在Hadoop中JT（JobTracker）與TT（TaskTracker）之間的通信是通過心跳機制完成的。JT實現InterTrackerProtocol協議，該協議定義了JT與TT之間的通信機制——心跳。心跳機制實際上就是一個RPC請求，JT作爲Server，而TT作爲Client，TT通過RPC調用JT的heartbeat方法，將TT自身的一些狀態信息發送給JT，同時JT通過返回值返回對TT的指令。
心跳有三個作用：
1）判斷TT是否活着
2）報告TT的資源情況以及任務運行情況
3）爲TT發送指令（如運行task，kill task等）
下面詳細閱讀下涉及到心跳調用的源碼。
首先我們需要清楚，心跳機制是TT調用JT的方法，而非JT主動調用TT的方法。TT通過transmitHeartBeat方法調用JT的heartbeat方法。
1.TaskTracker.transmitHeartBeat：
// Send Counters in the status once every COUNTER_UPDATE_INTERVAL
boolean sendCounters;
if (now > (previousUpdate + COUNTER_UPDATE_INTERVAL)) {
sendCounters = true;
previousUpdate = now;
}
else {
sendCounters = false;
}2.TaskTracker.transmitHeartBeat：
1.TaskTracker.transmitHeartBeat：// Check if the last heartbeat got through...
// if so then build the heartbeat information for the JobTracker;
// else resend the previous status information.
//
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);
}private synchronized List<TaskStatus> cloneAndResetRunningTaskStatuses(
boolean sendCounters) {
List<TaskStatus> result = new ArrayList<TaskStatus>(runningTasks.size());
for(TaskInProgress tip: runningTasks.values()) {
TaskStatus status = tip.getStatus();
status.setIncludeCounters(sendCounters);
// send counters for finished or failed tasks and commit pending tasks
if (status.getRunState() != TaskStatus.State.RUNNING) {
status.setIncludeCounters(true);
}
result.add((TaskStatus)status.clone());
status.clearStatus();
}
return result;
}3.TaskTrackerStatus()：
public TaskTrackerStatus(String trackerName, String host,
int httpPort, List<TaskStatus> taskReports,
int taskFailures, int dirFailures,
int maxMapTasks, int maxReduceTasks) {
this.trackerName = trackerName;
this.host = host;
this.httpPort = httpPort;
this.taskReports = new ArrayList<TaskStatus>(taskReports);
this.taskFailures = taskFailures;
this.dirFailures = dirFailures;
this.maxMapTasks = maxMapTasks;
this.maxReduceTasks = maxReduceTasks;
this.resStatus = new ResourceStatus();
this.healthStatus = new TaskTrackerHealthStatus();
}1）taskReports：包含該TT上目前所有的Task狀態信息，其中的counters信息會根據之前判斷sendCounters值進行決定是否發送，上一步有提到。
2）taskFailures：該TT上失敗的Task總數（重啓會清空），該參數幫助JT決定是否向該TT提交Task，因爲失敗數越多表明該TT可能出現Task失敗的概率越大。
3）dirFailures：這個值是mapred.local.dir參數設置的目錄中有多少是不可用的（以後會詳細提到）
4）maxMapSlots/maxReduceSlots：這個值是TT可使用的最大map和reduce slot數量
初始化完成，繼續回到TaskTracker.transmitHeartBeat方法。
4.TaskTracker.transmitHeartBeat：
// 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);
}首先第一步status.countOccupiedMapSlots()獲得該TT上已佔用的map slot數量：
/**
* Get the number of occupied map slots.
