【SpringCloud技术专题「Hystrix技术分析」故障切换的运作流程（含源码）】提兵百万西湖上，立马吴山第一峰！这篇文章主要讲述SpringCloud技术专题「Hystrix技术分析」故障切换的运作流程（含源码）相关的知识，希望能为你提供帮助。
背景介绍
Hystrix说明
官方文档
为什么需要Hystrix?

• 在大中型分布式系统中，通常系统很多依赖(HTTP,hession,Netty,Dubbo等)，在高并发访问下,这些依赖的稳定性与否对系统的影响非常大,但是依赖有很多不可控问题：如网络连接缓慢，资源繁忙，暂时不可用，服务脱机等。
  
• 当依赖阻塞时,大多数服务器的线程池就出现阻塞(BLOCK),影响整个线上服务的稳定性，在复杂的分布式架构的应用程序有很多的依赖，都会不可避免地在某些时候失败。高并发的依赖失败时如果没有隔离措施，当前应用服务就有被拖垮的风险。

例如:一个依赖30个SOA服务的系统,每个服务99.99%可用。99.99%的30次方 ≈ 99.7%0.3% 意味着一亿次请求 会有 3,000,00次失败换算成时间大约每月有2个小时服务不稳定.随着服务依赖数量的变多，服务不稳定的概率会成指数性提高.解决问题方案：对依赖做隔离。

Hystrix设计理念

文章图片

• 可见，Command是在Receiver和Invoker之间添加的中间层，Command实现了对Receiver的封装。
  
• API既可以是Invoker又可以是reciever，通过继承Hystrix核心类HystrixCommand来封装这些API（例如，远程接口调用，数据库查询之类可能会产生延时的操作）。
  
• 就可以为API提供弹性保护了。

Hystrix如何解决依赖隔离

1. Hystrix使用命令模式HystrixCommand(Command)包装依赖调用逻辑，每个命令在单独线程中/信号授权下执行。
   
2. 可配置依赖调用超时时间，超时时间一般设为比99.5%平均时间略高即可。当调用超时时，直接返回或执行fallback逻辑。
   
3. 为每个依赖提供一个小的线程池（或信号），如果线程池已满调用将被立即拒绝，默认不采用排队，加速失败判定时间。
   
4. 依赖调用结果分，成功，失败（抛出异常），超时，线程拒绝，短路。 请求失败(异常，拒绝，超时，短路)时执行fallback(降级)逻辑。
   
5. 提供熔断器组件，可以自动运行或手动调用，停止当前依赖一段时间(10秒)，熔断器默认错误率阈值为50%，超过将自动运行。
   
6. 提供近实时依赖的统计和监控

Hystrix流程结构解析

文章图片
、
流程说明:

1. 每次调用构建HystrixCommand或者HystrixObservableCommand对象，把依赖调用封装在run()方法中.
   
2. 结果是否有缓存如果没有执行execute()/queue做sync或async调用，对应真正的run()/construct()
   
3. 判断熔断器(circuit-breaker)是否打开，如果打开跳到步骤8，进行降级策略，如果关闭进入步骤.
   
4. 判断线程池/队列/信号量是否跑满，如果跑满进入降级步骤8，否则继续后续步骤.
   
5. 使用HystrixObservableCommand.construct()还是HystrixCommand.run()，运行依赖逻辑
   
6. 依赖逻辑调用超时，进入步骤8
   
7. 判断逻辑是否调用成功
   
   • 6a 返回成功调用结果
     
   • 6b 调用出错，进入步骤8.
8. 计算熔断器状态,所有的运行状态(成功, 失败, 拒绝,超时)上报给熔断器，用于统计从而判断熔断器状态.
   
9. getFallback()降级逻辑.
   a. 没有实现getFallback的Command将直接抛出异常
   b. fallback降级逻辑调用成功直接返回
   c. 降级逻辑调用失败抛出异常
   
10. 返回执行成功结果

以下四种情况将触发getFallback调用：

1. run()方法抛出非HystrixBadRequestException异常。
   
2. run()方法调用超时
   
3. 熔断器开启短路调用
   
4. 线程池/队列/信号量是否跑满

熔断器:Circuit Breaker
文章图片

Hystrix隔离分析
线程隔离

• 执行依赖代码的线程与请求线程(如:jetty线程)分离，请求线程可以自由控制离开的时间(异步过程)。
  
• 通过线程池大小可以控制并发量，当线程池饱和时可以提前拒绝服务,防止依赖问题扩散。
  
• 线上建议线程池不要设置过大，否则大量堵塞线程有可能会拖慢服务器。

实际案例：信号隔离线程隔离与信号隔离区别如下图:

