Hystrix运行原理

构造一个HystrixCommand或HystrixObservableCommand对象
执行命令。
检查是否已命中缓存，如果命中直接返回。
检查断路器开关是否打开，如果打开，直接熔断，走fallback逻辑。
检查线程池/队列/信号量是否已满，如果已满，直接拒绝请求，走fallback逻辑。
上面条件都不满足，调用HystrixObservableCommand.construct()方法HystrixCommand.run()方法，执行业务逻辑。
判断运行业务逻辑方法是否出现异常或者超时，如果出现，直接降级，走fallback逻辑。
上报统计数据，用户计算断路器状态。
返回结果

从流程图可以发现，只有出现5和7两种情况时，才会上报错误统计数据。

断路器运行原理

断路器的开关控制逻辑如下：

***在一个统计时间窗口内（HystrixCommandProperties.metricsRollingStatisticalWindowInMilliseconds()）***，处理的请求数量达到设定的最小阈值（HystrixCommandProperties.circuitBreakerRequestVolumeThreshold()），并且错误百分比超过设定的最大阈值（HystrixCommandProperties.circuitBreakerErrorThresholdPercentage()），这时断路器开关就会打开，断路器状态从转换CLOSED切换为OPEN。
当断路器为打开状态时，它他会直接熔断所有请求（快速失败），走fallback逻辑。
经过一个睡眠窗口时间后（HystrixCommandProperties.circuitBreakerSleepWindowInMilliseconds()），Hystrix会放行一个请到后续服务，并将断路器开关切换为半开状态（HALF-OPEN）。如果该请求失败，则断路器会将熔断开关切换为打开状态（OPEN），继续熔断所有请求，直到下一个睡眠时间窗口的到来；如果该请求成功，则断路器会切换到关闭状态（CLOSED）,这时将允许所有请求通过，直到出现1步骤的情况，断路器开关会切换为打开状态(OPEN)。

断路器源码

Hystrix断路器的实现类是HystrixCircuitBreaker，源码如下：

/*** Circuit-breaker logic that is hooked into {@link HystrixCommand} execution and will stop allowing executions if failures have gone past the defined threshold.* 断路器，在HystrixCommand执行时会调用断路器逻辑，如果故障超过定义的阈值，断路器熔断开关将会打开，这时将阻止任务执行。* <p>* The default (and only) implementation  will then allow a single retry after a defined sleepWindow until the execution* succeeds at which point it will again close the circuit and allow executions again.* <p>* 默认（且唯一）实现将允许在定义的sleepWindow之后进行单次重试，直到执行成功，此时它将再次关闭电路并允许再次执行。*/
public interface HystrixCircuitBreaker {/*** Every {@link HystrixCommand} requests asks this if it is allowed to proceed or not.  It is idempotent and does* not modify any internal state, and takes into account the half-open logic which allows some requests through* after the circuit has been opened* <p>* 每个HystrixCommand请求都会询问是否允许继续（当断路器开关为OPEN和HALF_OPEN都时返回false，当断路器开关是CLOSE时或者到了下一个睡眠窗口时返回true）。* 它是幂等的，不会修改任何内部状态，并考虑到半开逻辑，当一个睡眠窗口到来时他会放行一些请求到后续逻辑** @return boolean whether a request should be permitted (是否应允许请求)*/boolean allowRequest();/*** Whether the circuit is currently open (tripped).* 判断熔断开关是否打开（有无副作用，不是幂等方式）。** @return boolean state of circuit breaker（返回断路器的状态）*/boolean isOpen();/*** Invoked on successful executions from {@link HystrixCommand} as part of feedback mechanism when in a half-open state.* <p>* 断路器在处于半开状态时，作为反馈机制的一部分，从HystrixCommand成功执行时调用。*/void markSuccess();/*** Invoked on unsuccessful executions from {@link HystrixCommand} as part of feedback mechanism when in a half-open state.* 断路器当处于半开状态时，作为反馈机制的一部分，从HystrixCommand执行不成功的调用。*/void markNonSuccess();/*** Invoked at start of command execution to attempt an execution.  This is non-idempotent - it may modify internal* state.* <p>* 在命令执行开始时调用以尝试执行，主要所用时判断该请求是否可以执行。这是非幂等的 - 它可能会修改内部状态。*/boolean attemptExecution();
}

