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【一起学源码-微服务】Hystrix 源码三:Hystrix核心流程:Hystix降级、熔断等原理剖析

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前言

前情回顾

上一讲我们讲解了Hystrix在配合feign的过程中,一个正常的请求逻辑该怎样处理,这里涉及到线程池的创建、HystrixCommand的执行等逻辑。

如图所示:

高清大图:https://www.processon.com/view/link/5e1c128ce4b0169fb51ce77e

本讲目录

这一讲开始讲解Hystrix的看家本领:熔断+降级。
熔断功能是Hystrix最核心的组件,当然也是最复杂的一块。
源码中细节太多,本讲我们主要还是专注于它的设计思想去学习。

目录如下:

  1. HystrixCircuitBreaker初始化过程
  2. Hystrix熔断机制(CLOSED/OPEN/HALF_OPEN)
  3. fallback降级机制

源码分析

HystrixCircuitBreaker初始化过程

我们还是会以AbstractCommand为突破口,这里继续看它的构造函数,其中里面有初始化熔断器initCircuitBreaker()的过程,具体代码如下:

abstract class AbstractCommand<R> implements HystrixInvokableInfo<R>, HystrixObservable<R> {

    private static HystrixCircuitBreaker initCircuitBreaker(boolean enabled, HystrixCircuitBreaker fromConstructor,
                                                            HystrixCommandGroupKey groupKey, HystrixCommandKey commandKey,
                                                            HystrixCommandProperties properties, HystrixCommandMetrics metrics) {
        if (enabled) {
            if (fromConstructor == null) {
                // 构建默认的HystrixCircuitBreaker
                return HystrixCircuitBreaker.Factory.getInstance(commandKey, groupKey, properties, metrics);
            } else {
                return fromConstructor;
            }
        } else {
            return new NoOpCircuitBreaker();
        }
    }
}


public interface HystrixCircuitBreaker {
    public static HystrixCircuitBreaker getInstance(HystrixCommandKey key, HystrixCommandGroupKey group, HystrixCommandProperties properties, HystrixCommandMetrics metrics) {
        // circuitBreakersByCommand是一个map,key为commandKey,也就是FeignClient中定义的方法名
        // 类似于ServiceAFeignClient.sayHello(String)
        HystrixCircuitBreaker previouslyCached = circuitBreakersByCommand.get(key.name());
        if (previouslyCached != null) {
            return previouslyCached;
        }

        // 每个commandKey都对应着自己的熔断器,如果没有则会构造一个HystrixCircuitBreaker
        HystrixCircuitBreaker cbForCommand = circuitBreakersByCommand.putIfAbsent(key.name(), new HystrixCircuitBreakerImpl(key, group, properties, metrics));
        if (cbForCommand == null) {
            return circuitBreakersByCommand.get(key.name());
        } else {
            return cbForCommand;
        }
    }

    class HystrixCircuitBreakerImpl implements HystrixCircuitBreaker {
        private final HystrixCommandProperties properties;
        private final HystrixCommandMetrics metrics;

        private Subscription subscribeToStream() {
            // 对HealthCounts进行订阅
            // HealthCounts中包含 总请求次数、总失败次数、失败率
            // HealthCounts 统计数据有变化则会回调到这里来
            return metrics.getHealthCountsStream()
                    .observe()
                    .subscribe(new Subscriber<HealthCounts>() {
                        @Override
                        public void onCompleted() {

                        }

                        @Override
                        public void onError(Throwable e) {

                        }

                        // 判断是否要降级的核心逻辑
                        @Override
                        public void onNext(HealthCounts hc) {
                            // 一个时间窗口(默认10s钟)总请求次数是否大于circuitBreakerRequestVolumeThreshold 默认为20s
                            if (hc.getTotalRequests() < properties.circuitBreakerRequestVolumeThreshold().get()) {
                                
                            } else {
                                // 错误率(总错误次数/总请求次数)小于circuitBreakerErrorThresholdPercentage(默认50%)
                                if (hc.getErrorPercentage() < properties.circuitBreakerErrorThresholdPercentage().get()) {

                                } else {
                                    // 反之,熔断状态将从CLOSED变为OPEN,且circuitOpened==>当前时间戳
                                    if (status.compareAndSet(Status.CLOSED, Status.OPEN)) {
                                        circuitOpened.set(System.currentTimeMillis());
                                    }
                                }
                            }
                        }
                    });
        }
    }
}

上面就是熔断器初始化过程,这里面做了几件事:

  1. 每个commandKey都有自己的一个熔断器
    commandKey表现形式为:ServiceAFeignClient#sayHello(String)

  2. 如果commandKey不存在熔断器,则构建默认熔断器
    默认熔断器会对HealthCounts进行订阅。HealthCounts中包含时间窗口内(默认10s钟)请求的总次数、失败次数、失败率

