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hadoop运行原理之Job运行(四) JobTracker端心跳机制分析

  接着上篇来说,TaskTracker端的transmitHeartBeat()方法通过RPC调用JobTracker端的heartbeat()方法来接收心跳并返回心跳应答。还是先看看这张图,对它的大概流程有个了解。

  下面来一段一段的分析该方法。

 1 public synchronized HeartbeatResponse heartbeat(TaskTrackerStatus status,  2                                                   boolean restarted, 3                                                   boolean initialContact, 4                                                   boolean acceptNewTasks,  5                                                   short responseId)   6 throws IOException { 7      if (LOG.isDebugEnabled()) { 8        LOG.debug("Got heartbeat from: " + status.getTrackerName() +  " (restarted: " + restarted +  9                  " initialContact: " + initialContact + 10                  " acceptNewTasks: " + acceptNewTasks + ")" +11                  " with responseId: " + responseId);12      }13      // 检查该主机是否允许与JobTracker通信14     if (!acceptTaskTracker(status)) {15       throw new DisallowedTaskTrackerException(status);16     }

  上面这段代码首先检查该TaskTracker是否能与JobTracker通信。确保该TaskTracker在允许的主机列表里(即inHostsList(),由”mapred.hosts”指定),不在排除的主机列表里(即inExcludedHostsList(),由” mapred.hosts.exclude” 指定)。

 

1  String trackerName = status.getTrackerName();2     long now = clock.getTime();3     if (restarted) {4       faultyTrackers.markTrackerHealthy(status.getHost());5     } else {6       faultyTrackers.checkTrackerFaultTimeout(status.getHost(), now);7     }

  上面这段代码表示:如果该TaskTracker被重启了,则将其标注为健康的,然后从黑名单和灰名单中移除,并从potentiallyFaultyTrackers(潜在有错误的Tracker。当有task运行失败时,就将其加入该队列中)集合中移除。否则,启动TaskTracker容错机制以检查它是否处于健康状态。

 

 1 HeartbeatResponse prevHeartbeatResponse = 2       trackerToHeartbeatResponseMap.get(trackerName);  //获取上次心跳应答 3     boolean addRestartInfo = false;  //表示JobTracker是否重启 4  5     if (initialContact != true) { //该TaskTracker不是初次连接JobTracker 6       // If this isn‘t the ‘initial contact‘ from the tasktracker, 7       // there is something seriously wrong if the JobTracker has 8       // no record of the ‘previous heartbeat‘; if so, ask the  9       // tasktracker to re-initialize itself.10       if (prevHeartbeatResponse == null) { //TaskTracker不是初次连接JobTracker,并且上一次心跳应答对象为空11         // This is the first heartbeat from the old tracker to the newly 12         // started JobTracker13         if (hasRestarted()) {  //JobTracker重启了14           addRestartInfo = true;15           // inform the recovery manager about this tracker joining back16           recoveryManager.unMarkTracker(trackerName);17         } else {  //JobTracker没有重启18           // Jobtracker might have restarted but no recovery is needed19           // otherwise this code should not be reached20           LOG.warn("Serious problem, cannot find record of ‘previous‘ " +21                    "heartbeat for ‘" + trackerName + 22                    "‘; reinitializing the tasktracker");23           return new HeartbeatResponse(responseId, 24               new TaskTrackerAction[] {new ReinitTrackerAction()}); //重新初始化该TaskTracker25         }26 27       } else { //该TaskTracker不是第一次连接JobTracker,并且上次心跳应答不为空28                 29         // It is completely safe to not process a ‘duplicate‘ heartbeat from a 30         // {@link TaskTracker} since it resends the heartbeat when rpcs are 31         // lost see {@link TaskTracker.transmitHeartbeat()};32         // acknowledge it by re-sending the previous response to let the 33         // {@link TaskTracker} go forward. 34         if (prevHeartbeatResponse.getResponseId() != responseId) {35           LOG.info("Ignoring ‘duplicate‘ heartbeat from ‘" + 36               trackerName + "‘; resending the previous ‘lost‘ response");37           return prevHeartbeatResponse;  //心跳丢失,返回上次心跳应答38         }39       }40     }

  上面这部分用来检测上次心跳是否结束。首先获取该TaskTracker的上次心跳应答响应。正常情况下在JobTracker中的trackerToHeartbeatResponseMap对象中会存在该TaskTracker上一次的心跳应答对象信息HeartbeatResponse,初次心跳连接则不会有该TaskTracker上一次的心跳应答对象。

