Hystrix失败处理逻辑解析

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);
                }
            }
        };

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);
    }

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);
            }
        }
    }

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_EMIT、FALLBACK_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);
                }
            }
        });
    }

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