断路器的默认实现就是它的一个内部类：

/*** @ExcludeFromJavadoc* @ThreadSafe*/
class Factory {// String is HystrixCommandKey.name() (we can't use HystrixCommandKey directly as we can't guarantee it implements hashcode/equals correctly)// key是HystrixCommandKey.name()（我们不能直接使用HystrixCommandKey，因为我们无法保证它正确实现hashcode / equals）private static ConcurrentHashMap<String, HystrixCircuitBreaker> circuitBreakersByCommand = new ConcurrentHashMap<String, HystrixCircuitBreaker>();/*** 根据HystrixCommandKey获取HystrixCircuitBreaker* Get the {@link HystrixCircuitBreaker} instance for a given {@link HystrixCommandKey}.* <p>* This is thread-safe and ensures only 1 {@link HystrixCircuitBreaker} per {@link HystrixCommandKey}.** @param key        {@link HystrixCommandKey} of {@link HystrixCommand} instance requesting the {@link HystrixCircuitBreaker}* @param group      Pass-thru to {@link HystrixCircuitBreaker}* @param properties Pass-thru to {@link HystrixCircuitBreaker}* @param metrics    Pass-thru to {@link HystrixCircuitBreaker}* @return {@link HystrixCircuitBreaker} for {@link HystrixCommandKey}*/public static HystrixCircuitBreaker getInstance(HystrixCommandKey key, HystrixCommandGroupKey group, HystrixCommandProperties properties, HystrixCommandMetrics metrics) {// this should find it for all but the first time// 根据HystrixCommandKey获取断路器HystrixCircuitBreaker previouslyCached = circuitBreakersByCommand.get(key.name());if (previouslyCached != null) {return previouslyCached;}// if we get here this is the first time so we need to initialize// Create and add to the map ... use putIfAbsent to atomically handle the possible race-condition of// 2 threads hitting this point at the same time and let ConcurrentHashMap provide us our thread-safety// If 2 threads hit here only one will get added and the other will get a non-null response instead.// 第一次没有获取到断路器，那么我们需要取初始化它// 这里直接利用ConcurrentHashMap的putIfAbsent方法，它是原子操作，加入有两个线程执行到这里，将会只有一个线程将值放到容器中// 让我们省掉了加锁的步骤HystrixCircuitBreaker cbForCommand = circuitBreakersByCommand.putIfAbsent(key.name(), new HystrixCircuitBreakerImpl(key, group, properties, metrics));if (cbForCommand == null) {// this means the putIfAbsent step just created a new one so let's retrieve and return itreturn circuitBreakersByCommand.get(key.name());} else {// this means a race occurred and while attempting to 'put' another one got there before// and we instead retrieved it and will now return itreturn cbForCommand;}}/*** 根据HystrixCommandKey获取HystrixCircuitBreaker，如果没有返回NULL* Get the {@link HystrixCircuitBreaker} instance for a given {@link HystrixCommandKey} or null if none exists.** @param key {@link HystrixCommandKey} of {@link HystrixCommand} instance requesting the {@link HystrixCircuitBreaker}* @return {@link HystrixCircuitBreaker} for {@link HystrixCommandKey}*/public static HystrixCircuitBreaker getInstance(HystrixCommandKey key) {return circuitBreakersByCommand.get(key.name());}/*** Clears all circuit breakers. If new requests come in instances will be recreated.* 清除所有断路器。如果有新的请求将会重新创建断路器放到容器。*//* package */static void reset() {circuitBreakersByCommand.clear();}
}/**1. 默认的断路器实现2. The default production implementation of {@link HystrixCircuitBreaker}.3.  4. @ExcludeFromJavadoc5. @ThreadSafe*/
/* package */class HystrixCircuitBreakerImpl implements HystrixCircuitBreaker {private final HystrixCommandProperties properties;private final HystrixCommandMetrics metrics;enum Status {// 断路器状态，关闭，打开，半开CLOSED, OPEN, HALF_OPEN;}// 赋值操作不是线程安全的。若想不用锁来实现，可以用AtomicReference<V>这个类，实现对象引用的原子更新。// AtomicReference 原子引用，保证Status原子性修改private final AtomicReference<Status> status = new AtomicReference<Status>(Status.CLOSED);// 记录断路器打开的时间点（时间戳）,如果这个时间大于0表示断路器处于打开状态或半开状态private final AtomicLong circuitOpened = new AtomicLong(-1);private final AtomicReference<Subscription> activeSubscription = new AtomicReference<Subscription>(null);protected HystrixCircuitBreakerImpl(HystrixCommandKey key, HystrixCommandGroupKey commandGroup, final HystrixCommandProperties properties, HystrixCommandMetrics metrics) {this.properties = properties;this.metrics = metrics;//On a timer, this will set the circuit between OPEN/CLOSED as command executions occur// 在定时器上，当命令执行发生时，这将在OPEN / CLOSED之间设置电路Subscription s = subscribeToStream();activeSubscription.set(s);}private Subscription subscribeToStream() {/** This stream will recalculate the OPEN/CLOSED status on every onNext from the health stream* 此流将重新计算运行状况流中每个onNext上的OPEN / CLOSED状态*/return metrics.getHealthCountsStream().observe().subscribe(new Subscriber<HealthCounts>() {@Overridepublic void onCompleted() {}@Overridepublic void onError(Throwable e) {}@Overridepublic void onNext(HealthCounts hc) {// check if we are past the statisticalWindowVolumeThreshold// 检查一个时间窗口内的最小请求数if (hc.getTotalRequests() < properties.circuitBreakerRequestVolumeThreshold().get()) {// we are not past the minimum volume threshold for the stat window,// so no change to circuit status.