性能优化之异步日志

Posted 2022-11-24 Dreamer who

          你听说过打印日志能把系统拖垮的情况吗。

          所以项目中一般打印日志会使用异步AsyncAppender打印日志。

          那么使用了AsyncAppender，会不会性能就好了，就不会阻塞业务流程了，会不会丢失日志呢，我们来看一下logback的实现。

        

 

                                                                                 

          先看下官方文档的介绍，AsyncAppender 会把处理的事件缓存到一个阻塞队列，默认情况下达到队列容量的80%的时候，会丢弃TRACE, DEBUG and INFO级别的事件，

根据默认配置neverBlock=false，队列也会发生阻塞，所以设置true，虽然完全是非阻塞但会丢失日志，特别是error级别的日志。

       我们看下源码如何实现的。

public class AsyncAppender extends AsyncAppenderBase<ILoggingEvent> 

    boolean includeCallerData = false;

    /**
     * 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;
    

    protected void preprocess(ILoggingEvent eventObject) 
        eventObject.prepareForDeferredProcessing();
        if (includeCallerData)
            eventObject.getCallerData();
    

    public boolean isIncludeCallerData() 
        return includeCallerData;
    

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

根据实现：

protected boolean isDiscardable(ILoggingEvent event) 
        Level level = event.getLevel();
        return level.toInt() <= Level.INFO_INT;
    

 public static final int OFF_INT = Integer.MAX_VALUE;
    public static final int ERROR_INT = 40000;
    public static final int WARN_INT = 30000;
    public static final int INFO_INT = 20000;
    public static final int DEBUG_INT = 10000;
    public static final int TRACE_INT = 5000;
    public static final int ALL_INT = Integer.MIN_VALUE;

TRACE, DEBUG and INFO级别的事件，在容量默认到达80%的时候会丢弃。

事件放入阻塞队列的时候即调用put方法时会判断容量及是否可以丢弃条件的判断，如下：

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

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

    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();
            
        
    

当事件真正放入队列，是调用阻塞方法put，还是非阻塞方法offer，是由neverBlock配置决定的，默认false，会阻塞，所以为了性能，我们可以设置为true，也会丢弃日志。

真正处理队列事件的线程是默无闻的后台（Daemon）消费线程，

Worker worker = new Worker();

worker.setDaemon(true);
        worker.setName("AsyncAppender-Worker-" + getName());
        // make sure this instance is marked as "started" before staring the worker Thread
        super.start();
        worker.start()

 

 事件的最终归宿AppenderAttachableImpl<E> aa。

 

class Worker extends Thread 

        public void run() 
            AsyncAppenderBase<E> parent = AsyncAppenderBase.this;
            AppenderAttachableImpl<E> aai = parent.aai;

            // loop while the parent is started
            while (parent.isStarted()) 
                try 
                    E e = parent.blockingQueue.take();
                    aai.appendLoopOnAppenders(e);
                 catch (InterruptedException ie) 
                    break;
                
            

            addInfo("Worker thread will flush remaining events before exiting. ");

            for (E e : parent.blockingQueue) 
                aai.appendLoopOnAppenders(e);
                parent.blockingQueue.remove(e);
            

            aai.detachAndStopAllAppenders();
        
    

 

 

     异步打印日志是否非阻塞，是否会丢弃日志，是完全可以配置的，如果可以丢弃，依据日志的报警系统就不太准了，只能靠打点系统在系统内部aop的方式或其他方式打点上报了。