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本文主要研究一下LoggingEventAsyncDisruptorAppender

LoggingEventAsyncDisruptorAppender

net/logstash/logback/appender/LoggingEventAsyncDisruptorAppender.java

public class LoggingEventAsyncDisruptorAppender extends DelegatingAsyncDisruptorAppender<ILoggingEvent, AppenderListener<ILoggingEvent>> {

    /**
     * Set to true if the caller data should be captured before publishing the event
     * to the {@link RingBuffer}
     */
    private boolean includeCallerData;

    protected void prepareForDeferredProcessing(ILoggingEvent event) {
        super.prepareForDeferredProcessing(event);
        if (includeCallerData) {
            event.getCallerData();
        }
    }

    public boolean isIncludeCallerData() {
        return includeCallerData;
    }

    public void setIncludeCallerData(boolean includeCallerData) {
        this.includeCallerData = includeCallerData;
    }

}

LoggingEventAsyncDisruptorAppender继承了DelegatingAsyncDisruptorAppender，它定义了includeCallerData属性，其prepareForDeferredProcessing在includeCallerData为true时执行event.getCallerData()

DelegatingAsyncDisruptorAppender

net/logstash/logback/appender/DelegatingAsyncDisruptorAppender.java

public abstract class DelegatingAsyncDisruptorAppender<Event extends DeferredProcessingAware, Listener extends AppenderListener<Event>> extends AsyncDisruptorAppender<Event, Listener> implements AppenderAttachable<Event> {
    
    /**
     * The delegate appenders.
     */
    private final AppenderAttachableImpl<Event> appenders = new AppenderAttachableImpl<>();
    
    @Override
    protected EventHandler<LogEvent<Event>> createEventHandler() {
        return new DelegatingEventHandler();
    }
    
    @Override
    public void start() {
        startDelegateAppenders();
        super.start();
    }
    
    @Override
    public void stop() {
        if (!isStarted()) {
            return;
        }
        super.stop();
        stopDelegateAppenders();
    }

    private void startDelegateAppenders() {
        for (Iterator<Appender<Event>> appenderIter = appenders.iteratorForAppenders(); appenderIter.hasNext();) {
            Appender<Event> appender = appenderIter.next();
            if (appender.getContext() == null) {
                appender.setContext(getContext());
            }
            if (!appender.isStarted()) {
                appender.start();
            }
        }
    }

    private void stopDelegateAppenders() {
        for (Iterator<Appender<Event>> appenderIter = appenders.iteratorForAppenders(); appenderIter.hasNext();) {
            Appender<Event> appender = appenderIter.next();
            if (appender.isStarted()) {
                appender.stop();
            }
        }
    }

    @Override
    public void addAppender(Appender<Event> newAppender) {
        appenders.addAppender(newAppender);
    }

    @Override
    public Iterator<Appender<Event>> iteratorForAppenders() {
        return appenders.iteratorForAppenders();
    }

    @Override
    public Appender<Event> getAppender(String name) {
        return appenders.getAppender(name);
    }

    @Override
    public boolean isAttached(Appender<Event> appender) {
        return appenders.isAttached(appender);
    }

    @Override
    public void detachAndStopAllAppenders() {
        appenders.detachAndStopAllAppenders();
    }

    @Override
    public boolean detachAppender(Appender<Event> appender) {
        return appenders.detachAppender(appender);
    }

    @Override
    public boolean detachAppender(String name) {
        return appenders.detachAppender(name);
    }

}

DelegatingAsyncDisruptorAppender继承了AsyncDisruptorAppender，它定义了AppenderAttachableImpl，其createEventHandler创建的是DelegatingEventHandler，其start方法会执行startDelegateAppenders，其stop方法会执行stopDelegateAppenders，其addAppender会添加appender到AppenderAttachableImpl中

DelegatingEventHandler

    private class DelegatingEventHandler implements EventHandler<LogEvent<Event>> {
        /**
         * Whether exceptions should be reported with a error status or not.
         */
        private boolean silentError;
        