文章图片

fallback故障切换降级机制
有兴趣的小伙伴可以看看：官方参考文档
源码分析executeCommandAndObserve

/** * This decorates "Hystrix" functionality around the run() Observable. * @return R */ private Observable< R> executeCommandAndObserve(final AbstractCommand< R> _cmd) { //...... final Func1< Throwable, Observable< R> > handleFallback = new Func1< Throwable, Observable< R> > () { @Override public Observable< R> call(Throwable t) { circuitBreaker.markNonSuccess(); Exception e = getExceptionFromThrowable(t); executionResult = executionResult.setExecutionException(e); if (e instanceof RejectedExecutionException) { return handleThreadPoolRejectionViaFallback(e); } else if (t instanceof HystrixTimeoutException) { return handleTimeoutViaFallback(); } else if (t instanceof HystrixBadRequestException) { return handleBadRequestByEmittingError(e); } else { /* * Treat HystrixBadRequestException from ExecutionHook like a plain * HystrixBadRequestException. */ if (e instanceof HystrixBadRequestException) { eventNotifier.markEvent(HystrixEventType.BAD_REQUEST, commandKey); return Observable.error(e); } return handleFailureViaFallback(e); } } }; //...... Observable< R> execution; if (properties.executionTimeoutEnabled().get()) { execution = executeCommandWithSpecifiedIsolation(_cmd).lift(new HystrixObservableTimeoutOperator< R> (_cmd)); } else { execution = executeCommandWithSpecifiedIsolation(_cmd); } return execution.doOnNext(markEmits) .doOnCompleted(markOnCompleted) .onErrorResumeNext(handleFallback) .doOnEach(setRequestContext); }

使用Observable的onErrorResumeNext，里头调用了handleFallback，handleFallback中区分不同的异常来调用不同的fallback。

• RejectedExecutionException调用handleThreadPoolRejectionViaFallback
  
• HystrixTimeoutException调用handleTimeoutViaFallback
  
• 非HystrixBadRequestException的调用handleFailureViaFallback

applyHystrixSemantics

private Observable< R> applyHystrixSemantics(final AbstractCommand< R> _cmd) { // mark that we\'re starting execution on the ExecutionHook // if this hook throws an exception, then a fast-fail occurs with no fallback.No state is left inconsistent executionHook.onStart(_cmd); /* determine if we\'re allowed to execute */ if (circuitBreaker.attemptExecution()) { final TryableSemaphore executionSemaphore = getExecutionSemaphore(); final AtomicBoolean semaphoreHasBeenReleased = new AtomicBoolean(false); final Action0 singleSemaphoreRelease = new Action0() { @Override public void call() { if (semaphoreHasBeenReleased.compareAndSet(false, true)) { executionSemaphore.release(); } } }; final Action1< Throwable> markExceptionThrown = new Action1< Throwable> () { @Override public void call(Throwable t) { eventNotifier.markEvent(HystrixEventType.EXCEPTION_THROWN, commandKey); } }; if (executionSemaphore.tryAcquire()) { try { /* used to track userThreadExecutionTime */ executionResult = executionResult.setInvocationStartTime(System.currentTimeMillis()); return executeCommandAndObserve(_cmd) .doOnError(markExceptionThrown) .doOnTerminate(singleSemaphoreRelease) .doOnUnsubscribe(singleSemaphoreRelease); } catch (RuntimeException e) { return Observable.error(e); } } else { return handleSemaphoreRejectionViaFallback(); } } else { return handleShortCircuitViaFallback(); } }

• applyHystrixSemantics方法针对executionSemaphore.tryAcquire()没通过的调用
  
• handleSemaphoreRejectionViaFallback
  
• applyHystrixSemantics方法针对circuitBreaker.attemptExecution()没通过的调用handleShortCircuitViaFallback()

