Flume-ng源码解析之Source组件

如果你还没看过Flume-ng源码解析系列中的启动流程、Channel组件和Sink组件，可以点击下面链接：
Flume-ng源码解析之启动流程
Flume-ng源码解析之Channel组件
Flume-ng源码解析之Sink组件

在前面三篇文章中我们初步了解了Flume的启动流程、Channel组件和Sink组件，接下来我们一起来看看agent三大组件中Source组件。

1 Source

Source，作为agent中的消息来源组件，我们来看看它是如何将event传递给channel的和它的特性。

依然先看代码：

@InterfaceAudience.Public
@InterfaceStability.Stable
public interface Source extends LifecycleAware, NamedComponent {
  public void setChannelProcessor(ChannelProcessor channelProcessor);
  public ChannelProcessor getChannelProcessor();
}

我们可以看到它里面定义的两个需要实现方法是getChannelProcessor和setChannelProcessor，我们大概可以猜到，source就是通过ChannelProcessor将event传输给channel的。

这里先来了解一下Source的类型，Flume根据数据来源的特性将Source分成两类类，像Http、netcat和exec等就是属于事件驱动型（EventDrivenSource），而kafka和Jms等就是属于轮询拉取型（PollableSource）。

据我们在启动流程中了解到的，Application是先启动SourceRunner，再由SourceRunner来启动source，那么既然source有两种类型，那么Sourcerunner也分为EventDrivenSourceRunner和PollableSourceRunner，我们来看看它们的start()：

EventDrivenSourceRunner

public class EventDrivenSourceRunner extends SourceRunner {
  …
  @Override
  public void start() {
    Source source = getSource();
    ChannelProcessor cp = source.getChannelProcessor();
    cp.initialize();
    source.start();
    lifecycleState = LifecycleState.START;
  }
  …
}

PollableSourceRunner

public class PollableSourceRunner extends SourceRunner {

  …
  @Override
  public void start() {
    PollableSource source = (PollableSource) getSource();
    ChannelProcessor cp = source.getChannelProcessor();
    cp.initialize();
    source.start();

    runner = new PollingRunner();

    runner.source = source;
    runner.counterGroup = counterGroup;
    runner.shouldStop = shouldStop;

    runnerThread = new Thread(runner);
    runnerThread.setName(getClass().getSimpleName() + "-" + 
        source.getClass().getSimpleName() + "-" + source.getName());
    runnerThread.start();

    lifecycleState = LifecycleState.START;
  }

  …
  public static class PollingRunner implements Runnable {

    private PollableSource source;
    private AtomicBoolean shouldStop;
    private CounterGroup counterGroup;

    @Override
    public void run() {
      logger.debug("Polling runner starting. Source:{}", source);

      while (!shouldStop.get()) {
        counterGroup.incrementAndGet("runner.polls");

        try {
          if (source.process().equals(PollableSource.Status.BACKOFF)) {
            counterGroup.incrementAndGet("runner.backoffs");

            Thread.sleep(Math.min(
                counterGroup.incrementAndGet("runner.backoffs.consecutive")
                * source.getBackOffSleepIncrement(), source.getMaxBackOffSleepInterval()));
          } else {
            counterGroup.set("runner.backoffs.consecutive", 0L);
          }
        } catch (InterruptedException e) {
          logger.info("Source runner interrupted. Exiting");
          counterGroup.incrementAndGet("runner.interruptions");
        } catch (EventDeliveryException e) {
          logger.error("Unable to deliver event. Exception follows.", e);
          counterGroup.incrementAndGet("runner.deliveryErrors");
        } catch (Exception e) {
          counterGroup.incrementAndGet("runner.errors");
          logger.error("Unhandled exception, logging and sleeping for " +
              source.getMaxBackOffSleepInterval() + "ms", e);
          try {
            Thread.sleep(source.getMaxBackOffSleepInterval());
          } catch (InterruptedException ex) {
            Thread.currentThread().interrupt();
          }
        }
      }

      logger.debug("Polling runner exiting. Metrics:{}", counterGroup);
    }

  }

}

无论是PollableSourceRunner还是EventDrivenSourceRunner，都是调用它里面的source的start()。这个时候我们看到ChannelProcessor的存在，那么就会有疑惑，这ChannelProcessor哪来的？我们还是得看回AbstarctConfigurationProvider，查看里面的loadSources()，我们就会发现下面这段代码：

ChannelSelectorConfiguration selectorConfig = config.getSelectorConfiguration();
ChannelSelector selector = ChannelSelectorFactory.create(sourceChannels, selectorConfig);
ChannelProcessor channelProcessor = new ChannelProcessor(selector);
Configurables.configure(channelProcessor, config);
source.setChannelProcessor(channelProcessor);

到这里我们基本已经了解了Source的启动流程，下面以AvroSource为例看看，source是在哪里调用ChannelProcessor的插入方法。

2 AvroSource

public class AvroSource extends AbstractSource implements EventDrivenSource,
    Configurable, AvroSourceProtocol {
  …
  @Override
  public Status append(AvroFlumeEvent avroEvent) {
    if (logger.isDebugEnabled()) {
      if (LogPrivacyUtil.allowLogRawData()) {
        logger.debug("Avro source {}: Received avro event: {}", getName(), avroEvent);
      } else {
        logger.debug("Avro source {}: Received avro event", getName());
      }
    }

    sourceCounter.incrementAppendReceivedCount();
    sourceCounter.incrementEventReceivedCount();

    Event event = EventBuilder.withBody(avroEvent.getBody().array(),
        toStringMap(avroEvent.getHeaders()));

    try {
      getChannelProcessor().processEvent(event);
    } catch (ChannelException ex) {
      logger.warn("Avro source " + getName() + ": Unable to process event. " +
          "Exception follows.", ex);
      return Status.FAILED;
    }

    sourceCounter.incrementAppendAcceptedCount();
    sourceCounter.incrementEventAcceptedCount();

    return Status.OK;
  }

  @Override
  public Status appendBatch(List events) {
    logger.debug("Avro source {}: Received avro event batch of {} events.",
        getName(), events.size());
    sourceCounter.incrementAppendBatchReceivedCount();
    sourceCounter.addToEventReceivedCount(events.size());

    List batch = new ArrayList();

    for (AvroFlumeEvent avroEvent : events) {
      Event event = EventBuilder.withBody(avroEvent.getBody().array(),
          toStringMap(avroEvent.getHeaders()));

      batch.add(event);
    }

    try {
      getChannelProcessor().processEventBatch(batch);
    } catch (Throwable t) {
      logger.error("Avro source " + getName() + ": Unable to process event " +
          "batch. Exception follows.", t);
      if (t instanceof Error) {
        throw (Error) t;
      }
      return Status.FAILED;
    }

    sourceCounter.incrementAppendBatchAcceptedCount();
    sourceCounter.addToEventAcceptedCount(events.size());

    return Status.OK;
  }
  …
}

在append方法中我们可以看到getChannelProcessor().processEvent(event);，所以不同的Source根据它的不同触发机制和拉取机制，在特定的时候调用ChannelProcessor来执行event的插入。 ·

到此为止，我们就完成了对Flume启动流程和三大组件的研究，鉴于能力，其中有些细节没办法深入研究，希望以后有时间能够继续深入分析下去。