* @return the number of occupied map slots
*/
public int countOccupiedMapSlots() {
int mapSlotsCount = 0;
for (TaskStatus ts : taskReports) {
if (ts.getIsMap() && isTaskRunning(ts)) {
mapSlotsCount += ts.getNumSlots();
}
}
return mapSlotsCount;
}localMinSpaceStart = minSpaceStart，minSpaceStart由mapred.local.dir.minspacestart參數決定，默認是0，即無限制，該值的意思應該是可接收新任務的localDirs最小的可用空間大小。接下來可以看到該值能夠影響acceptNewTasks值。
當acceptNewTasks==true時，即初步判斷可以接收新任務，會再次根據localMinSpaceStart判斷是否可接收新任務。
/**
* Check if any of the local directories has enough
* free space (more than minSpace)
*
* If not, do not try to get a new task assigned
* @return
* @throws IOException
*/
private boolean enoughFreeSpace(long minSpace) throws IOException {
if (minSpace == 0) {
return true;
}
return minSpace < getFreeSpace();
}private long getFreeSpace() throws IOException {
long biggestSeenSoFar = 0;
String[] localDirs = localStorage.getDirs();
for (int i = 0; i < localDirs.length; i++) {
DF df = null;
if (localDirsDf.containsKey(localDirs[i])) {
df = localDirsDf.get(localDirs[i]);
} else {
df = new DF(new File(localDirs[i]), fConf);
localDirsDf.put(localDirs[i], df);
}
long availOnThisVol = df.getAvailable();
if (availOnThisVol > biggestSeenSoFar) {
biggestSeenSoFar = availOnThisVol;
}
}
//Should ultimately hold back the space we expect running tasks to use but
//that estimate isn't currently being passed down to the TaskTrackers
return biggestSeenSoFar;
}接下來就是獲取TT的一些資源信息，如總虛擬內存，總物理內存，可用的虛擬內存，可用的物理內存，CPU使用情況等。接着將這些值添加到status中去，發送給JT。
5.TaskTracker.transmitHeartBeat：
//add node health information
TaskTrackerHealthStatus healthStatus = status.getHealthStatus();
synchronized (this) {
if (healthChecker != null) {
healthChecker.setHealthStatus(healthStatus);
} else {
healthStatus.setNodeHealthy(true);
healthStatus.setLastReported(0L);
healthStatus.setHealthReport("");
}
}6.TaskTracker.transmitHeartBeat：
//
// Xmit the heartbeat
//
HeartbeatResponse heartbeatResponse = jobClient.heartbeat(status,
justStarted,
justInited,
askForNewTask,
heartbeatResponseId);7.TaskTracker.transmitHeartBeat：
//
// The heartbeat got through successfully!
//
heartbeatResponseId = heartbeatResponse.getResponseId();
synchronized (this) {
for (TaskStatus taskStatus : status.getTaskReports()) {
if (taskStatus.getRunState() != TaskStatus.State.RUNNING &&
taskStatus.getRunState() != TaskStatus.State.UNASSIGNED &&
taskStatus.getRunState() != TaskStatus.State.COMMIT_PENDING &&
!taskStatus.inTaskCleanupPhase()) {
if (taskStatus.getIsMap()) {
mapTotal--;
} else {
reduceTotal--;
}
myInstrumentation.completeTask(taskStatus.getTaskID());
runningTasks.remove(taskStatus.getTaskID());
}
}
// Clear transient status information which should only
// be sent once to the JobTracker
for (TaskInProgress tip: runningTasks.values()) {
tip.getStatus().clearStatus();
}
}
// Force a rebuild of 'status' on the next iteration
status = null;
return heartbeatResponse;myInstrumentation.completeTask(taskStatus.getTaskID())此處將該TT所有完成任務數加一，runningTasks.remove(taskStatus.getTaskID())則是將該task從runningTasks隊列中移除，所以可以知道runningTasks中只包含未完成的task信息。
接下來是清除TaskInProgress的TaskStatus的臨時信息（diagnosticInfo），從clearStatus()方法的註釋可以看出diagnosticInfo信息只是在Task向TaskTracker，或者TaskTracker向JobTracker發送一個狀態更新信息時的臨時診斷信息，所以在發送完成之後需要清除。
到這裏整個TaskTracker發送心跳信息的過程就完成了，方法返回值是HeartbeatResponse對象，即心跳的返回值。