  当该TaskTracker与JobTracker不是初次连接时:如果JobTracker中没有上次与该TaskTracker通信的心跳应答记录(即prevHeartbeatResponse == null),那么再检查JobTracker若重启了,则(来自该TaskTracker的心跳表明与JobTracker已经重新连接了)从recoveryManager中删除这个TaskTracker;否则重新初始化该TaskTracker。如果JobTracker有与TaskTracker通信的上次心跳记录(即prevHeartbeatResponse != null),但是JobTracker记录的心跳ID与TaskTracker发送过来的心跳ID不一致,说明发生了心跳丢失,此时返回上一次的心跳应答,这样可以防止处理重复的心跳请求。

 

 1    // Process this heartbeat  2     short newResponseId = (short)(responseId + 1); 3     status.setLastSeen(now); 4     if (!processHeartbeat(status, initialContact, now)) { //心跳处理失败 5       if (prevHeartbeatResponse != null) {  //上次心跳应答不存在 6         trackerToHeartbeatResponseMap.remove(trackerName); 7       } 8       return new HeartbeatResponse(newResponseId,  9                    new TaskTrackerAction[] {new ReinitTrackerAction()});  //重新初始化该TaskTracker10     }

  这一部分来处理心跳信息。首先是心跳响应的ID加1,然后将心跳发送时间设置为当前时间。processHeartbeat()方法用来处理心跳。外层if表示心跳处理失败,内层if表示如果上次心跳应答存在的话,则从trackerToHeartbeatResponseMap中移除,然后重新初始化TaskTracker。最后分析processHeartbeat()方法。

  

 1  // Initialize the response to be sent for the heartbeat 2     HeartbeatResponse response = new HeartbeatResponse(newResponseId, null); 3     List<TaskTrackerAction> actions = new ArrayList<TaskTrackerAction>(); 4     boolean isBlacklisted = faultyTrackers.isBlacklisted(status.getHost());  //检查该结点是否在黑名单中 5     // Check for new tasks to be executed on the tasktracker 6     if (recoveryManager.shouldSchedule() && acceptNewTasks && !isBlacklisted) { 7       TaskTrackerStatus taskTrackerStatus = getTaskTrackerStatus(trackerName); 8       if (taskTrackerStatus == null) { 9         LOG.warn("Unknown task tracker polling; ignoring: " + trackerName);10       } else {  //首先执行cleanup和setup task11         List<Task> tasks = getSetupAndCleanupTasks(taskTrackerStatus);12         if (tasks == null ) {  //如果没有cleanup和setup task,则由作业调度器分配map和reduce task13           tasks = taskScheduler.assignTasks(taskTrackers.get(trackerName));14         }15         if (tasks != null) {16           for (Task task : tasks) {17             expireLaunchingTasks.addNewTask(task.getTaskID());18             if(LOG.isDebugEnabled()) {19               LOG.debug(trackerName + " -> LaunchTask: " + task.getTaskID());20             }21             actions.add(new LaunchTaskAction(task));22           }23         }24       }25     }

   上面这部分主要用来构造心跳应答,其中包含对TaskTracker下达的命令。首先优先执行辅助型任务,其优先级为job-cleanup task、task-cleanup task(主要作用是清理失败的map或者reduce任务的部分结果数据)和job-setup task。它们由jobTracker直接调度,而且其调度的优先级比map和reduce任务都要高。从getSetupAndCleanupTasks()方法可以看出,执行时首先运行Map的辅助型任务,再执行Reduce的辅助型任务。然后将任务添加到expireLaunchingTasks队列中,监测其是否超时未汇报。最后为task构造LaunchTaskAction指令。如果没有辅助型任务,则由作业调度器TaskScheduler(默认为JobQueueTaskScheduler)来为TaskTracker分配任务。

  捎带分析下getSetupAndCleanupTasks()方法,顾名思义,这个方法是用来获得setup task和cleanup task的。在JobInProgress类的initTasks()方法中,我们看到有4类task,分别是setup、map、reduce、cleanup,而setup和cleanup是针对Job的,并没有task-cleanup的task,但在getSetupAndCleanupTasks()方法中却多出了这个task,这是为什么?原来在JobInProgress中维护了两个队列:mapCleanupTasks和reduceCleanupTasks,分别用来存放map和reduce需要清理的task。当task处于FAILED_UNCLEAN或KILLED_UNCLEAN状态时,则根据task的类型将其添加到对应的队列中。getSetupAndCleanupTasks()方法的执行过程为:先检查map的job-cleanup task(对应JobInProgress中的cleanup[0])、task-cleanup task和job-setup task(对应JobInProgress中的setup[0]),再检查reduce的job-cleanup task(对应JobInProgress中的cleanup[1])、task-cleanup task和job-setup task(对应JobInProgress中的setup[1])。

 

