上一节分析到了JobTracker把作业从队列里取出来并进行了初始化，所谓的初始化，主要是获取了Map、Reduce任务的数量，并统计了哪些DataNode所在的服务器可以处理哪些Split等等，将这些信息缓存起来，但还没有进行实质的分配。等待TaskTracker跟自己通信。
TaskTracker一般运行于DataNode之上，下面是它的声明，可见，是一个线程类：
/*******************************************************
* TaskTracker is a process that starts and tracks MR Tasks
* in a networked environment. It contacts the JobTracker
* for Task assignments and reporting results.
*
*******************************************************/
public class TaskTracker implements MRConstants, TaskUmbilicalProtocol,
Runnable, TaskTrackerMXBean
。。。。。
TaskTracker里面存在一个main入口，当启动TaskTracker时进入该方法：
/**
* Start the TaskTracker, point toward the indicated JobTracker
*/
public static void main(String argv[]) throws Exception {
StringUtils.startupShutdownMessage(TaskTracker.class, argv, LOG);
if (argv.length != 0) {
System.out.println("usage: TaskTracker");
System.exit(-1);
}
try {
JobConf conf=new JobConf();
// enable the server to track time spent waiting on locks
ReflectionUtils.setContentionTracing
(conf.getBoolean("tasktracker.contention.tracking", false));
DefaultMetricsSystem.initialize("TaskTracker");
TaskTracker tt = new TaskTracker(conf);
MBeans.register("TaskTracker", "TaskTrackerInfo", tt);
tt.run();
} catch (Throwable e) {
LOG.error("Can not start task tracker because "+
StringUtils.stringifyException(e));
System.exit(-1);
}
}
可见，在里面会执行TaskTracker tt = new TaskTracker(conf);创建一个TaskTracker对象，并利用tt.run()进入其run方法：
/**
* The server retry loop.
* This while-loop attempts to connect to the JobTracker. It only
* loops when the old TaskTracker has gone bad (its state is
* stale somehow) and we need to reinitialize everything.
*/
public void run() {
try {
getUserLogManager().start();
startCleanupThreads();
boolean denied = false;
while (running && !shuttingDown) {
boolean staleState = false;
try {
// This while-loop attempts reconnects if we get network errors
while (running && !staleState && !shuttingDown && !denied) {
try {
State osState = offerService();
............
前面是获得写日志管理器，开启清理线程等操作。之后则进入offerService这个方法，这个方法比较长，offerService基本就是一个永远的循环，这从开始的代码可以看出：
/**
* Main service loop. Will stay in this loop forever.
*/
State offerService() throws Exception {
long lastHeartbeat = System.currentTimeMillis(); while (running && !shuttingDown) {
。。。。
// Send the heartbeat and process the jobtracker's directives
HeartbeatResponse heartbeatResponse = transmitHeartBeat(now);
。。。。
在这个方法里，只要TaskTracker还没有停止，就会一直循环，在循环里面，如果超过了心跳间隔，就会执行transmitHeartBeat这个心跳方法。我们逐步来分析。
首先TaskTracker会往JobTracker请求，查看版本信息和系统目录等。这里采用的也是RPC机制，并且与前面各节分析的客户端的机制相同，TaskTracker里面也包含一个jobClient，其类型是InterTrackerProtocol，这个协议其实和客户端那个JobSubmissionProtocol类似，都是一个继承于VersionedProtocol的接口，其定义为：
interface InterTrackerProtocol extends VersionedProtocol {
HeartbeatResponse heartbeat(TaskTrackerStatus status,
boolean restarted,
boolean initialContact,
boolean acceptNewTasks,
short responseId)
throws IOException;
public String getFilesystemName() throws IOException;