// if it was CLOSED, it stays CLOSED// IF IT WAS HALF-OPEN, WE NEED TO WAIT FOR A SUCCESSFUL COMMAND EXECUTION// if it was open, we need to wait for sleep window to elapse// 我们没有超过统计窗口的最小音量阈值，所以我们不会去改变断路器状态，如果是closed状态，他将保持这个状态// 如果是半开状态，那么她需要等到一个成功的 Command执行// 如果是打开状态，那么它需要等到这个时间窗口过去} else {// 检查错误比例阀值if (hc.getErrorPercentage() < properties.circuitBreakerErrorThresholdPercentage().get()) {//we are not past the minimum error threshold for the stat window,// so no change to circuit status.// if it was CLOSED, it stays CLOSED// if it was half-open, we need to wait for a successful command execution// if it was open, we need to wait for sleep window to elapse} else {// our failure rate is too high, we need to set the state to OPEN// 我们的失败率太高，我们需要将状态设置为OPENif (status.compareAndSet(Status.CLOSED, Status.OPEN)) {circuitOpened.set(System.currentTimeMillis());}}}}});}@Overridepublic void markSuccess() {// 断路器是处理半开并且HystrixCommand执行成功，将状态设置成关闭if (status.compareAndSet(Status.HALF_OPEN, Status.CLOSED)) {//This thread wins the race to close the circuit - it resets the stream to start it over from 0//该线程赢得了关闭电路的竞争 - 它重置流以从0开始metrics.resetStream();Subscription previousSubscription = activeSubscription.get();if (previousSubscription != null) {previousSubscription.unsubscribe();}Subscription newSubscription = subscribeToStream();activeSubscription.set(newSubscription);circuitOpened.set(-1L);}}@Overridepublic void markNonSuccess() {// 断路器是处理半开并且HystrixCommand执行成功，将状态设置成打开if (status.compareAndSet(Status.HALF_OPEN, Status.OPEN)) {//This thread wins the race to re-open the circuit - it resets the start time for the sleep window// 该线程赢得了重新打开电路的竞争 - 它重置了睡眠窗口的开始时间circuitOpened.set(System.currentTimeMillis());}}@Overridepublic boolean isOpen() {// 获取配置判断断路器是否强制打开if (properties.circuitBreakerForceOpen().get()) {return true;}// 获取配置判断断路器是否强制关闭if (properties.circuitBreakerForceClosed().get()) {return false;}return circuitOpened.get() >= 0;}@Overridepublic boolean allowRequest() {// 获取配置判断断路器是否强制打开if (properties.circuitBreakerForceOpen().get()) {return false;}// 获取配置判断断路器是否强制关闭if (properties.circuitBreakerForceClosed().get()) {return true;}if (circuitOpened.get() == -1) {return true;} else {// 如果是半开状态则返回不允许Command执行if (status.get().equals(Status.HALF_OPEN)) {return false;} else {// 检查睡眠窗口是否过了return isAfterSleepWindow();}}}private boolean isAfterSleepWindow() {final long circuitOpenTime = circuitOpened.get();final long currentTime = System.currentTimeMillis();// 获取配置的一个睡眠的时间窗口final long sleepWindowTime = properties.circuitBreakerSleepWindowInMilliseconds().get();return currentTime > circuitOpenTime + sleepWindowTime;}@Overridepublic boolean attemptExecution() {// 获取配置判断断路器是否强制打开if (properties.circuitBreakerForceOpen().get()) {return false;}// 获取配置判断断路器是否强制关闭if (properties.circuitBreakerForceClosed().get()) {return true;}if (circuitOpened.get() == -1) {return true;} else {if (isAfterSleepWindow()) {//only the first request after sleep window should execute//if the executing command succeeds, the status will transition to CLOSED//if the executing command fails, the status will transition to OPEN//if the executing command gets unsubscribed, the status will transition to OPEN// 只有一个睡眠窗口后的第一个请求会被执行// 如果执行命令成功，状态将转换为CLOSED// 如果执行命令失败，状态将转换为OPEN// 如果执行命令取消订阅，状态将过渡到OPENif (status.compareAndSet(Status.OPEN, Status.HALF_OPEN)) {return true;} else {6. List itemreturn false;}} else {return false;}}}
}

isOpen()：判断熔断开关是否打开（该方法是否幂等和Hystrix版本相关）。
allowRequest()：每个HystrixCommand请求都会询问是否允许继续执行（当断路器开关为OPEN和HALF_OPEN都时返回false，当断路器开关是CLOSE或到了下一个睡眠窗口时返回true），它是幂等的，不会修改任何内部状态，并考虑到半开逻辑，当一个睡眠窗口到来时他会放行一些请求到后续逻辑。
attemptExecution()：在命令执行开始时调用以尝试执行，主要所用时判断该请求是否可以执行。这是非幂等的，它可能会修改内部状态。

这里需要注意的是allowRequest()方法时幂等的，可以重复调用；attemptExecution()方法是有副作用的，不可以重复调用；isOpen()是否幂等和Hystrix版本有关。

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Hystrix核心原理和断路器源码解析

Hystrix运行原理

断路器运行原理

断路器源码

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