ViaFallback方法

private Observable< R> handleSemaphoreRejectionViaFallback() { Exception semaphoreRejectionException = new RuntimeException("could not acquire a semaphore for execution"); executionResult = executionResult.setExecutionException(semaphoreRejectionException); eventNotifier.markEvent(HystrixEventType.SEMAPHORE_REJECTED, commandKey); logger.debug("HystrixCommand Execution Rejection by Semaphore."); // debug only since we\'re throwing the exception and someone higher will do something with it // retrieve a fallback or throw an exception if no fallback available return getFallbackOrThrowException(this, HystrixEventType.SEMAPHORE_REJECTED, FailureType.REJECTED_SEMAPHORE_EXECUTION, "could not acquire a semaphore for execution", semaphoreRejectionException); }private Observable< R> handleShortCircuitViaFallback() { // record that we are returning a short-circuited fallback eventNotifier.markEvent(HystrixEventType.SHORT_CIRCUITED, commandKey); // short-circuit and go directly to fallback (or throw an exception if no fallback implemented) Exception shortCircuitException = new RuntimeException("Hystrix circuit short-circuited and is OPEN"); executionResult = executionResult.setExecutionException(shortCircuitException); try { return getFallbackOrThrowException(this, HystrixEventType.SHORT_CIRCUITED, FailureType.SHORTCIRCUIT, "short-circuited", shortCircuitException); } catch (Exception e) { return Observable.error(e); } }private Observable< R> handleThreadPoolRejectionViaFallback(Exception underlying) { eventNotifier.markEvent(HystrixEventType.THREAD_POOL_REJECTED, commandKey); threadPool.markThreadRejection(); // use a fallback instead (or throw exception if not implemented) return getFallbackOrThrowException(this, HystrixEventType.THREAD_POOL_REJECTED, FailureType.REJECTED_THREAD_EXECUTION, "could not be queued for execution", underlying); }private Observable< R> handleTimeoutViaFallback() { return getFallbackOrThrowException(this, HystrixEventType.TIMEOUT, FailureType.TIMEOUT, "timed-out", new TimeoutException()); }private Observable< R> handleFailureViaFallback(Exception underlying) { /** * All other error handling */ logger.debug("Error executing HystrixCommand.run(). Proceeding to fallback logic ...", underlying); // report failure eventNotifier.markEvent(HystrixEventType.FAILURE, commandKey); // record the exception executionResult = executionResult.setException(underlying); return getFallbackOrThrowException(this, HystrixEventType.FAILURE, FailureType.COMMAND_EXCEPTION, "failed", underlying); }

• handleSemaphoreRejectionViaFallback、handleShortCircuitViaFallback、handleThreadPoolRejectionViaFallback、handleTimeoutViaFallback、handleFailureViaFallback这几个方法调用了getFallbackOrThrowException
• 其eventType分别是SEMAPHORE_REJECTED、SHORT_CIRCUITED、THREAD_POOL_REJECTED、TIMEOUT、FAILURE
• AbstractCommand.getFallbackOrThrowException

