Hystrix工做流程解析

搭建Hystrix源碼閱讀環境

引入依賴

<dependency>
            <groupId>com.netflix.hystrix</groupId>
            <artifactId>hystrix-core</artifactId>
            <version>1.5.12</version>
        </dependency>

建立Command類

public class HelloCommand extends HystrixCommand<String> {

    public HelloCommand() {
        super(HystrixCommandGroupKey.Factory.asKey("test"));
    }

    @Override
    protected String run() throws Exception {
        Thread.sleep(800);
        return "sucess";
    }

    @Override
    protected String getFallback() {
        System.out.println("執行了回退方法");
        return "error";
    }

}

建立測試類

public class CommandTest {
    public static void main(String[] args) {
        HelloCommand command = new HelloCommand();
        String result = command.execute();
        System.out.println(result);
    }
}

Hystrix工做流程

首先咱們看一下上方的這張圖，這個圖完整的描述了Hystrix的工做流程：
1.每次調用都會建立一個HystrixCommand
2.執行execute或queue作同步異步調用
3.判斷熔斷器是否打開,若是打開跳到步驟8，不然進入步驟4
4.判斷線程池/信號量是否跑滿，若是跑滿進入步驟8,不然進入步驟5
5.調用HystrixCommand的run方法，若是調用超時進入步驟8
6.判斷是否調用成功，返回成功調用結果，若是失敗進入步驟8
7.計算熔斷器狀態,全部的運行狀態(成功, 失敗, 拒絕,超時)上報給熔斷器，用於統計從而判斷熔斷器狀態
8.降級處理邏輯，根據上方的步驟能夠得出如下四種狀況會進入降級處理：

1. 熔斷器打開
2. 線程池/信號量跑滿
3. 調用超時
4. 調用失敗

9.返回執行成功結果

建立HystrixCommand

接着咱們結合源碼看一下這個調用流程，直接執行測試類的main方法，能夠看到入口就在execute方法上

public R execute() {
        try {
            return queue().get();
        } catch (Exception e) {
            throw Exceptions.sneakyThrow(decomposeException(e));
        }
    }

執行同步方法

public Future<R> queue() {
        final Future<R> delegate = toObservable().toBlocking().toFuture();
        //省略。。。
};

接着看toObservable()方法

public Observable<R> toObservable() {
        //省略。。。
        return Observable.defer(new Func0<Observable<R>>() {
            @Override
            public Observable<R> call() {
                if (!commandState.compareAndSet(CommandState.NOT_STARTED, CommandState.OBSERVABLE_CHAIN_CREATED)) {
                    IllegalStateException ex = new IllegalStateException("This instance can only be executed once. Please instantiate a new instance.");
                    //TODO make a new error type for this
                    throw new HystrixRuntimeException(FailureType.BAD_REQUEST_EXCEPTION, _cmd.getClass(), getLogMessagePrefix() + " command executed multiple times - this is not permitted.", ex, null);
                }

                commandStartTimestamp = System.currentTimeMillis();

                if (properties.requestLogEnabled().get()) {
                    // log this command execution regardless of what happened
                    if (currentRequestLog != null) {
                        currentRequestLog.addExecutedCommand(_cmd);
                    }
                }

                final boolean requestCacheEnabled = isRequestCachingEnabled();
                final String cacheKey = getCacheKey();
                //若是開啓請求緩存則查詢緩存是否存在
                if (requestCacheEnabled) {
                    HystrixCommandResponseFromCache<R> fromCache = (HystrixCommandResponseFromCache<R>) requestCache.get(cacheKey);
                    if (fromCache != null) {
                        isResponseFromCache = true;
                        return handleRequestCacheHitAndEmitValues(fromCache, _cmd);
                    }
                }

                Observable<R> hystrixObservable =
                        Observable.defer(applyHystrixSemantics)
                                .map(wrapWithAllOnNextHooks);

                Observable<R> afterCache;

                if (requestCacheEnabled && cacheKey != null) { 
                    HystrixCachedObservable<R> toCache = HystrixCachedObservable.from(hystrixObservable, _cmd);
                    HystrixCommandResponseFromCache<R> fromCache = (HystrixCommandResponseFromCache<R>) requestCache.putIfAbsent(cacheKey, toCache);
                    if (fromCache != null) {
                        // another thread beat us so we'll use the cached value instead
                        toCache.unsubscribe();
                        isResponseFromCache = true;
                        return handleRequestCacheHitAndEmitValues(fromCache, _cmd);
                    } else {
                        // we just created an ObservableCommand so we cast and return it
                        afterCache = toCache.toObservable();
                    }
                } else {
                    afterCache = hystrixObservable;
                }

                return afterCache
                        .doOnTerminate(terminateCommandCleanup)     // perform cleanup once (either on normal terminal state (this line), or unsubscribe (next line))
                        .doOnUnsubscribe(unsubscribeCommandCleanup) // perform cleanup once
                        .doOnCompleted(fireOnCompletedHook);
            }
        });
    }

在上面這個方法中會有一個緩存的判斷，若是存在緩存的話直接返回結果，不然進入方法applyHystrixSemantics方法

判斷熔斷器和線程池以及信號量

private Observable<R> applyHystrixSemantics(final AbstractCommand<R> _cmd) {
        executionHook.onStart(_cmd);

        /* determine if we're allowed to execute */
        //判斷是否開啓熔斷器
        if (circuitBreaker.attemptExecution()) {
            //獲取信號量實例
            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);
                }
            };
            //嘗試獲取信號量
            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 {
                //拒絕
                return handleSemaphoreRejectionViaFallback();
            }
        } else {
            //失敗
            return handleShortCircuitViaFallback();
        }
    }

在applyHystrixSemantics方法中，首先會判斷是否開啓熔斷器，若是開啓則直接進入失敗處理的邏輯。不然會嘗試獲取信號量（若是使用的是線程池的模式則默認獲取成功），獲取成功進入executeCommandAndObserve方法

判斷超時

private Observable<R> executeCommandAndObserve(final AbstractCommand<R> _cmd) {
        final HystrixRequestContext currentRequestContext = HystrixRequestContext.getContextForCurrentThread();
        //省略...

