聊聊HystrixCommandExecutionHook

序

本文主要研究一下HystrixCommandExecutionHook

HystrixCommandExecutionHook

hystrix-core-1.5.12-sources.jar!/com/netflix/hystrix/strategy/executionhook/HystrixCommandExecutionHook.java

/**
 * Abstract ExecutionHook with invocations at different lifecycle points of {@link HystrixCommand}
 * and {@link HystrixObservableCommand} execution with default no-op implementations.
 * <p>
 * See {@link HystrixPlugins} or the Hystrix GitHub Wiki for information on configuring plugins: <a
 * href="https://github.com/Netflix/Hystrix/wiki/Plugins">https://github.com/Netflix/Hystrix/wiki/Plugins</a>.
 * <p>
 * <b>Note on thread-safety and performance</b>
 * <p>
 * A single implementation of this class will be used globally so methods on this class will be invoked concurrently from multiple threads so all functionality must be thread-safe.
 * <p>
 * Methods are also invoked synchronously and will add to execution time of the commands so all behavior should be fast. If anything time-consuming is to be done it should be spawned asynchronously
 * onto separate worker threads.
 * 
 * @since 1.2
 * */
public abstract class HystrixCommandExecutionHook {

    /**
     * Invoked before {@link HystrixInvokable} begins executing.
     *
     * @param commandInstance The executing HystrixInvokable instance.
     *
     * @since 1.2
     */
    public <T> void onStart(HystrixInvokable<T> commandInstance) {
        //do nothing by default
    }

    /**
     * Invoked when {@link HystrixInvokable} emits a value.
     *
     * @param commandInstance The executing HystrixInvokable instance.
     * @param value value emitted
     *
     * @since 1.4
     */
    public <T> T onEmit(HystrixInvokable<T> commandInstance, T value) {
        return value; //by default, just pass through
    }

    /**
     * Invoked when {@link HystrixInvokable} fails with an Exception.
     *
     * @param commandInstance The executing HystrixInvokable instance.
     * @param failureType {@link FailureType} enum representing which type of error
     * @param e exception object
     *
     * @since 1.2
     */
    public <T> Exception onError(HystrixInvokable<T> commandInstance, FailureType failureType, Exception e) {
        return e; //by default, just pass through
    }

    /**
     * Invoked when {@link HystrixInvokable} finishes a successful execution.
     *
     * @param commandInstance The executing HystrixInvokable instance.
     *
     * @since 1.4
     */
    public <T> void onSuccess(HystrixInvokable<T> commandInstance) {
        //do nothing by default
    }

    /**
     * Invoked at start of thread execution when {@link HystrixCommand} is executed using {@link ExecutionIsolationStrategy#THREAD}.
     *
     * @param commandInstance The executing HystrixCommand instance.
     *
     * @since 1.2
     */
    public <T> void onThreadStart(HystrixInvokable<T> commandInstance) {
        //do nothing by default
    }

    /**
     * Invoked at completion of thread execution when {@link HystrixCommand} is executed using {@link ExecutionIsolationStrategy#THREAD}.
     * This will get invoked whenever the Hystrix thread is done executing, regardless of whether the thread finished
     * naturally, or was unsubscribed externally
     *
     * @param commandInstance The executing HystrixCommand instance.
     *
     * @since 1.2
     */
    public <T> void onThreadComplete(HystrixInvokable<T> commandInstance) {
        // do nothing by default
    }

    /**
     * Invoked when the user-defined execution method in {@link HystrixInvokable} starts.
     *
     * @param commandInstance The executing HystrixInvokable instance.
     *
     * @since 1.4
     */
    public <T> void onExecutionStart(HystrixInvokable<T> commandInstance) {
        //do nothing by default
    }

    /**
     * Invoked when the user-defined execution method in {@link HystrixInvokable} emits a value.
     *
     * @param commandInstance The executing HystrixInvokable instance.
     * @param value value emitted
     *
     * @since 1.4
     */
    public <T> T onExecutionEmit(HystrixInvokable<T> commandInstance, T value) {
        return value; //by default, just pass through
    }

    /**
     * Invoked when the user-defined execution method in {@link HystrixInvokable} fails with an Exception.
     *
     * @param commandInstance The executing HystrixInvokable instance.
     * @param e exception object
     *
     * @since 1.4
     */
    public <T> Exception onExecutionError(HystrixInvokable<T> commandInstance, Exception e) {
        return e; //by default, just pass through
    }

    /**
     * Invoked when the user-defined execution method in {@link HystrixInvokable} completes successfully.
     *
     * @param commandInstance The executing HystrixInvokable instance.
     *
     * @since 1.4
     */
    public <T> void onExecutionSuccess(HystrixInvokable<T> commandInstance) {
        //do nothing by default
    }

    /**
     * Invoked when the fallback method in {@link HystrixInvokable} starts.
     *
     * @param commandInstance The executing HystrixInvokable instance.
     *
     * @since 1.2
     */
    public <T> void onFallbackStart(HystrixInvokable<T> commandInstance) {
        //do nothing by default
    }