/** * Execute < code> getFallback()< /code> within protection of a semaphore that limits number of concurrent executions. * < p> * Fallback implementations shouldn\'t perform anything that can be blocking, but we protect against it anyways in case someone doesn\'t abide by the contract. * < p> * If something in the < code> getFallback()< /code> implementation is latent (such as a network call) then the semaphore will cause us to start rejecting requests rather than allowing potentially * all threads to pile up and block. * * @return K * @throws UnsupportedOperationException *if getFallback() not implemented * @throws HystrixRuntimeException *if getFallback() fails (throws an Exception) or is rejected by the semaphore */ private Observable< R> getFallbackOrThrowException(final AbstractCommand< R> _cmd, final HystrixEventType eventType, final FailureType failureType, final String message, final Exception originalException) { final HystrixRequestContext requestContext = HystrixRequestContext.getContextForCurrentThread(); long latency = System.currentTimeMillis() - executionResult.getStartTimestamp(); // record the executionResult // do this before executing fallback so it can be queried from within getFallback (see See https://github.com/Netflix/Hystrix/pull/144) executionResult = executionResult.addEvent((int) latency, eventType); if (isUnrecoverable(originalException)) { logger.error("Unrecoverable Error for HystrixCommand so will throw HystrixRuntimeException and not apply fallback. ", originalException); /* executionHook for all errors */ Exception e = wrapWithOnErrorHook(failureType, originalException); return Observable.error(new HystrixRuntimeException(failureType, this.getClass(), getLogMessagePrefix() + " " + message + " and encountered unrecoverable error.", e, null)); } else { if (isRecoverableError(originalException)) { logger.warn("Recovered from java.lang.Error by serving Hystrix fallback", originalException); }if (properties.fallbackEnabled().get()) { /* fallback behavior is permitted so attempt */final Action1< Notification< ? super R> > setRequestContext = new Action1< Notification< ? super R> > () { @Override public void call(Notification< ? super R> rNotification) { setRequestContextIfNeeded(requestContext); } }; final Action1< R> markFallbackEmit = new Action1< R> () { @Override public void call(R r) { if (shouldOutputOnNextEvents()) { executionResult = executionResult.addEvent(HystrixEventType.FALLBACK_EMIT); eventNotifier.markEvent(HystrixEventType.FALLBACK_EMIT, commandKey); } } }; final Action0 markFallbackCompleted = new Action0() { @Override public void call() { long latency = System.currentTimeMillis() - executionResult.getStartTimestamp(); eventNotifier.markEvent(HystrixEventType.FALLBACK_SUCCESS, commandKey); executionResult = executionResult.addEvent((int) latency, HystrixEventType.FALLBACK_SUCCESS); } }; final Func1< Throwable, Observable< R> > handleFallbackError = new Func1< Throwable, Observable< R> > () { @Override public Observable< R> call(Throwable t) { /* executionHook for all errors */ Exception e = wrapWithOnErrorHook(failureType, originalException); Exception fe = getExceptionFromThrowable(t); long latency = System.currentTimeMillis() - executionResult.getStartTimestamp(); Exception toEmit; if (fe instanceof UnsupportedOperationException) { logger.debug("No fallback for HystrixCommand. ", fe); // debug only since we\'re throwing the exception and someone higher will do something with it eventNotifier.markEvent(HystrixEventType.FALLBACK_MISSING, commandKey); executionResult = executionResult.addEvent((int) latency, HystrixEventType.FALLBACK_MISSING); toEmit = new HystrixRuntimeException(failureType, _cmd.getClass(), getLogMessagePrefix() + " " + message + " and no fallback available.", e, fe); } else { logger.debug("HystrixCommand execution " + failureType.name() + " and fallback failed.", fe); eventNotifier.markEvent(HystrixEventType.FALLBACK_FAILURE, commandKey); executionResult = executionResult.addEvent((int) latency, HystrixEventType.FALLBACK_FAILURE); toEmit = new HystrixRuntimeException(failureType, _cmd.getClass(), getLogMessagePrefix() + " " + message + " and fallback failed.", e, fe); }// NOTE: we\'re suppressing fallback exception here if (shouldNotBeWrapped(originalException)) { return Observable.error(e); }return Observable.error(toEmit); } }; final TryableSemaphore fallbackSemaphore = getFallbackSemaphore(); final AtomicBoolean semaphoreHasBeenReleased = new AtomicBoolean(false); final Action0 singleSemaphoreRelease = new Action0() { @Override public void call() { if (semaphoreHasBeenReleased.compareAndSet(false, true)) { fallbackSemaphore.release(); } } }; Observable< R> fallbackExecutionChain; // acquire a permit if (fallbackSemaphore.tryAcquire()) { try { if (isFallbackUserDefined()) { executionHook.onFallbackStart(this); fallbackExecutionChain = getFallbackObservable(); } else { //same logic as above without the hook invocation fallbackExecutionChain = getFallbackObservable(); } } catch (Throwable ex) { //If hook or user-fallback throws, then use that as the result of the fallback lookup fallbackExecutionChain = Observable.error(ex); }return fallbackExecutionChain .doOnEach(setRequestContext) .lift(new FallbackHookApplication(_cmd)) .lift(new DeprecatedOnFallbackHookApplication(_cmd)) .doOnNext(markFallbackEmit) .doOnCompleted(markFallbackCompleted) .onErrorResumeNext(handleFallbackError) .doOnTerminate(singleSemaphoreRelease) .doOnUnsubscribe(singleSemaphoreRelease); } else { return handleFallbackRejectionByEmittingError(); } } else { return handleFallbackDisabledByEmittingError(originalException, failureType, message); } } }

• fallbackExecutionChain的onErrorResumeNext，调用了handleFallbackError
• fallbackExecutionChain的doOnCompleted，调用了markFallbackCompleted
• AbstractCommand.getFallbackSemaphore

/** * Get the TryableSemaphore this HystrixCommand should use if a fallback occurs. * * @return TryableSemaphore */ protected TryableSemaphore getFallbackSemaphore() { if (fallbackSemaphoreOverride == null) { TryableSemaphore _s = fallbackSemaphorePerCircuit.get(commandKey.name()); if (_s == null) { // we didn\'t find one cache so setup fallbackSemaphorePerCircuit.putIfAbsent(commandKey.name(), new TryableSemaphoreActual(properties.fallbackIsolationSemaphoreMaxConcurrentRequests())); // assign whatever got set (this or another thread) return fallbackSemaphorePerCircuit.get(commandKey.name()); } else { return _s; } } else { return fallbackSemaphoreOverride; } }

fallback源码分析小结hystrix的fallback主要分为5种类型：

• SEMAPHORE_REJECTED对应handleSemaphoreRejectionViaFallback
• SHORT_CIRCUITED对应handleShortCircuitViaFallback
• THREAD_POOL_REJECTED对应handleThreadPoolRejectionViaFallback
• TIMEOUT对应handleTimeoutViaFallback
• FAILURE对应handleFailureViaFallback
• 这几个方法最后都调用了getFallbackOrThrowException方法。

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