 1    // Check for tasks to be killed 2     List<TaskTrackerAction> killTasksList = getTasksToKill(trackerName); 3     if (killTasksList != null) { 4       actions.addAll(killTasksList); 5     } 6       7     // Check for jobs to be killed/cleanedup 8     List<TaskTrackerAction> killJobsList = getJobsForCleanup(trackerName); 9     if (killJobsList != null) {10       actions.addAll(killJobsList);11     }12 13     // Check for tasks whose outputs can be saved14     List<TaskTrackerAction> commitTasksList = getTasksToSave(status);15     if (commitTasksList != null) {16       actions.addAll(commitTasksList);17     }

  这三段代码分别构造KillTaskAction、KillJobAction和CommitTaskAction指令。

 

 1    // calculate next heartbeat interval and put in heartbeat response 2     int nextInterval = getNextHeartbeatInterval();  //计算下次心跳时间间隔 3     response.setHeartbeatInterval(nextInterval); 4     response.setActions(  //将下达给TaskTracker的指令封装到心跳应答中 5                         actions.toArray(new TaskTrackerAction[actions.size()])); 6      7     // check if the restart info is req 8     if (addRestartInfo) {  //将需要恢复的Job告知放入心跳应答中 9       response.setRecoveredJobs(recoveryManager.getJobsToRecover());10     }11         12     // Update the trackerToHeartbeatResponseMap13     trackerToHeartbeatResponseMap.put(trackerName, response);  //将本次心跳应答更新到trackerToHeartbeatResponseMap,作为该TaskTracker最新的心跳应答14 15     // Done processing the hearbeat, now remove ‘marked‘ tasks16     removeMarkedTasks(trackerName);  //从trackerToMarkedTasksMap移除已被标记为完成的task17         18     return response;

  上面这段代码首先计算下次心跳间隔时间,JobTracker会根据集群规模(TaskTracker的数目)动态调整心跳时间间隔。如果JobTracker重启过的话,则将需要恢复的Job告知放入心跳应答中。然后将本次心跳应答更新到trackerToHeartbeatResponseMap,作为该TaskTracker最新的心跳应答。最后从trackerToMarkedTasksMap移除已被标记为完成的task。返回心跳应答。

  心跳时间间隔的计算方法如下所示:

 1   /** 2    * Calculates next heartbeat interval using cluster size. 3    * Heartbeat interval is incremented by 1 second for every 100 nodes by default.  4    * @return next heartbeat interval. 5    */ 6   public int getNextHeartbeatInterval() {  //默认情况下,处理每个RPC大约要10ms 7     // get the no of task trackers 8     int clusterSize = getClusterStatus().getTaskTrackers();  // 9     int heartbeatInterval =  Math.max(10                                 (int)(1000 * HEARTBEATS_SCALING_FACTOR *11                                       ((double)clusterSize / 12                                                 NUM_HEARTBEATS_IN_SECOND)),13                                 HEARTBEAT_INTERVAL_MIN) ;14     return heartbeatInterval;15   }

  HEARTBEATS_SCALING_FACTOR(mapreduce.jobtracker.heartbeats.scaling.factor):规模因子,默认为1,最小是0.01

  NUM_HEARTBEATS_IN_SECOND(mapred.heartbeats.in.second):JobTracker每秒能处理的RPC数目,处理每个RPC约10ms时间。默认为100,最小为1。

  所以这个公式表示,当集群增加了mapred.heartbeats.in.second个结点,心跳间隔增加mapreduce.jobtracker.heartbeats.scaling.factor秒。但为了防止设置不合理而对JobTracker产生较大负载,JobTracker要求心跳间隔至少为300ms。

 

  最后分析下processHeartbeat()方法,还是先看图。

  processHeartbeat()方法用来处理心跳。首先会更新TaskTracker的状态资源信息(如task、slot数目,在updateTaskTrackerStatus()方法中实现),返回值seenBefore代表JobTracker上是否存在该TaskTracker的上次status信息。然后判断该TaskTracker与JobTracker如果是初次连接并且存在上次status信息,则将其清空;如果不是初次连接并且不存在上次status信息,则更新TaskTracker的资源状态信息并返回false,表示处理心跳出错。接着再次判断如果是初次连接并且该TaskTracker在黑名单中,则黑名单中host的数目加1;然后将该TaskTracker添加JobTracker中。最后更新该TaskTracker的Task信息和健康状态。

 

  本文基于hadoop1.2.1

  如有错误,还请指正

  参考文章:《Hadoop技术内幕 深入理解MapReduce架构设计与实现原理》 董西成

       http://zengzhaozheng.blog.51cto.com/8219051/1359887

       http://www.cnblogs.com/lxf20061900/p/3775963.html

  转载请注明出处:http://www.cnblogs.com/gwgyk/p/4055133.html

 

hadoop运行原理之Job运行(四) JobTracker端心跳机制分析