  3. HealthCounts中统计数据有变化则会回调subscribe.onNext()方法进行熔断开启判断

  4. 熔断开启条件:
  • 时间窗口内(默认10s钟)总请求次数大于20次
  • 时间窗口内(默认10s钟)失败率大于50%
  • 满足上述两个条件后熔断器状态从CLOSED变成OPEN

熔断器在第一次请求时会初始化AbtractCommand,同时也会创建对应commandKey的熔断器 ,熔断器默认都是关闭的(可配置为强制开启),只有满足触发条件才会被开启。下面就一起来看下熔断、半开等状态是如何触发的吧。

Hystrix熔断机制(CLOSED/OPEN/HALF_OPEN)

这里我们以AbstractCommand.applyHystrixSemantics() 为入口,一步步往下探究,这个方法在上一讲已经提到过,一个正常的Feign请求都会调用此方法。

abstract class AbstractCommand<R> implements HystrixInvokableInfo<R>, HystrixObservable<R> {
    private Observable<R> applyHystrixSemantics(final AbstractCommand<R> _cmd) {
        // 如果熔断了,这这里返回为false
        // 这里也包含HALF_OPEN逻辑
        if (circuitBreaker.attemptExecution()) {
            final Action1<Throwable> markExceptionThrown = new Action1<Throwable>() {
                @Override
                public void call(Throwable t) {
                    eventNotifier.markEvent(HystrixEventType.EXCEPTION_THROWN, commandKey);
                }
            };

            if (executionSemaphore.tryAcquire()) {
                try {
                    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();
        }
    }
}

circuitBreaker.attemptExecution() 这个逻辑就是判断,如果熔断了,那么返回false。而且这里还包含HALF_OPEN的逻辑,我们先看如何触发熔断的,这个后面再接着看。

接着往下跟进executeCommandAndObserve() 方法:

abstract class AbstractCommand<R> implements HystrixInvokableInfo<R>, HystrixObservable<R> {
    private Observable<R> executeCommandAndObserve(final AbstractCommand<R> _cmd) {
        final HystrixRequestContext currentRequestContext = HystrixRequestContext.getContextForCurrentThread();

        // 省略部分代码...

        // 运行过程中,出现异常等都会进入此回调函数
        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()) {
            // 这里创建一个 HystrixObservableTimeoutOperator
            execution = executeCommandWithSpecifiedIsolation(_cmd)
                    .lift(new HystrixObservableTimeoutOperator<R>(_cmd));
        } else {
            execution = executeCommandWithSpecifiedIsolation(_cmd);
        }

        return execution.doOnNext(markEmits)
                .doOnCompleted(markOnCompleted)
                .onErrorResumeNext(handleFallback)
                .doOnEach(setRequestContext);
    }
}

当我们服务调用中出现异常都会进入handleFallback()中,里面的方法我们就不继续跟入了,猜测里面会有更新HealthCounts中的属性,然后触发 HystrixCircuitBreaker中的onNext()方法,当满足熔断条件时 则会将熔断状态从CLOSED变成OPEN

这里我们会跟进下HystrixObservableTimeoutOperator 代码,这个是对我们执行过程中判断是否超时。
上面代码中,执行executeCommandWithSpecifiedIsolation() 方法时也会创建一个超时监视器:

private static class HystrixObservableTimeoutOperator<R> implements Operator<R, R> {

    final AbstractCommand<R> originalCommand;

    public HystrixObservableTimeoutOperator(final AbstractCommand<R> originalCommand) {
        this.originalCommand = originalCommand;
    }

    @Override
    public Subscriber<? super R> call(final Subscriber<? super R> child) {
        TimerListener listener = new TimerListener() {

            @Override
            public void tick() {
                // 判断command的timeOut状态,如果是未执行状态,则更新为已超时
                if (originalCommand.isCommandTimedOut.compareAndSet(TimedOutStatus.NOT_EXECUTED, TimedOutStatus.TIMED_OUT)) {
                    originalCommand.eventNotifier.markEvent(HystrixEventType.TIMEOUT, originalCommand.commandKey);
                    s.unsubscribe();

                    final HystrixContextRunnable timeoutRunnable = new HystrixContextRunnable(originalCommand.concurrencyStrategy, hystrixRequestContext, new Runnable() {
                        @Override
                        public void run() {
                            child.onError(new HystrixTimeoutException());
                        }
                    });


                    timeoutRunnable.run();
                }
            }

            @Override
            public int getIntervalTimeInMilliseconds() {
                return originalCommand.properties.executionTimeoutInMilliseconds().get();
            }
        };

        final Reference<TimerListener> tl = HystrixTimer.getInstance().addTimerListener(listener);
        originalCommand.timeoutTimer.set(tl);

