但是比配置的最小请求数还小，那不需要使用断路器
        if (totalCount < minRequestAmount) {
            return;
        }
        double curCount = errCount;
		// 如果策略是:按照异常率的话，计算概率
        if (strategy == DEGRADE_GRADE_EXCEPTION_RATIO) {
            // Use errorRatio
            curCount = errCount * 1.0d / totalCount;
        }
		// 这里开启断路器
        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 +
                '}';
        }
    }
}