        @Override
        public void onEvent(LogEvent<Event> logEvent, long sequence, boolean endOfBatch) throws Exception {
            
            boolean exceptionThrown = false;
            for (Iterator<Appender<Event>> it = appenders.iteratorForAppenders(); it.hasNext();) {
                Appender<Event> appender = it.next();
                
                try {
                    appender.doAppend(logEvent.event);
                    
                    /*
                     * Optimization:
                     *
                     * If any of the delegate appenders are instances of OutputStreamAppender or Flushable,
                     * then flush them at the end of the batch.
                     */
                    if (endOfBatch) {
                        flushAppender(appender);
                    }
                } catch (Exception e) {
                    exceptionThrown = true;
                    if (!this.silentError) {
                        addError(String.format("Unable to forward event to appender [%s]: %s", appender.getName(), e.getMessage()), e);
                    }
                }
            }
            
            this.silentError = exceptionThrown;
        }
        
        
        private void flushAppender(Appender<Event> appender) throws IOException {
            // Similar to #doAppend() - don't flush if appender is stopped
            if (!appender.isStarted()) {
                return;
            }
            if (appender instanceof Flushable) {
                flushAppender((Flushable) appender);
            } else if (appender instanceof OutputStreamAppender) {
                flushAppender((OutputStreamAppender<Event>) appender);
            }
        }
        
        private void flushAppender(OutputStreamAppender<Event> appender) throws IOException {
            if (!appender.isImmediateFlush()) {
                OutputStream os = appender.getOutputStream();
                if (os != null) {
                    os.flush();
                }
            }
        }
        
        private void flushAppender(Flushable appender) throws IOException {
            appender.flush();
        }
    }

DelegatingEventHandler实现了EventHandler接口，其onEvent方法主要是遍历AppenderAttachableImpl，挨个执行appender.doAppend(logEvent.event)，在endOfBatch的时候会执行flushAppender

AsyncDisruptorAppender

net/logstash/logback/appender/AsyncDisruptorAppender.java

public abstract class AsyncDisruptorAppender<Event extends DeferredProcessingAware, Listener extends AppenderListener<Event>> extends UnsynchronizedAppenderBase<Event> {
    /**
     * Time in nanos to wait between drain attempts during the shutdown phase
     */
    private static final long SLEEP_TIME_DURING_SHUTDOWN = 50 * 1_000_000L; // 50ms

    protected static final String APPENDER_NAME_FORMAT = "%1$s";
    protected static final String THREAD_INDEX_FORMAT = "%2$d";
    public static final String DEFAULT_THREAD_NAME_FORMAT = "logback-appender-" + APPENDER_NAME_FORMAT + "-" + THREAD_INDEX_FORMAT;

    public static final int DEFAULT_RING_BUFFER_SIZE = 8192;
    public static final ProducerType DEFAULT_PRODUCER_TYPE = ProducerType.MULTI;
    public static final WaitStrategy DEFAULT_WAIT_STRATEGY = new BlockingWaitStrategy();
    public static final int DEFAULT_DROPPED_WARN_FREQUENCY = 1000;

    private static final RingBufferFullException RING_BUFFER_FULL_EXCEPTION = new RingBufferFullException();
    static {
        RING_BUFFER_FULL_EXCEPTION.setStackTrace(new StackTraceElement[] {new StackTraceElement(AsyncDisruptorAppender.class.getName(), "append(..)", null, -1)});
    }

    /**
     * The size of the {@link RingBuffer}.
     * Defaults to {@value #DEFAULT_RING_BUFFER_SIZE}.
     * <p>
     * Must be a positive power of 2.
     */
    private int ringBufferSize = DEFAULT_RING_BUFFER_SIZE;

    /**
     * The {@link ProducerType} to use to configure the Disruptor.
     * Only set to {@link ProducerType#SINGLE} if only one thread
     * will ever be appending to this appender.
     */
    private ProducerType producerType = DEFAULT_PRODUCER_TYPE;