public void reportTaskTrackerError(String taskTracker,
String errorClass,
String errorMessage) throws IOException;
TaskCompletionEvent[] getTaskCompletionEvents(JobID jobid, int fromEventId , int maxEvents) throws IOException;
public String getSystemDir();
public String getVIVersion() throws IOException;
}
这里面的方法实际上都由JobTracker实现，这可以从JobTracker的定义看出来，它实现了InterTrackerProtocol、JobSubmissionProtocol等接口，同时为客户端、TaskTracker等提供RPC服务：
public class JobTracker implements MRConstants, InterTrackerProtocol,
JobSubmissionProtocol, TaskTrackerManager, RefreshUserMappingsProtocol,
RefreshAuthorizationPolicyProtocol, AdminOperationsProtocol,
JobTrackerMXBean
。。。。。
利用这些RPC服务，TaskTracker如果是第一次启动，会查看版本信息和系统目录信息（后面再执行offerService时则跳过这些步骤）：
// If the TaskTracker is just starting up:
// 1. Verify the versions matches with the JobTracker
// 2. Get the system directory & filesystem
if(justInited) {
String jtBuildVersion = jobClient.getBuildVersion();
String jtVersion = jobClient.getVIVersion();
。。。。。。。。。
String dir = jobClient.getSystemDir();
。。。。。。。。。
systemDirectory = new Path(dir);
systemFS = systemDirectory.getFileSystem(fConf);
}
之后，会进行心跳方法transmitHeartBeat，这个方法也比较长，也分为几步，首先会判断是否存在任务状态的对象，如果不存在就创建一个：
if (status == null) {
synchronized (this) {
status = new TaskTrackerStatus(taskTrackerName, localHostname,
httpPort,
cloneAndResetRunningTaskStatuses(
sendCounters),
taskFailures,
localStorage.numFailures(),
maxMapSlots,
maxReduceSlots);
}
} else {
LOG.info("Resending 'status' to '" + jobTrackAddr.getHostName() +
"' with reponseId '" + heartbeatResponseId);
}
TaskTrackerStatus记录了TaskTracker当前的任务状态，可以看出，里面包含两个参数：maxMapSlots、maxReduceSlots，分别代表了本机可以最多执行的Map任务和Reduce任务数量，这两个数量是配置的，从代码里看出，默认都是2：
maxMapSlots = conf.getInt( "mapred.tasktracker.map.tasks.maximum", 2);
maxReduceSlots = conf.getInt( "mapred.tasktracker.reduce.tasks.maximum", 2);
那么，在初始化TaskTrackerStatus的时候，就把这两个参数记录下来。另外，从TaskTracker与JobTracker之间的心跳RPC服务来看，TaskTrackerStatus对象会被传递至JobTracker，供其分配任务作为参考：
HeartbeatResponse heartbeat(TaskTrackerStatus status,
boolean restarted,
boolean initialContact,
boolean acceptNewTasks,
short responseId)
throws IOException;
接下来，TaskTracker准备给TaskTrackerStatus对象赋值，然后传递至JobTracker。在赋值时，首先判断下是否能接收新任务，通过判断当前的任务数量是否超过了配置的最大数量得到，如果可以接收新任务，则把本机的内存信息等写入到TaskTrackerStatus相关的变量中：
//
// Check if we should ask for a new Task
//
boolean askForNewTask;
long localMinSpaceStart;
synchronized (this) {
askForNewTask =
((status.countOccupiedMapSlots() < maxMapSlots ||
status.countOccupiedReduceSlots() < maxReduceSlots) &&
acceptNewTasks);
localMinSpaceStart = minSpaceStart;
}
if (askForNewTask) {
askForNewTask = enoughFreeSpace(localMinSpaceStart);
long freeDiskSpace = getFreeSpace();
long totVmem = getTotalVirtualMemoryOnTT();
long totPmem = getTotalPhysicalMemoryOnTT();
long availableVmem = getAvailableVirtualMemoryOnTT();
long availablePmem = getAvailablePhysicalMemoryOnTT();
long cumuCpuTime = getCumulativeCpuTimeOnTT();
long cpuFreq = getCpuFrequencyOnTT();
