Hadoop MapReduce之ReduceTask任务执行（二）

  reduce任务为获得map的输出要和TaskTracker 通信以获得map的完成情况，负责这个工作的是GetMapEventsThread。线程运行时会通过TT的代理调用TaskUmbilicalProtocol协议的getMapCompletionEvents来获得MapTaskCompletionEventsUpdate，该返回数据总包含了最新map任务的完成状态：SUCCEEDED FAILED KILLED OBSOLETE TIPFAILED，线程会根据事件的不同状态更新相关集合。总之该线程总会不停的获得MapTask的完成信息，以便为其他线程(如copier)服务。

public void run() {
  LOG.info(reduceTask.getTaskID() + " Thread started: " + getName());
  
  do {
    try {
    	//获得新完成的MapTask的数量
      int numNewMaps = getMapCompletionEvents();
      if (LOG.isDebugEnabled()) {
        if (numNewMaps > 0) {
          LOG.debug(reduceTask.getTaskID() + ": " +  
              "Got " + numNewMaps + " new map-outputs"); 
        }
      }
      //线程暂停
      Thread.sleep(SLEEP_TIME);
    } 
    catch (InterruptedException e) {
      LOG.warn(reduceTask.getTaskID() +
          " GetMapEventsThread returning after an " +
          " interrupted exception");
      return;
    }
    catch (Throwable t) {
      String msg = reduceTask.getTaskID()
                   + " GetMapEventsThread Ignoring exception : " 
                   + StringUtils.stringifyException(t);
      reportFatalError(getTaskID(), t, msg);
    }
  } while (!exitGetMapEvents);


  LOG.info("GetMapEventsThread exiting");


}

主要逻辑包含在getMapCompletionEvents函数中

  private int getMapCompletionEvents() throws IOException {
    
    int numNewMaps = 0;
    //与TT通信获得事件数据
    MapTaskCompletionEventsUpdate update = 
      umbilical.getMapCompletionEvents(reduceTask.getJobID(), 
                                       fromEventId.get(), 
                                       MAX_EVENTS_TO_FETCH,
                                       reduceTask.getTaskID(), jvmContext);
                                       
    //提取事件数组
    TaskCompletionEvent events[] = update.getMapTaskCompletionEvents();
      
    // Check if the reset is required.
    // Since there is no ordering of the task completion events at the 
    // reducer, the only option to sync with the new jobtracker is to reset 
    // the events index
    if (update.shouldReset()) {
      fromEventId.set(0);
      obsoleteMapIds.clear(); // clear the obsolete map
      mapLocations.clear(); // clear the map locations mapping
    }
    
    // Update the last seen event ID
    fromEventId.set(fromEventId.get() + events.length);
    
    // Process the TaskCompletionEvents:
    // 1. Save the SUCCEEDED maps in knownOutputs to fetch the outputs.
    // 2. Save the OBSOLETE/FAILED/KILLED maps in obsoleteOutputs to stop 
    //    fetching from those maps.
    // 3. Remove TIPFAILED maps from neededOutputs since we don't need their
    //    outputs at all.
    for (TaskCompletionEvent event : events) {
      switch (event.getTaskStatus()) {
        case SUCCEEDED:
        {
        	//如果该MapTask执行成功，则构建新的mapOutputLocation，并更新mapLocations
        	//该集合会在主线程的fetchOutputs函数中取出有效map位置，并将其放入scheduledCopies
        	//集合中供copier线程使用
          URI u = URI.create(event.getTaskTrackerHttp());
          String host = u.getHost();
          TaskAttemptID taskId = event.getTaskAttemptId();
          URL mapOutputLocation = new URL(event.getTaskTrackerHttp() + 
                                  "/mapOutput?job=" + taskId.getJobID() +
                                  "&map=" + taskId + 
                                  "&reduce=" + getPartition());
          List<MapOutputLocation> loc = mapLocations.get(host);
          if (loc == null) {
            loc = Collections.synchronizedList
              (new LinkedList<MapOutputLocation>());
            mapLocations.put(host, loc);
           }
          loc.add(new MapOutputLocation(taskId, host, mapOutputLocation));
          numNewMaps ++;
        }
        break;
        case FAILED:
        case KILLED:
        case OBSOLETE:
        {
        	//不能获取则加入过期集合
          obsoleteMapIds.add(event.getTaskAttemptId());
          LOG.info("Ignoring obsolete output of " + event.getTaskStatus() + 
                   " map-task: '" + event.getTaskAttemptId() + "'");
        }
        break;
        case TIPFAILED:
        {
        	//任务失败则忽略该输出
          copiedMapOutputs.add(event.getTaskAttemptId().getTaskID());
          LOG.info("Ignoring output of failed map TIP: '" +  
               event.getTaskAttemptId() + "'");
        }
        break;
      }
    }
    return numNewMaps;
  }
 }
}