    /**
     * The {@link WaitStrategy} to used by the RingBuffer
     * when pulling events to be processed by {@link #eventHandler}.
     * <p>
     * By default, a {@link BlockingWaitStrategy} is used, which is the most
     * CPU conservative, but results in a higher latency.
     * If you need lower latency (at the cost of higher CPU usage),
     * consider using a {@link SleepingWaitStrategy} or a {@link PhasedBackoffWaitStrategy}.
     */
    private WaitStrategy waitStrategy = DEFAULT_WAIT_STRATEGY;

    /**
     * Pattern used by the {@link WorkerThreadFactory} to set the
     * handler thread name.
     * Defaults to {@value #DEFAULT_THREAD_NAME_FORMAT}.
     * <p>
     *
     * If you change the {@link #threadFactory}, then this
     * value may not be honored.
     * <p>
     *
     * The string is a format pattern understood by {@link Formatter#format(String, Object...)}.
     * {@link Formatter#format(String, Object...)} is used to
     * construct the actual thread name prefix.
     * The first argument (%1$s) is the string appender name.
     * The second argument (%2$d) is the numerical thread index.
     * Other arguments can be made available by subclasses.
     */
    private String threadNameFormat = DEFAULT_THREAD_NAME_FORMAT;

    /**
     * When true, child threads created by this appender will be daemon threads,
     * and therefore allow the JVM to exit gracefully without
     * needing to explicitly shut down the appender.
     * Note that in this case, it is possible for log events to not
     * be handled.
     * <p>
     *
     * When false, child threads created by this appender will not be daemon threads,
     * and therefore will prevent the JVM from shutting down
     * until the appender is explicitly shut down.
     * Set this to false if you want to ensure that every log event
     * prior to shutdown is handled.
     * <p>
     *
     * If you change the {@link #threadFactory}, then this
     * value may not be honored.
     */
    private boolean useDaemonThread = true;

    /**
     * When true, if no status listener is registered, then a default {@link OnConsoleStatusListener}
     * will be registered, so that error messages are seen on the console.
     */
    private boolean addDefaultStatusListener = true;

    /**
     * For every droppedWarnFrequency consecutive dropped events, log a warning.
     * Defaults to {@value #DEFAULT_DROPPED_WARN_FREQUENCY}.
     */
    private int droppedWarnFrequency = DEFAULT_DROPPED_WARN_FREQUENCY;

    /**
     * The {@link ThreadFactory} used to create the handler thread.
     */
    private ThreadFactory threadFactory = new WorkerThreadFactory();

    /**
     * The {@link Disruptor} containing the {@link RingBuffer} onto
     * which to publish events.
     */
    private Disruptor<LogEvent<Event>> disruptor;

    /**
     * Sets the {@link LogEvent#event} to the logback Event.
     * Used when publishing events to the {@link RingBuffer}.
     */
    private EventTranslatorOneArg<LogEvent<Event>, Event> eventTranslator = new LogEventTranslator<>();

    /**
     * Defines what happens when there is an exception during
     * {@link RingBuffer} processing.
     */
    private ExceptionHandler<LogEvent<Event>> exceptionHandler = new LogEventExceptionHandler();

    /**
     * Consecutive number of dropped events.
     */
    private final AtomicLong consecutiveDroppedCount = new AtomicLong();

    /**
     * The {@link EventFactory} used to create {@link LogEvent}s for the RingBuffer.
     */
    private LogEventFactory<Event> eventFactory = new LogEventFactory<>();

    /**
     * Incrementor number used as part of thread names for uniqueness.
     */
    private final AtomicInteger threadNumber = new AtomicInteger(1);

    /**
     * These listeners will be notified when certain events occur on this appender.
     */
    protected final List<Listener> listeners = new ArrayList<>();

    /**
     * Maximum time to wait when appending events to the ring buffer when full before the event
     * is dropped. Use the following values:
     * <ul>
     * <li>{@code -1} to disable timeout and wait until space becomes available.
     * <li>{@code 0} for no timeout and drop the event immediately when the buffer is full.
     * <li>{@code > 0} to retry during the specified amount of time.
     * </ul>
     */
    private Duration appendTimeout = Duration.buildByMilliseconds(0);