int numCpu = getNumProcessorsOnTT();
float cpuUsage = getCpuUsageOnTT(); status.getResourceStatus().setAvailableSpace(freeDiskSpace);
status.getResourceStatus().setTotalVirtualMemory(totVmem);
status.getResourceStatus().setTotalPhysicalMemory(totPmem);
status.getResourceStatus().setMapSlotMemorySizeOnTT(
mapSlotMemorySizeOnTT);
status.getResourceStatus().setReduceSlotMemorySizeOnTT(
reduceSlotSizeMemoryOnTT);
status.getResourceStatus().setAvailableVirtualMemory(availableVmem);
status.getResourceStatus().setAvailablePhysicalMemory(availablePmem);
status.getResourceStatus().setCumulativeCpuTime(cumuCpuTime);
status.getResourceStatus().setCpuFrequency(cpuFreq);
status.getResourceStatus().setNumProcessors(numCpu);
status.getResourceStatus().setCpuUsage(cpuUsage);
}
这些资源信息包括物理内存、虚拟内存、Map/Reduce任务槽的内存占用、累积的CPU时间、CPU频率、CPU数量、使用率等各种资源状态。
另外，还进行了健康检查，把信息也记录在里面，这里用一个TaskTrackerHealthStatus记录健康状态。这里进行健康检查实际上让系统运行一个脚本：
TaskTrackerHealthStatus healthStatus = status.getHealthStatus();
synchronized (this) {
if (healthChecker != null) {
healthChecker.setHealthStatus(healthStatus);
} else {
healthStatus.setNodeHealthy(true);
healthStatus.setLastReported(0L);
healthStatus.setHealthReport("");
}
}
记录了这些信息后，就执行心跳方法：
HeartbeatResponse heartbeatResponse = jobClient.heartbeat(status,
justStarted,
justInited,
askForNewTask,
heartbeatResponseId);
前面看到，jobClient实际上是一个接口，当调用该方法的时候，利用动态代理，会将该方法交由JobTracker端的同名方法执行。
这个方法较为复杂，我们分几个步骤分别分析。
1、检查TaskTracker的合法性
JobTracker会首先检查该TaskTracker的合法性：
// Make sure heartbeat is from a tasktracker allowed by the jobtracker.
if (!acceptTaskTracker(status)) {
throw new DisallowedTaskTrackerException(status);
} /**
* Returns true if the tasktracker is in the hosts list and
* not in the exclude list.
*/
private boolean acceptTaskTracker(TaskTrackerStatus status) {
return (inHostsList(status) && !inExcludedHostsList(status));
}
inHostsList和inExcludedHostsList的代码为：
/**
* Return if the specified tasktracker is in the hosts list,
* if one was configured. If none was configured, then this
* returns true.
*/
private boolean inHostsList(TaskTrackerStatus status) {
Set<String> hostsList = hostsReader.getHosts();
return (hostsList.isEmpty() || hostsList.contains(status.getHost()));
} /**
* Return if the specified tasktracker is in the exclude list.
*/
private boolean inExcludedHostsList(TaskTrackerStatus status) {
Set<String> excludeList = hostsReader.getExcludedHosts();
return excludeList.contains(status.getHost());
}
JobTracker里面存在一个HostsFileReader对象hostsReader，用于记录是否合法的TaskTracker，该对象在JobTracker创建时会被创建出来，该对象存在两个队列，其定义为：
// Keeps track of which datanodes/tasktrackers are allowed to connect to the
// namenode/jobtracker.
public class HostsFileReader {
private Set<String> includes;
private Set<String> excludes;
private String includesFile;
private String excludesFile;
。。。。。。
在这个对象里，includes表示可以访问的host列表，excludes则表示不可访问的host列表，最初，这两个列表的内容是根据两个配置文件生成的：
// Read the hosts/exclude files to restrict access to the jobtracker.
