在上篇文章Hystrix工作流程解析中,我们整体介绍了Hystrix的工作流程,知道了Hystrix会在下面四种情况下发生降级:

  1. 熔断器打开
  2. 线程池/信号量跑满
  3. 调用超时
  4. 调用失败

本篇文章则介绍一下在发生降级时Hystrix的处理细节,下面的方法异常的处理逻辑:

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);
                }
            }
        };
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这里我们拿线程池跑满的逻辑来进行分析

线程池跑满
    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);
    }
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  1. 第一行发布了一个线程池拒绝的事件
  2. 第二行记录了线程池拒绝的次数
  3. 获取Fallback方法
获取Fallback方法
    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);
            }
        }
    }
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方法比较长,主要做了以下事情:

  1. 直接看isUnrecoverable方法,判断异常是否为不可恢复异常,如果不可恢复则直接返回失败
  2. 如果是可恢复异常则打印日志
  3. 判断是否开启执行回退方法,如果开启进入步骤4
  4. 创建开始和完成需要发送的两个事件:FALLBACK_EMITFALLBACK_SUCCESS
  5. 创建调用回退方法出现异常时的处理逻辑:handleFallbackError,而这种场景发生的异常只有两种情况:
    1. UnsupportedOperationException异常:未实现getFallback抽象方法
    2. 其他异常
  6. 创建释放信号量的Action:singleSemaphoreRelease
  7. 获取信号量,如果成功执行回退逻辑,也就是调用用户实现的getFallback方法
final protected Observable<R> getFallbackObservable() {
        return Observable.defer(new Func0<Observable<R>>() {
            @Override
            public Observable<R> call() {
                try {
                    return Observable.just(getFallback());
                } catch (Throwable ex) {
                    return Observable.error(ex);
                }
            }
        });
    }
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对于其他异常类型的处理感兴趣的同学可以继续基于Func1 handleFallback研究

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目录