    /**
     * Delay between consecutive attempts to append an event in the ring buffer when
     * full.
     */
    private Duration appendRetryFrequency = Duration.buildByMilliseconds(5);
    
    /**
     * How long to wait for in-flight events during shutdown.
     */
    private Duration shutdownGracePeriod = Duration.buildByMinutes(1);

    /**
     * Lock used to limit the number of concurrent threads retrying at the same time
     */
    private final ReentrantLock lock = new ReentrantLock();

    //......
}    

AsyncDisruptorAppender继承了logback的UnsynchronizedAppenderBase，它使用了Disruptor的RingBuffer来进行异步，其默认的WaitStrategy为BlockingWaitStrategy，默认的ringBufferSize为8192，默认的producerType为ProducerType.MULTI，droppedWarnFrequency为1000

start

    public void start() {
        if (addDefaultStatusListener && getStatusManager() != null && getStatusManager().getCopyOfStatusListenerList().isEmpty()) {
            LevelFilteringStatusListener statusListener = new LevelFilteringStatusListener();
            statusListener.setLevelValue(Status.WARN);
            statusListener.setDelegate(new OnConsoleStatusListener());
            statusListener.setContext(getContext());
            statusListener.start();
            getStatusManager().add(statusListener);
        }

        this.disruptor = new Disruptor<>(
                this.eventFactory,
                this.ringBufferSize,
                this.threadFactory,
                this.producerType,
                this.waitStrategy);

        /*
         * Define the exceptionHandler first, so that it applies
         * to all future eventHandlers.
         */
        this.disruptor.setDefaultExceptionHandler(this.exceptionHandler);

        this.disruptor.handleEventsWith(new EventClearingEventHandler<>(createEventHandler()));

        this.disruptor.start();
        super.start();
        fireAppenderStarted();
    }

其start方法根据eventFactory、ringBufferSize、threadFactory、producerType、waitStrategy创建Disruptor，然后设置defaultExceptionHandler，设置EventHandler为EventClearingEventHandler，然后执行disruptor.start()，再执行super.start()

stop

    public void stop() {
        /*
         * Check super.isStarted() instead of isStarted() because subclasses
         * might override isStarted() to perform other comparisons that we don't
         * want to check here.  Those should be checked by subclasses
         * prior to calling super.stop()
         */
        if (!super.isStarted()) {
            return;
        }
        
        /*
         * Don't allow any more events to be appended.
         */
        super.stop();

        
        /*
         * Shutdown Disruptor
         *
         * Calling Disruptor#shutdown() will wait until all enqueued events are fully processed,
         * but this waiting happens in a busy-spin. To avoid wasting CPU we wait for at most the configured
         * grace period before asking the Disruptor for an immediate shutdown.
         */
        long deadline = getShutdownGracePeriod().getMilliseconds() < 0 ? Long.MAX_VALUE : System.currentTimeMillis() + getShutdownGracePeriod().getMilliseconds();
        while (!isRingBufferEmpty() && (System.currentTimeMillis() < deadline)) {
            LockSupport.parkNanos(SLEEP_TIME_DURING_SHUTDOWN);
        }
        
        this.disruptor.halt();

        if (!isRingBufferEmpty()) {
            addWarn("Some queued events have not been logged due to requested shutdown");
        }
        fireAppenderStopped();
    }

stop方法先执行super.stop()不让event再进来，然后根据shutdownGracePeriod计算deadline，在isRingBufferEmpty为false的时候进行等待，最后执行disruptor.halt()

append

    protected void append(Event event) {
        long startTime = System.nanoTime();
        
        try {
            prepareForDeferredProcessing(event);
        } catch (RuntimeException e) {
            addWarn("Unable to prepare event for deferred processing. Event output might be missing data.", e);
        }

        try {
            if (enqueue(event)) {
                // Log warning if we had drop before
                //
                long consecutiveDropped = this.consecutiveDroppedCount.get();
                if (consecutiveDropped != 0 && this.consecutiveDroppedCount.compareAndSet(consecutiveDropped, 0L)) {
                    addWarn("Dropped " + consecutiveDropped + " total events due to ring buffer at max capacity [" + this.ringBufferSize + "]");
                }
                