this.hostsReader = new HostsFileReader(conf.get("mapred.hosts", ""),
conf.get("mapred.hosts.exclude", ""));
2.更新TaskTracker的健康状态
如果是一个合法的TaskTracker，接下来还会检查是否在灰名单里，对于JobTracker而言，不是每个TaskTracker都适合分配任务，比如有的TaskTracker心跳间隔超时了， 出现过故障等等，JobTracker使用一个FaultyTrackersInfo对象记录这些TaskTracker：
// statistics about TaskTrackers with faults; may lead to graylisting
private FaultyTrackersInfo faultyTrackers = new FaultyTrackersInfo();
其检查代码为：
// First check if the last heartbeat response got through
String trackerName = status.getTrackerName();
long now = clock.getTime();
if (restarted) {
faultyTrackers.markTrackerHealthy(status.getHost());
} else {
faultyTrackers.checkTrackerFaultTimeout(status.getHost(), now);
}
restarted是TaskTracker通过heartbeat方法传递过来的，这说明如果TaskTracker是重启了的，则认为该TaskTracker处于健康状态，进行标记：
/**
* Removes the tracker from the blacklist, graylist, and
* potentially-faulty list, when it is restarted.
*
* Assumes JobTracker is locked on the entry.
*
* @param hostName
*/
void markTrackerHealthy(String hostName) {
synchronized (potentiallyFaultyTrackers) {
FaultInfo fi = potentiallyFaultyTrackers.remove(hostName);
if (fi != null) {
// a tracker can be both blacklisted and graylisted, so check both
if (fi.isGraylisted()) {
LOG.info("Marking " + hostName + " healthy from graylist");
decrGraylistedTrackers(getNumTaskTrackersOnHost(hostName));
}
if (fi.isBlacklisted()) {
LOG.info("Marking " + hostName + " healthy from blacklist");
addHostCapacity(hostName);
}
// no need for fi.unBlacklist() for either one: fi is already gone
}
}
}
从上面的代码看出，对于刚刚重启的主机，把它从出错的TaskTracker集合potentiallyFaultyTrackers里面删除。并更新numGraylistedTrackers、numBlacklistedTrackers等数量以及totalMapTaskCapacity、totalReduceTaskCapacity的数量。
对于不是重启的主机，检查是否应该放入黑名单：
void checkTrackerFaultTimeout(String hostName, long now) {
synchronized (potentiallyFaultyTrackers) {
FaultInfo fi = potentiallyFaultyTrackers.get(hostName);
// getFaultCount() auto-rotates the buckets, clearing out the oldest
// as needed, before summing the faults:
if (fi != null && fi.getFaultCount(now) < TRACKER_FAULT_THRESHOLD) {
unBlacklistTracker(hostName, ReasonForBlackListing.EXCEEDING_FAILURES,
true, now);
}
}
}
所谓黑名单，其判断标准是当前TaskTracker所在的主机是否发生了超过了TRACKER_FAULT_THRESHOLD=4次失败的情况。按照Hadoop代码，应该是以滑窗方式检测一个主机是否应该放入黑名单，从下面的说明可以看出，如果在3小时范围内，出现了大于4次失败，则应该放入黑名单。
// Fault threshold (number occurring within TRACKER_FAULT_TIMEOUT_WINDOW)
// to consider a task tracker bad enough to blacklist heuristically. This
// is functionally the same as the older "MAX_BLACKLISTS_PER_TRACKER" value.
private int TRACKER_FAULT_THRESHOLD; // = 4; // Width of overall fault-tracking sliding window (in minutes). (Default
// of 24 hours matches previous "UPDATE_FAULTY_TRACKER_INTERVAL" value that
// was used to forgive a single fault if no others occurred in the interval.)
private int TRACKER_FAULT_TIMEOUT_WINDOW; // = 180 (3 hours)
3.判断JobTracker自身的故障、心跳是否重复等
JobTracker会保存一个映射表，记录了各个TaskTracker上一次的心跳响应：
// (trackerID --> last sent HeartBeatResponse)
Map<String, HeartbeatResponse> trackerToHeartbeatResponseMap =
new TreeMap<String, HeartbeatResponse>();
从该表中取出对应于该TaskTracker的上一次心跳响应：
HeartbeatResponse prevHeartbeatResponse =
trackerToHeartbeatResponseMap.get(trackerName);
如果TaskTracker不是第一次联系JobTracker（这个信息TaskTracker通过心跳传递过来），且JobTracker没有保存着对应于该TaskTracker的上一次响应，一种可能是JobTracker刚刚重启了，重启与否可以判断出来；如果也不是重启，那表明出现了严重问题。出现这种情况时，JobTracker也不进行任务分配了，直接给TaskTracker返回一个响应。如下面代码所示：
// If this isn't the 'initial contact' from the tasktracker,
// there is something seriously wrong if the JobTracker has
// no record of the 'previous heartbeat'; if so, ask the
// tasktracker to re-initialize itself.
