源码分析Flume启动过程

时间：2021-01-01 04:22:59

对Flume-NG的agent启动过程进行详细的分析。

启动过程

flume的main函数在Application.java中，在flume-ng的shell启动脚本中会用java来起flume：

$EXEC $JAVA_HOME/bin/java $JAVA_OPTS $FLUME_JAVA_OPTS "${arr_java_props[@]}" -cp "$FLUME_CLASSPATH" -Djava.library.path=$FLUME_JAVA_LIBRARY_PATH "$FLUME_APPLICATION_CLASS" $*

在main函数中，会检查一系列参数，最重要的是no-reload-conf，根据reload的不同，判断是否动态加载配置文件，然后start：

List<LifecycleAware> components = Lists.newArrayList();

if (reload) {
          EventBus eventBus = new EventBus(agentName + "-event-bus");
          PollingPropertiesFileConfigurationProvider configurationProvider =
new PollingPropertiesFileConfigurationProvider(
              agentName, configurationFile, eventBus, 30);
          components.add(configurationProvider);
          application = new Application(components);
          eventBus.register(application);
        } else {
          PropertiesFileConfigurationProvider configurationProvider =
new PropertiesFileConfigurationProvider(
              agentName, configurationFile);
          application = new Application();
          application.handleConfigurationEvent(configurationProvider
            .getConfiguration());
        }
        application.start();

这里面有很多需要讲解的：


1.1 LifecycleAware接口

实现这个接口的类是有定义好的一系列状态转化的生命周期的：

@InterfaceAudience.Public
@InterfaceStability.Stable
public interface LifecycleAware {

/**
 * <p>
 * Starts a service or component.
 * </p>
 * <p>
 * Implementations should determine the result of any start logic and effect
 * the return value of {@link #getLifecycleState()} accordingly.
 * </p>
 *
 * @throws LifecycleException
 * @throws InterruptedException
 */
public void start();

/**
 * <p>
 * Stops a service or component.
 * </p>
 * <p>
 * Implementations should determine the result of any stop logic and effect
 * the return value of {@link #getLifecycleState()} accordingly.
 * </p>
 *
 * @throws LifecycleException
 * @throws InterruptedException
 */
public void stop();

/**
 * <p>
 * Return the current state of the service or component.
 * </p>
 */
public LifecycleState getLifecycleState();

}

比如给的样例：

 * Example usage
 * </p>
 * <code>
 *  public class MyService implements LifecycleAware {
 *
 *    private LifecycleState lifecycleState;
 *
 *    public MyService() {
 *      lifecycleState = LifecycleState.IDLE;
 *    }
 *
 *    @Override
 *    public void start(Context context) throws LifecycleException,
 *      InterruptedException {
 *
 *      ...your code does something.
 *
 *      lifecycleState = LifecycleState.START;
 *    }
 *
 *    @Override
 *    public void stop(Context context) throws LifecycleException,
 *      InterruptedException {
 *
 *      try {
 *        ...you stop services here.
 *      } catch (SomethingException) {
 *        lifecycleState = LifecycleState.ERROR;
 *      }
 *
 *      lifecycleState = LifecycleState.STOP;
 *    }
 *
 *    @Override
 *    public LifecycleState getLifecycleState() {
 *      return lifecycleState;
 *    }
 *
 *  }
 * </code>
 */

每一个flume node，每一个flume component(source, channel, sink)都实现了这个接口。


1.2 EventBus事件监听和发布订阅模式

回到上面，再往前判断是否设置了no-reload-conf，如果设置了，会新建一个EventBus，它是Guava(Google写的java库)的事件处理机制，是设计模式中的观察者模式（生产/消费者编程模型）的优雅实现。

使用Guava之后, 如果要订阅消息, 就不用再继承指定的接口, 只需要在指定的方法上加上@Subscribe注解即可。

我们看flume是如何用的：

PollingPropertiesFileConfigurationProvider configurationProvider =
 new PollingPropertiesFileConfigurationProvider(
 agentName, configurationFile, eventBus, 30);

将eventBus传给PollingPropertiesFileConfigurationProvider，一方面它继承了PropertiesFileConfigurationProvider类，说明它是配置文件的提供者，另一方面，它实现了LifecycleAware接口，说明它是有生命周期的。

那么它在生命周期做了什么？

  @Override
  public void start() {
    LOGGER.info("Configuration provider starting");

    Preconditions.checkState(file != null,
"The parameter file must not be null");

    executorService = Executors.newSingleThreadScheduledExecutor(
            new ThreadFactoryBuilder().setNameFormat("conf-file-poller-%d")
.build());

    FileWatcherRunnable fileWatcherRunnable =
        new FileWatcherRunnable(file, counterGroup);

    executorService.scheduleWithFixedDelay(fileWatcherRunnable, 0, interval,
        TimeUnit.SECONDS);

    lifecycleState = LifecycleState.START;

    LOGGER.debug("Configuration provider started");
  }

  @Override
  public void stop() {
    LOGGER.info("Configuration provider stopping");

    executorService.shutdown();
    try{
      while(!executorService.awaitTermination(500, TimeUnit.MILLISECONDS)) {
        LOGGER.debug("Waiting for file watcher to terminate");
      }
    } catch (InterruptedException e) {
      LOGGER.debug("Interrupted while waiting for file watcher to terminate");
      Thread.currentThread().interrupt();
    }
    lifecycleState = LifecycleState.STOP;
    LOGGER.debug("Configuration provider stopped");
  }