    /**
     * Invoked when the fallback method in {@link HystrixInvokable} emits a value.
     *
     * @param commandInstance The executing HystrixInvokable instance.
     * @param value value emitted
     *
     * @since 1.4
     */
    public <T> T onFallbackEmit(HystrixInvokable<T> commandInstance, T value) {
        return value; //by default, just pass through
    }

    /**
     * Invoked when the fallback method in {@link HystrixInvokable} fails with an Exception.
     *
     * @param commandInstance The executing HystrixInvokable instance.
     * @param e exception object
     *
     * @since 1.2
     */
    public <T> Exception onFallbackError(HystrixInvokable<T> commandInstance, Exception e) {
        //by default, just pass through
        return e;
    }

    /**
     * Invoked when the user-defined execution method in {@link HystrixInvokable} completes successfully.
     *
     * @param commandInstance The executing HystrixInvokable instance.
     *
     * @since 1.4
     */
    public <T> void onFallbackSuccess(HystrixInvokable<T> commandInstance) {
        //do nothing by default
    }

    /**
     * Invoked when the command response is found in the {@link com.netflix.hystrix.HystrixRequestCache}.
     *
     * @param commandInstance The executing HystrixCommand
     *
     * @since 1.4
     */
    public <T> void onCacheHit(HystrixInvokable<T> commandInstance) {
        //do nothing by default
    }

    /**
     * Invoked with the command is unsubscribed before a terminal state
     *
     * @param commandInstance The executing HystrixInvokable instance.
     *
     * @since 1.5.9
     */
    public <T> void onUnsubscribe(HystrixInvokable<T> commandInstance) {
        //do nothing by default
    }

}

主要规范了如下钩子方法：

• onStart
• onEmit
• onError
• onSuccess
• onThreadStart
• onThreadComplete
• onExecutionStart
• onExecutionEmit
• onExecutionError
• onExecutionSuccess
• onFallbackStart
• onFallbackEmit
• onFallbackError
• onFallbackSuccess
• onCacheHit
• onUnsubscribe

HystrixCommandExecutionHookDefault

hystrix-core-1.5.12-sources.jar!/com/netflix/hystrix/strategy/executionhook/HystrixCommandExecutionHookDefault.java

/**
 * Default implementations of {@link HystrixCommandExecutionHook} that does nothing.
 * 
 * @ExcludeFromJavadoc
 */
public class HystrixCommandExecutionHookDefault extends HystrixCommandExecutionHook {

    private static HystrixCommandExecutionHookDefault INSTANCE = new HystrixCommandExecutionHookDefault();

    private HystrixCommandExecutionHookDefault() {

    }

    public static HystrixCommandExecutionHook getInstance() {
        return INSTANCE;
    }

}

默认的实现并没有重新钩子方法

回调

在hystrix-core-1.5.12-sources.jar!/com/netflix/hystrix/AbstractCommand.java这个类中，在相应的方法里头对executionHook进行了相应的回调，比如：

    private Observable<R> executeCommandWithSpecifiedIsolation(final AbstractCommand<R> _cmd) {
        if (properties.executionIsolationStrategy().get() == ExecutionIsolationStrategy.THREAD) {
            // mark that we are executing in a thread (even if we end up being rejected we still were a THREAD execution and not SEMAPHORE)
            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) {
                        // the command timed out in the wrapping thread so we will return immediately
                        // and not increment any of the counters below or other such logic
                        return Observable.error(new RuntimeException("timed out before executing run()"));
                    }
                    if (threadState.compareAndSet(ThreadState.NOT_USING_THREAD, ThreadState.STARTED)) {
                        //we have not been unsubscribed, so should proceed
                        HystrixCounters.incrementGlobalConcurrentThreads();
                        threadPool.markThreadExecution();
                        // store the command that is being run
                        endCurrentThreadExecutingCommand = Hystrix.startCurrentThreadExecutingCommand(getCommandKey());
                        executionResult = executionResult.setExecutedInThread();
                        /**
                         * If any of these hooks throw an exception, then it appears as if the actual execution threw an error
                         */
                        try {
                            executionHook.onThreadStart(_cmd);
                            executionHook.onRunStart(_cmd);
                            executionHook.onExecutionStart(_cmd);
                            return getUserExecutionObservable(_cmd);
                        } catch (Throwable ex) {
                            return Observable.error(ex);
                        }
                    } else {
                        //command has already been unsubscribed, so return immediately
                        return Observable.error(new RuntimeException("unsubscribed before executing run()"));
                    }
                }
            }).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)) {
                        //if it was never started and received terminal, then no need to clean up (I don't think this is possible)
                    }
                    //if it was unsubscribed, then other cleanup handled it
                }
            }).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)) {
                        //if it was never started and was cancelled, then no need to clean up
                    }
                    //if it was terminal, then other cleanup handled it
                }
            }).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);
                    }
                }
            });
        }
    }

这个方法回调了executionHook的onThreadStart、onRunStart、onExecutionStart方法。

小结

HystrixCommandExecutionHook提供了对HystrixCommand及HystrixObservableCommand生命周期的钩子方法，开发者可以自定义实现，做一些额外的处理，比如日志打印、覆盖response、更改线程状态等等。

doc

• Command Execution Hook