if (prevHeartbeatResponse == null) {
// This is the first heartbeat from the old tracker to the newly
// started JobTracker
if (hasRestarted()) {
addRestartInfo = true;
// inform the recovery manager about this tracker joining back
recoveryManager.unMarkTracker(trackerName);
} else {
// Jobtracker might have restarted but no recovery is needed
// otherwise this code should not be reached
LOG.warn("Serious problem, cannot find record of 'previous' " +
"heartbeat for '" + trackerName +
"'; reinitializing the tasktracker");
return new HeartbeatResponse(responseId,
new TaskTrackerAction[] {new ReinitTrackerAction()});
}
如果JobTracker确实也保存了对应于该TaskTracker的上一次响应，那么检查一下两者是否是重复，JobTracker理论上会为每次心跳响应赋一个递增的值：
// It is completely safe to not process a 'duplicate' heartbeat from a
// {@link TaskTracker} since it resends the heartbeat when rpcs are
// lost see {@link TaskTracker.transmitHeartbeat()};
// acknowledge it by re-sending the previous response to let the
// {@link TaskTracker} go forward.
if (prevHeartbeatResponse.getResponseId() != responseId) {
LOG.info("Ignoring 'duplicate' heartbeat from '" +
trackerName + "'; resending the previous 'lost' response");
return prevHeartbeatResponse;
}
4、处理心跳，更新TaskTracker状态等：
当合法性检查通过后，JobTracker会更新自己记录的关于TaskTracker的状态：
// Process this heartbeat
short newResponseId = (short)(responseId + 1);
status.setLastSeen(now);
if (!processHeartbeat(status, initialContact, now)) {
if (prevHeartbeatResponse != null) {
trackerToHeartbeatResponseMap.remove(trackerName);
}
return new HeartbeatResponse(newResponseId,
new TaskTrackerAction[] {new ReinitTrackerAction()});
}
首先递增响应ID，调用processHeartbeat更新TaskTracker的状态，如果不成功，则将其从trackerToHeartbeatResponseMap中移除；但仍然返回一个响应给TaskTracker。
在processHeartbeat方法里，主要是更新JobTracker自己的数据结构：
/**
* Process incoming heartbeat messages from the task trackers.
*/
private synchronized boolean processHeartbeat(
TaskTrackerStatus trackerStatus,
boolean initialContact,
long timeStamp) throws UnknownHostException {
。。。。。。。。。。。。
updateTaskStatuses(trackerStatus);
updateNodeHealthStatus(trackerStatus, timeStamp); return true;
}
JobTracker里存在要执行的Map和Reduce Task，每个Task的实例不止一个，一方面是考虑到容错，当一个机器挂掉的时候，让其它机器来执行这个任务，同时，在多个机器上分配，让他们执行一样的任务，这样谁先执行完，这个任务也就执行完，避免某些机器出了点问题，速度太慢，影响了整体的进度，所以，一个任务Task实际上有多个实例，称为TaskAttemptID，TaskAttemptID的格式类似这样的格式：
attempt_200707121733_0003_m_000005_0
这代表任务2007年7月12日17时33分启动的JobTracker中第0003号作业的第000005号map task的第0号task attempt。
JobTracker会去查看TaskTracker传递过来的TaskAttemptID，如果该任务对应的Job都不存在了，表名执行完了等等，就会把它放入一个清空队列里面：
void updateTaskStatuses(TaskTrackerStatus status) {
String trackerName = status.getTrackerName();
for (TaskStatus report : status.getTaskReports()) {
report.setTaskTracker(trackerName);
TaskAttemptID taskId = report.getTaskID();
JobInProgress job = getJob(taskId.getJobID());
if (job == null) {
// if job is not there in the cleanup list ... add it
synchronized (trackerToJobsToCleanup) {
Set<JobID> jobs = trackerToJobsToCleanup.get(trackerName);
if (jobs == null) {