        // 省略部分代码...
        s.add(parent);

        return parent;
    }
}

public class HystrixTimer {
    public Reference<TimerListener> addTimerListener(final TimerListener listener) {
        startThreadIfNeeded();
        Runnable r = new Runnable() {
            @Override
            public void run() {
                try {
                    // 执行上面的tick方法,改变command timeout状态
                    listener.tick();
                } catch (Exception e) {
                    logger.error("Failed while ticking TimerListener", e);
                }
            }
        };

        // 执行调度任务,延迟加载,延迟时间和调度时间默认都为1s钟
        // 这里使用线程池,coreSize=cpu核心数 maxSize为Integer.Max
        ScheduledFuture<?> f = executor.get().getThreadPool().scheduleAtFixedRate(r, listener.getIntervalTimeInMilliseconds(), listener.getIntervalTimeInMilliseconds(), TimeUnit.MILLISECONDS);
        return new TimerReference(listener, f);
    }
}

这里面核心业务是起一个调度任务,默认每秒钟执行一次,然后调用tick()方法,如果当前command状态还是NOT_EXECUTED状态,那么将command状态改为TIMED_OUT 。此时会进入到之前的handleFallback回调函数中,这里又会更新HealthCounts中的数据,对应的触发之前熔断的判断条件:

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
  Subscription s = subscribeToStream();
  activeSubscription.set(s);
}

private Subscription subscribeToStream() {
  //这里会在每次执行onNext()事件的时候来评估是否需要打开或者关闭断路器
  return metrics.getHealthCountsStream()
    .observe()
    .subscribe(new Subscriber<HealthCounts>() {
      @Override
      public void onCompleted() {

      }

      @Override
      public void onError(Throwable e) {

      }

      @Override
      public void onNext(HealthCounts hc) {
        //首先校验的时在时间窗范围内的请求次数,如果低于阈值(默认是20),不做处理,如果高于阈值,则去判断接口请求的错误率
        if (hc.getTotalRequests() < properties.circuitBreakerRequestVolumeThreshold().get()) {           // 如果没有超过统计阈值的最低窗口值,就没有必要去改变断路器的状态
          // 当前如果断路器是关闭的,那么就保持关闭状态无需更改;
          // 如果断路器状态为半开状态,需要等待直到有成功的命令执行;
          // 如果断路器是打开状态,需要等待休眠窗口过期。
        } else {
          //判断接口请求的错误率(阈值默认是50),如果高于这个值,则断路器打开
          if (hc.getErrorPercentage() < properties.circuitBreakerErrorThresholdPercentage().get()) {
    
            // 如果当前请求的错误率小于断路器设置的容错率百分比,也不会拦截请求
          } else {
            // 如果当前错误率太高则打开断路器
            if (status.compareAndSet(Status.CLOSED, Status.OPEN)) {
              circuitOpened.set(System.currentTimeMillis());
            }
          }
        }
      }
    });
}

如果符合熔断条件,那么command熔断状态就会变为OPEN,此时熔断器打开。

如果我们command执行成功,那么就会清理掉这个timeout timer schedule任务。

abstract class AbstractCommand<R> implements HystrixInvokableInfo<R>, HystrixObservable<R> {
    private void handleCommandEnd(boolean commandExecutionStarted) {
        Reference<TimerListener> tl = timeoutTimer.get();
        // 如果timeOutTimer不为空,这里则clear一下
        // clear会关闭启动的调度任务
        if (tl != null) {
            tl.clear();
        }

        long userThreadLatency = System.currentTimeMillis() - commandStartTimestamp;
        executionResult = executionResult.markUserThreadCompletion((int) userThreadLatency);
        if (executionResultAtTimeOfCancellation == null) {
            // metrics统计数据
            metrics.markCommandDone(executionResult, commandKey, threadPoolKey, commandExecutionStarted);
        } else {
            metrics.markCommandDone(executionResultAtTimeOfCancellation, commandKey, threadPoolKey, commandExecutionStarted);
        }

        if (endCurrentThreadExecutingCommand != null) {
            endCurrentThreadExecutingCommand.call();
        }
    }
}

如上所属,我们已经知道了熔断开启的触发时机,那么如果一个commandKey开启了熔断,下次的请求是该如何直接降级呢?我们来看下代码:

abstract class AbstractCommand<R> implements HystrixInvokableInfo<R>, HystrixObservable<R> {
    private Observable<R> applyHystrixSemantics(final AbstractCommand<R> _cmd) {