                // Notify listeners
                //
                fireEventAppended(event, System.nanoTime() - startTime);
                
            } else {
                // Log a warning status about the failure
                //
                long consecutiveDropped = this.consecutiveDroppedCount.incrementAndGet();
                if ((consecutiveDropped % this.droppedWarnFrequency) == 1) {
                    addWarn("Dropped " + consecutiveDropped + " events (and counting...) due to ring buffer at max capacity [" + this.ringBufferSize + "]");
                }
                
                // Notify listeners
                //
                fireEventAppendFailed(event, RING_BUFFER_FULL_EXCEPTION);
            }
            
        } catch (ShutdownInProgressException e) {
            // Same message as if Appender#append is called after the appender is stopped...
            addWarn("Attempted to append to non started appender [" + this.getName() + "].");
            
        } catch (InterruptedException e) {
            // be silent but re-interrupt the thread
            Thread.currentThread().interrupt();
        }
    }

其append方法先执行prepareForDeferredProcessing，再执行enqueue，enqueue不成功则递增consecutiveDroppedCount，再根据droppedWarnFrequency判断是否需要打印warning日志

enqueue

    private boolean enqueue(Event event) throws ShutdownInProgressException, InterruptedException {
        // Try enqueue the "normal" way
        //
        if (this.disruptor.getRingBuffer().tryPublishEvent(this.eventTranslator, event)) {
            return true;
        }
        
        // Drop event immediately when no retry
        //
        if (this.appendTimeout.getMilliseconds() == 0) {
            return false;
        }
        
        // Limit retries to a single thread at once to avoid burning CPU cycles "for nothing"
        // in CPU constraint environments.
        //
        long deadline = Long.MAX_VALUE;
        if (this.appendTimeout.getMilliseconds() < 0) {
            lock.lockInterruptibly();
            
        } else {
            deadline = System.currentTimeMillis() + this.appendTimeout.getMilliseconds();
            if (!lock.tryLock(this.appendTimeout.getMilliseconds(), TimeUnit.MILLISECONDS)) {
                return false;
            }
        }
        
        // Retry until deadline
        //
        long backoff = 1L;
        long backoffLimit = TimeUnit.MILLISECONDS.toNanos(this.appendRetryFrequency.getMilliseconds());
        
        try {
            do {
                if (!isStarted()) {
                    throw new ShutdownInProgressException();
                }

                if (deadline <= System.currentTimeMillis()) {
                    return false;
                }
                
                if (Thread.currentThread().isInterrupted()) {
                    throw new InterruptedException();
                }
                
                LockSupport.parkNanos(backoff);
                backoff = Math.min(backoff * 2, backoffLimit);
                
            } while (!this.disruptor.getRingBuffer().tryPublishEvent(this.eventTranslator, event));

            return true;
            
        } finally {
            lock.unlock();
        }
    }

enqueue方法先执行disruptor.getRingBuffer().tryPublishEvent，成功则返回true，否则根据appendTimeout判断是否需要重试，为0则返回false，否则根据appendTimeout计算deadline，然后进行lock.tryLock，再循环尝试disruptor.getRingBuffer().tryPublishEvent，直到成功或者deadline小于等于当前时间

小结

reactor-logback已经不在维护了EOL reactor-logback in 3.3+ #204，官方推荐使用logstash-logback-encoder。LoggingEventAsyncDisruptorAppender继承了DelegatingAsyncDisruptorAppender，主要是根据includeCallerData属性判断是否需要计算callerData；DelegatingAsyncDisruptorAppender继承了AsyncDisruptorAppender，它主要是组合了AppenderAttachableImpl实现了AppenderAttachable接口；AsyncDisruptorAppender则是使用了Disruptor的RingBuffer来进行异步，其默认的WaitStrategy为BlockingWaitStrategy，默认的ringBufferSize为8192，默认的producerType为ProducerType.MULTI，droppedWarnFrequency为1000。