jobs = new HashSet<JobID>();
trackerToJobsToCleanup.put(trackerName, jobs);
}
jobs.add(taskId.getJobID());
}
continue;
}
。。。。。。
如果Job都没有完成初始化，那么也要将其清理掉：
if (!job.inited()) {
// if job is not yet initialized ... kill the attempt
synchronized (trackerToTasksToCleanup) {
Set<TaskAttemptID> tasks = trackerToTasksToCleanup.get(trackerName);
if (tasks == null) {
tasks = new HashSet<TaskAttemptID>();
trackerToTasksToCleanup.put(trackerName, tasks);
}
tasks.add(taskId);
}
continue;
}
另外，如果JobTracker是重启过的，那么还会把TaskTracker报告的任务信息加进来：
TaskInProgress tip = taskidToTIPMap.get(taskId);
// Check if the tip is known to the jobtracker. In case of a restarted
// jt, some tasks might join in later
if (tip != null || hasRestarted()) {
if (tip == null) {
tip = job.getTaskInProgress(taskId.getTaskID());
job.addRunningTaskToTIP(tip, taskId, status, false);
} // Update the job and inform the listeners if necessary
JobStatus prevStatus = (JobStatus)job.getStatus().clone();
// Clone TaskStatus object here, because JobInProgress
// or TaskInProgress can modify this object and
// the changes should not get reflected in TaskTrackerStatus.
// An old TaskTrackerStatus is used later in countMapTasks, etc.
job.updateTaskStatus(tip, (TaskStatus)report.clone());
JobStatus newStatus = (JobStatus)job.getStatus().clone(); // Update the listeners if an incomplete job completes
if (prevStatus.getRunState() != newStatus.getRunState()) {
JobStatusChangeEvent event =
new JobStatusChangeEvent(job, EventType.RUN_STATE_CHANGED,
prevStatus, newStatus);
updateJobInProgressListeners(event);
}
} else {
LOG.info("Serious problem. While updating status, cannot find taskid "
+ report.getTaskID());
}
5、分配任务：
在上面的操作都做完了以后，JobTracker开始分配任务，首先创建一个心跳响应，以及关于这个心跳响应的任务分配队列List<TaskTrackerAction>：
// Initialize the response to be sent for the heartbeat
HeartbeatResponse response = new HeartbeatResponse(newResponseId, null);
List<TaskTrackerAction> actions = new ArrayList<TaskTrackerAction>();
boolean isBlacklisted = faultyTrackers.isBlacklisted(status.getHost());
// Check for new tasks to be executed on the tasktracker
if (recoveryManager.shouldSchedule() && acceptNewTasks && !isBlacklisted) {
TaskTrackerStatus taskTrackerStatus = getTaskTrackerStatus(trackerName);
if (taskTrackerStatus == null) {
LOG.warn("Unknown task tracker polling; ignoring: " + trackerName);
} else {
List<Task> tasks = getSetupAndCleanupTasks(taskTrackerStatus);
if (tasks == null ) {
tasks = taskScheduler.assignTasks(taskTrackers.get(trackerName));
}
if (tasks != null) {
for (Task task : tasks) {
expireLaunchingTasks.addNewTask(task.getTaskID());
if(LOG.isDebugEnabled()) {
LOG.debug(trackerName + " -> LaunchTask: " + task.getTaskID());
}
actions.add(new LaunchTaskAction(task));
}
}
}
}
如果该TaskTracker支持任务调度，且不在黑名单里面，则可以进行分配，核心的代码是：
tasks = taskScheduler.assignTasks(taskTrackers.get(trackerName));
这个方法用于任务分配。taskScheduler是一个TaskScheduler对象，而TaskScheduler是一个抽象类，有几种实现，默认的是JobQueueTaskScheduler。关于任务分配过程，也比较复杂，留作后续分析，这里先假定已经分配了任务，并被加入到队列List<TaskTrackerAction>中。