        // 这个if条件就代表是否开启熔断
        if (circuitBreaker.attemptExecution()) {
            // 执行业务逻辑代码...
        } else {
            return handleShortCircuitViaFallback();
        }
    }
}

class HystrixCircuitBreakerImpl implements HystrixCircuitBreaker {
    public boolean attemptExecution() {
            // 如果熔断配置的为强制开启,那么直接返回false执行熔断逻辑
            if (properties.circuitBreakerForceOpen().get()) {
                return false;
            }
            // 如果熔断配置为强制关闭,那么永远不走熔断逻辑
            if (properties.circuitBreakerForceClosed().get()) {
                return true;
            }
            // 熔断开启时 circuitOpened设置为当前时间戳
            if (circuitOpened.get() == -1) {
                return true;
            } else {
                // 如果当前时间距离熔断小于5s钟,那么将熔断状态从OPEN改为HALF_OPEN
                if (isAfterSleepWindow()) {
                    if (status.compareAndSet(Status.OPEN, Status.HALF_OPEN)) {
                        //only the first request after sleep window should execute
                        return true;
                    } else {
                        return false;
                    }
                } else {
                    return false;
                }
            }
        }
    }

    private boolean isAfterSleepWindow() {
        final long circuitOpenTime = circuitOpened.get();
        final long currentTime = System.currentTimeMillis();
        // circuitBreakerSleepWindowInMilliseconds 默认为5s钟
        final long sleepWindowTime = properties.circuitBreakerSleepWindowInMilliseconds().get();
        // 当前熔断距离熔断是否超过5s钟
        return currentTime > circuitOpenTime + sleepWindowTime;
    }
}

我们可以看到,在applyHystrixSemantics()这个核心的方法中,先判断是否熔断,如果熔断则直接走fallback逻辑。

attemptExecution()判断条件中还涉及到HALF_OPEN的逻辑,如果熔断开启,下一次请求的时候,会判断当前时间距离上一次时间是否超过了5s钟,如果没有超过,则会将熔断状态从OPEN变为HALF_OPEN,此时会放一个请求按照正常逻辑去执行:

  1. 执行失败,熔断状态又会从HALF_OPEN变成OPEN
  2. 执行成功,熔断状态从HALF_OPEN变成CLOSED,并清除熔断相关设置

执行成功后代码:

class HystrixCircuitBreakerImpl implements HystrixCircuitBreaker {
    public void markSuccess() {
        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
            metrics.resetStream();
            Subscription previousSubscription = activeSubscription.get();
            if (previousSubscription != null) {
                previousSubscription.unsubscribe();
            }
            Subscription newSubscription = subscribeToStream();
            activeSubscription.set(newSubscription);
            circuitOpened.set(-1L);
        }
    }
}

上面对整个熔断的状态:CLOSED、OPEN、HALF_OPEN梳理的已经很清楚了,下面看看降级是该如何处理的吧。

fallback降级机制

上面已经讲解了Hystrix 熔断开启的机制等内容,这里主要是说如果一个请求失败(线程池拒绝、超时、badRequest等),那么Hystrix是如何执行降级的呢?

还是回到我们最初的代码 HystrixInvocationHandler类中,看看其invoke()方法中的getFallback回调函数:

protected Object getFallback() {
    if (fallbackFactory == null) {
      return super.getFallback();
    }
    try {
      // 通过我们配置好的fallbackFactory找到对应的FeignClient,这里是获取ServiceAFeignClient
      Object fallback = fallbackFactory.create(getExecutionException());
      // fallbackMap中key为ServiceAFeignClient.sayHello(Integer)
      // 获取具体的降级method方法
      Object result = fallbackMethodMap.get(method).invoke(fallback, args);
      if (isReturnsHystrixCommand(method)) {
        return ((HystrixCommand) result).execute();
      } else if (isReturnsObservable(method)) {
        // Create a cold Observable
        return ((Observable) result).toBlocking().first();
      } else if (isReturnsSingle(method)) {
        // Create a cold Observable as a Single
        return ((Single) result).toObservable().toBlocking().first();
      } else if (isReturnsCompletable(method)) {
        ((Completable) result).await();
        return null;
      } else {
        return result;
      }
    } catch (IllegalAccessException e) {
      // shouldn't happen as method is public due to being an interface
      throw new AssertionError(e);
    } catch (InvocationTargetException e) {
      // Exceptions on fallback are tossed by Hystrix
      throw new AssertionError(e.getCause());
    }
  }
};

这里很简单,其实就是先获取到我们自己在FallbackFactory中配置的的降级方法,然后执行降级逻辑。

总结

这一讲核心逻辑主要是Hystrix熔断状态的变化,主要是CLOSED、OPEN、HALF_OPEN几种状态触发的时间,互相转变的流程,以及执行降级逻辑的原理。

我们仍然是用一个流程图来总结一下:

高清大图链接:
https://www.processon.com/view/link/5e1ee0afe4b0c62462aae684

(点击原文可以直接查看大图哦

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