        //判斷是否開啓超時設置
        if (properties.executionTimeoutEnabled().get()) {
           //list添加超時操做
            execution = executeCommandWithSpecifiedIsolation(_cmd)
                    .lift(new HystrixObservableTimeoutOperator<R>(_cmd));
        } else {
            execution = executeCommandWithSpecifiedIsolation(_cmd);
        }

這裏若是設置超時時間的話則會加上一個超時的處理，接着看executeCommandWithSpecifiedIsolation方法

private Observable<R> executeCommandWithSpecifiedIsolation(final AbstractCommand<R> _cmd) {
        if (properties.executionIsolationStrategy().get() == ExecutionIsolationStrategy.THREAD) {
            return Observable.defer(new Func0<Observable<R>>() {
                @Override
                public Observable<R> call() {
                    executionResult = executionResult.setExecutionOccurred();
                    if (!commandState.compareAndSet(CommandState.OBSERVABLE_CHAIN_CREATED, CommandState.USER_CODE_EXECUTED)) {
                        return Observable.error(new IllegalStateException("execution attempted while in state : " + commandState.get().name()));
                    }
                    metrics.markCommandStart(commandKey, threadPoolKey, ExecutionIsolationStrategy.THREAD);

                    if (isCommandTimedOut.get() == TimedOutStatus.TIMED_OUT) {
                        return Observable.error(new RuntimeException("timed out before executing run()"));
                    }
                    if (threadState.compareAndSet(ThreadState.NOT_USING_THREAD, ThreadState.STARTED)) {
                        HystrixCounters.incrementGlobalConcurrentThreads();
                        threadPool.markThreadExecution();
                        // store the command that is being run
                        endCurrentThreadExecutingCommand = Hystrix.startCurrentThreadExecutingCommand(getCommandKey());
                        executionResult = executionResult.setExecutedInThread();

                        try {
                            executionHook.onThreadStart(_cmd);
                            executionHook.onRunStart(_cmd);
                            executionHook.onExecutionStart(_cmd);
                            return getUserExecutionObservable(_cmd);
                        } catch (Throwable ex) {
                            return Observable.error(ex);
                        }
                    } else {
                        return Observable.empty();
                    }
                }
            }).doOnTerminate(new Action0() {
                @Override
                public void call() {
                    if (threadState.compareAndSet(ThreadState.STARTED, ThreadState.TERMINAL)) {
                        handleThreadEnd(_cmd);
                    }
                    if (threadState.compareAndSet(ThreadState.NOT_USING_THREAD, ThreadState.TERMINAL)) {
                    }
                }
            }).doOnUnsubscribe(new Action0() {
                @Override
                public void call() {
                    if (threadState.compareAndSet(ThreadState.STARTED, ThreadState.UNSUBSCRIBED)) {
                        handleThreadEnd(_cmd);
                    }
                    if (threadState.compareAndSet(ThreadState.NOT_USING_THREAD, ThreadState.UNSUBSCRIBED)) {
                    }
                }
            }).subscribeOn(threadPool.getScheduler(new Func0<Boolean>() {
                @Override
                public Boolean call() {
                    return properties.executionIsolationThreadInterruptOnTimeout().get() && _cmd.isCommandTimedOut.get() == TimedOutStatus.TIMED_OUT;
                }
            }));
        } else {
            return Observable.defer(new Func0<Observable<R>>() {
                @Override
                public Observable<R> call() {
                    executionResult = executionResult.setExecutionOccurred();
                    if (!commandState.compareAndSet(CommandState.OBSERVABLE_CHAIN_CREATED, CommandState.USER_CODE_EXECUTED)) {
                        return Observable.error(new IllegalStateException("execution attempted while in state : " + commandState.get().name()));
                    }

                    metrics.markCommandStart(commandKey, threadPoolKey, ExecutionIsolationStrategy.SEMAPHORE);
                    // semaphore isolated
                    // store the command that is being run
                    endCurrentThreadExecutingCommand = Hystrix.startCurrentThreadExecutingCommand(getCommandKey());
                    try {
                        executionHook.onRunStart(_cmd);
                        executionHook.onExecutionStart(_cmd);
                        return getUserExecutionObservable(_cmd);  //the getUserExecutionObservable method already wraps sync exceptions, so this shouldn't throw
                    } catch (Throwable ex) {
                        //If the above hooks throw, then use that as the result of the run method
                        return Observable.error(ex);
                    }
                }
            });
        }
    }

這段代碼比較長，具體的執行邏輯爲：

1. 進入方法會首先判斷隔離策略，若是是使用的信號量模式則在當前線程上執行，不然進入下方的線程池邏輯
2. 更改HystrixCommand的狀態爲USER_CODE_EXECUTED
3. 判斷HystrixCommand的超時狀態，若是超時則拋出異常
4. 更改當前command的線程執行狀態爲STARTED
5. 調用getUserExecutionObservable執行具體的業務邏輯，也就是咱們實現的那個run方法
6. doOnTerminate：執行完畢後更改線程狀態爲TERMINAL
7. doOnUnsubscribe：當Observable被取消訂閱，更改線程狀態爲TERMINAL
8. subscribeOn：指定scheduler