可以看到，在start时候，它起了一个周期调用线程executorService，这个周期调用线程又回每隔30s调用fileWatcherRunnable这个配置文件监控线程，在FileWatcherRunnable这里面，会去监听flume配置文件的变化，如果修改时间发生变化，eventBus会说我感兴趣的事件发生了！即eventBus.post(getConfiguration())

    @Override
public void run() {
      LOGGER.debug("Checking file:{} for changes", file);

      counterGroup.incrementAndGet("file.checks");

long lastModified = file.lastModified();

if (lastModified > lastChange) {
        LOGGER.info("Reloading configuration file:{}", file);

        counterGroup.incrementAndGet("file.loads");

        lastChange = lastModified;

try {
          eventBus.post(getConfiguration());
        } catch (Exception e) {
          LOGGER.error("Failed to load configuration data. Exception follows.",
              e);
        } catch (NoClassDefFoundError e) {
          LOGGER.error("Failed to start agent because dependencies were not " +
"found in classpath. Error follows.", e);
        } catch (Throwable t) {
// caught because the caller does not handle or log Throwables
          LOGGER.error("Unhandled error", t);
        }
      }
    }

之后对该事件感兴趣的listener就会进行事件处理，这里flume本身的Application对配置文件的变化感兴趣：

eventBus.register(application);
# 相当于 
eventBus.register(listener);

在application中，用注解@Subscribe标明的方法就告诉了我们，事件发生后，如何处理：

  @Subscribe
public synchronized void handleConfigurationEvent(MaterializedConfiguration conf) {
    stopAllComponents();
    startAllComponents(conf);
  }

到这里为止，讲清楚了如果启动flume时候配置了no-reload-con参数，flume就会动态加载配置文件，默认每30秒检查一次配置文件，如果有修改，会重启所有的components；如果没有配置该参数，则只会启动一次。


1.3 Application的start前与后

1.3.1start前：
components.add(configurationProvider);
application = new Application(components);

将监听文件变化的类作为一个component

  public Application(List<LifecycleAware> components) {
this.components = components;
    supervisor = new LifecycleSupervisor();
  }

然后这里出现了另一个重要的类LifecycleSupervisor：

  public LifecycleSupervisor() {
    lifecycleState = LifecycleState.IDLE;
    supervisedProcesses = new HashMap<LifecycleAware, Supervisoree>();
    monitorFutures = new HashMap<LifecycleAware, ScheduledFuture<?>>();
    monitorService = new ScheduledThreadPoolExecutor(10,
new ThreadFactoryBuilder().setNameFormat(
"lifecycleSupervisor-" + Thread.currentThread().getId() + "-%d")
            .build());
    monitorService.setMaximumPoolSize(20);
    monitorService.setKeepAliveTime(30, TimeUnit.SECONDS);
    purger = new Purger();
    needToPurge = false;
  }

可以看到，它构建了一个定期执行任务的线程池，大小是10


1.3.2 start后：application.start();

对每一个component使用SupervisorPolicy.AlwaysRestartPolicy的策略，有监督的执行：

public synchronized void start() {
for(LifecycleAware component : components) {
      supervisor.supervise(component,
new SupervisorPolicy.AlwaysRestartPolicy(), LifecycleState.START);
    }
  }

再看supervisor.supervise()方法的主要流程：

Supervisoree process = new Supervisoree();
    process.status = new Status();

    process.policy = policy;
    process.status.desiredState = desiredState;
    process.status.error = false;

    MonitorRunnable monitorRunnable = new MonitorRunnable();
    monitorRunnable.lifecycleAware = lifecycleAware;
    monitorRunnable.supervisoree = process;
    monitorRunnable.monitorService = monitorService;

    supervisedProcesses.put(lifecycleAware, process);

    ScheduledFuture<?> future = monitorService.scheduleWithFixedDelay(
        monitorRunnable, 0, 3, TimeUnit.SECONDS);
    monitorFutures.put(lifecycleAware, future);

对每一个component会通过线程池ScheduledThreadPoolExecutor monitorService起一个MonitorRunnable线程执行，在monitorRunnable中的run()方法中，是每一个component的执行逻辑，根据desiredState的不同，选择不同的状态执行方法去执行，比如：

  1. PollingPropertiesFileConfigurationProvider有对应监控配置文件的start、stop逻辑；
  2. ExecSource有对应的source的start、stop执行逻辑；
  3. MemoryChannel有对应channel的start、stop逻辑；
  4. ElasticSearchSink有对应sink的start、stop逻辑；

等等…

那每个component，也就是每个monitorRunnable隔多长时间会执行一次呢？

    ScheduledFuture<?> future = monitorService.scheduleWithFixedDelay(
        monitorRunnable, 0, 3, TimeUnit.SECONDS);

可以看出，这里设定的执行线程计划是：初始0s延迟，每次任务完成后延迟3s再执行一次任务，比如，ExecSource每次从数据源取数据，取完后回来隔3s再去取一次。

在application.start()后，还有一个钩子方法：

final Application appReference = application;
      Runtime.getRuntime().addShutdownHook(new Thread("agent-shutdown-hook") {
@Override
public void run() {
          appReference.stop();
        }
      });

它主要在agent停止后用来执行内存清理、对象销毁等操作。

好了，以上就是对flume agent启动过程的详细分析。

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