logback避坑指南：AsyncAppender配置避坑

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发布于 2023-06-19 16:50:39

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AsyncAppender可能出现OOM问题

AsyncAppender会使用BlockingQueue缓存事件，默认缓存大小为256。

 /**
     * The default buffer size.
     */
    public static final int DEFAULT_QUEUE_SIZE = 256;
    int queueSize = DEFAULT_QUEUE_SIZE;

blockingQueue = new ArrayBlockingQueue<E>(queueSize);

如果设置了过大的queueSize可能会导致OOM的发生。

同时我们也要设置合适的discardingThreshold。

 static final int UNDEFINED = -1;
 int discardingThreshold = UNDEFINED;

discardingThreshold的默认值为queueSize的 1/5。

 if (discardingThreshold == UNDEFINED)
            discardingThreshold = queueSize / 5;

discardingThreshold的作用就是当缓存BlockingQueue所剩余的空间不足discardingThreshold值时，会丢弃日志级别为TRACE, DEBUG 及 INFO的事件。

@Override
    protected void append(E eventObject) {
        if (isQueueBelowDiscardingThreshold() && isDiscardable(eventObject)) {
            return;
        }
        preprocess(eventObject);
        put(eventObject);
    }

    private boolean isQueueBelowDiscardingThreshold() {
        return (blockingQueue.remainingCapacity() < discardingThreshold);
    }

/**
     * Events of level TRACE, DEBUG and INFO are deemed to be discardable.
     * @param event
     * @return true if the event is of level TRACE, DEBUG or INFO false otherwise.
     */
    protected boolean isDiscardable(ILoggingEvent event) {
        Level level = event.getLevel();
        return level.toInt() <= Level.INFO_INT;
    }

LOSSY BY DEFAULT IF 80% FULL AsyncAppender buffers events in a BlockingQueue. A worker thread created byAsyncAppender takes events from the head of the queue, and dispatches them to the single appender attached toAsyncAppender. Note that by default, AsyncAppender will drop events of level TRACE, DEBUG and INFO if its queue is 80% full. This strategy has an amazingly favorable effect on performance at the cost of event loss.

https://logback.qos.ch/manual/appenders.html

AsyncAppender可能出现丢失日志的问题

上述上谈到了discardingThreshold值的作用，及会丢弃日志级别为TRACE, DEBUG 及 INFO的事件。

如果BlockingQueue队列的大小设置的比较小（默认值就很小了已经），且discardingThreshold设置的值大于0或者默认值，就很容易丢弃丢弃日志级别为TRACE, DEBUG 及 INFO的事件。

如果不想日志丢失，设置discardingThreshold值为0，而且最好把BlockingQueue队列的大小queueSize值设置的大一点。

AsyncAppender可能出现阻塞的问题

AsyncAppender的异步机制的出现本来就是优化日志阻塞问题的，但是使用不当还是出现阻塞问题。

 private void put(E eventObject) {
        if (neverBlock) {
            blockingQueue.offer(eventObject);
        } else {
            putUninterruptibly(eventObject);
        }
    }

  private void putUninterruptibly(E eventObject) {
        boolean interrupted = false;
        try {
            while (true) {
                try {
                    blockingQueue.put(eventObject);
                    break;
                } catch (InterruptedException e) {
                    interrupted = true;
                }
            }
        } finally {
            if (interrupted) {
                Thread.currentThread().interrupt();
            }
        }
    }

neverBlockd的默认值为false，会调用BlockingQueue的阻塞方法put。