6、将其他任务加入到队列中：
除了分配的任务，还需要把以下一些任务加入队列actions：
需要被kill的Task：
// Check for tasks to be killed
List<TaskTrackerAction> killTasksList = getTasksToKill(trackerName);
if (killTasksList != null) {
actions.addAll(killTasksList);
}
需要被kill或清理的任务：
// Check for jobs to be killed/cleanedup
List<TaskTrackerAction> killJobsList = getJobsForCleanup(trackerName);
if (killJobsList != null) {
actions.addAll(killJobsList);
}
需要保存输出的任务：
// Check for tasks whose outputs can be saved
List<TaskTrackerAction> commitTasksList = getTasksToSave(status);
if (commitTasksList != null) {
actions.addAll(commitTasksList);
}
7、获得心跳响应，返回：
将任务队列actions加入到心跳响应：
// calculate next heartbeat interval and put in heartbeat response
int nextInterval = getNextHeartbeatInterval();
response.setHeartbeatInterval(nextInterval);
response.setActions(actions.toArray(new TaskTrackerAction[actions.size()])); // check if the restart info is req
if (addRestartInfo) {
response.setRecoveredJobs(recoveryManager.getJobsToRecover());
} // Update the trackerToHeartbeatResponseMap
trackerToHeartbeatResponseMap.put(trackerName, response);
至此，TaskTracker这个通过RPC访问的heartbeat服务方法执行结束了，JobTracker向TaskTracker返回了心跳响应，将要执行的任务信息返回给TaskTracker。
回到TaskTracker，上一部分分析到发出心跳，接下来就对响应进行解析了，继续offerService方法的后续分析：
首先要看一下，是否有任务需要清理：
// Check if the map-event list needs purging
Set<JobID> jobs = heartbeatResponse.getRecoveredJobs();
if (jobs.size() > 0) {
synchronized (this) {
// purge the local map events list
for (JobID job : jobs) {
RunningJob rjob;
synchronized (runningJobs) {
rjob = runningJobs.get(job);
if (rjob != null) {
synchronized (rjob) {
FetchStatus f = rjob.getFetchStatus();
if (f != null) {
f.reset();
}
}
}
}
} // Mark the reducers in shuffle for rollback
synchronized (shouldReset) {
for (Map.Entry<TaskAttemptID, TaskInProgress> entry
: runningTasks.entrySet()) {
if (entry.getValue().getStatus().getPhase() == Phase.SHUFFLE) {
this.shouldReset.add(entry.getKey());
}
}
}
}
}
接下来获得所有任务信息：
TaskTrackerAction[] actions = heartbeatResponse.getActions();
对于每个任务，如果需要启动，就加入到队列addToTaskQueue中：
if (actions != null){
for(TaskTrackerAction action: actions) {
if (action instanceof LaunchTaskAction) {
addToTaskQueue((LaunchTaskAction)action);
} else if (action instanceof CommitTaskAction) {
CommitTaskAction commitAction = (CommitTaskAction)action;
if (!commitResponses.contains(commitAction.getTaskID())) {
LOG.info("Received commit task action for " +
commitAction.getTaskID());
commitResponses.add(commitAction.getTaskID());
}
} else {
addActionToCleanup(action);
}
}
}
如果需要提交的，就进行要提交的队列中，否则加入到要清理的队列中。
之后，杀死那些很久没有反馈进度的任务：
markUnresponsiveTasks();
当磁盘空间不够时，杀死某些任务以腾出空间：
killOverflowingTasks();
此时，TaskTracker利用心跳从JobTracker获得了任务，并加入了自己的各个队列，有的是待启动的队列，有的是要提交的队列，有的是要清理的队列，这些队列里面的任务，会有其他线程来取了以后去执行。这一部分暂时告一段落。
上面我们遗留了一个问题，就是JobTracker在调用JobQueueTaskScheduler的任务分配方法时，是如何按照什么策略分配的还没有涉及。这个在下一博文里面进行分析。
另外，关于TaskTracker怎么从队列里取出任务继续启动JAVA虚拟机执行等过程，我们也留作后续博文分析研究。