上一节分析了Job由JobClient提交到JobTracker的流程，利用RPC机制，JobTracker接收到Job ID和Job所在HDFS的目录，够早了JobInProgress对象，丢入队列，另一个线程从队列中取出JobInProgress对象，并丢入线程池中执行，执行JobInProgress的initJob方法，我们逐步分析。
public void initJob(JobInProgress job) {
if (null == job) {
LOG.info("Init on null job is not valid");
return;
}
try {
JobStatus prevStatus = (JobStatus)job.getStatus().clone();
LOG.info("Initializing " + job.getJobID());
job.initTasks();
// Inform the listeners if the job state has changed
// Note : that the job will be in PREP state.
JobStatus newStatus = (JobStatus)job.getStatus().clone();
if (prevStatus.getRunState() != newStatus.getRunState()) {
JobStatusChangeEvent event =
new JobStatusChangeEvent(job, EventType.RUN_STATE_CHANGED, prevStatus,
newStatus);
synchronized (JobTracker.this) {
updateJobInProgressListeners(event);
}
}
} catch (KillInterruptedException kie) {
// If job was killed during initialization, job state will be KILLED
LOG.error("Job initialization interrupted:\n" +
StringUtils.stringifyException(kie));
killJob(job);
} catch (Throwable t) {
String failureInfo =
"Job initialization failed:\n" + StringUtils.stringifyException(t);
// If the job initialization is failed, job state will be FAILED
LOG.error(failureInfo);
job.getStatus().setFailureInfo(failureInfo);
failJob(job);
}
}
可以看出，先进行 job.initTasks()，初始化Map和Reduce任务，之后更新所有
synchronized (JobTracker.this) {
updateJobInProgressListeners(event);
}
Map/Reduce Task初始化完毕是一个事件，下面的代码进行消息通知：
// Update the listeners about the job
// Assuming JobTracker is locked on entry.
private void updateJobInProgressListeners(JobChangeEvent event) {
for (JobInProgressListener listener : jobInProgressListeners) {
listener.jobUpdated(event);
}
}
可见，在Job放入队列时使用的是jobAdded，此时使用的是jobUpdated。我们在后面再分析jobUpdated后的细节，此时先分析从jobAdded到jobUpdated之间，Job的初始化过程，主要分为几个阶段。
首先执行的是获取Split信息，这一部分信息事先已经由JobClient上传至HDFS中。
1、读取Split信息：
//
// read input splits and create a map per a split
//
TaskSplitMetaInfo[] splits = createSplits(jobId);
if (numMapTasks != splits.length) {
throw new IOException("Number of maps in JobConf doesn't match number of " +
"recieved splits for job " + jobId + "! " +
"numMapTasks=" + numMapTasks + ", #splits=" + splits.length);
}
numMapTasks = splits.length;
createSplits方法的代码为：
TaskSplitMetaInfo[] createSplits(org.apache.hadoop.mapreduce.JobID jobId)
throws IOException {
TaskSplitMetaInfo[] allTaskSplitMetaInfo =
SplitMetaInfoReader.readSplitMetaInfo(jobId, fs, jobtracker.getConf(),
jobSubmitDir);
return allTaskSplitMetaInfo;
}
即读取job.splitmetainfo文件，获得Split信息：
public static JobSplit.TaskSplitMetaInfo[] readSplitMetaInfo(
JobID jobId, FileSystem fs, Configuration conf, Path jobSubmitDir)
throws IOException {
long maxMetaInfoSize = conf.getLong("mapreduce.jobtracker.split.metainfo.maxsize",
10000000L);
Path metaSplitFile = JobSubmissionFiles.getJobSplitMetaFile(jobSubmitDir);
FileStatus fStatus = fs.getFileStatus(metaSplitFile);
if (maxMetaInfoSize > 0 && fStatus.getLen() > maxMetaInfoSize) {
throw new IOException("Split metadata size exceeded " +
maxMetaInfoSize +". Aborting job " + jobId);
}
FSDataInputStream in = fs.open(metaSplitFile);
byte[] header = new byte[JobSplit.META_SPLIT_FILE_HEADER.length];
in.readFully(header);
if (!Arrays.equals(JobSplit.META_SPLIT_FILE_HEADER, header)) {
throw new IOException("Invalid header on split file");
}
int vers = WritableUtils.readVInt(in);
if (vers != JobSplit.META_SPLIT_VERSION) {
in.close();
throw new IOException("Unsupported split version " + vers);
}
int numSplits = WritableUtils.readVInt(in); //TODO: check for insane values
JobSplit.TaskSplitMetaInfo[] allSplitMetaInfo =
new JobSplit.TaskSplitMetaInfo[numSplits];
final int maxLocations =
conf.getInt(JobSplitWriter.MAX_SPLIT_LOCATIONS, Integer.MAX_VALUE);
for (int i = 0; i < numSplits; i++) {
JobSplit.SplitMetaInfo splitMetaInfo = new JobSplit.SplitMetaInfo();
splitMetaInfo.readFields(in);
final int numLocations = splitMetaInfo.getLocations().length;
if (numLocations > maxLocations) {
throw new IOException("Max block location exceeded for split: #" + i +
" splitsize: " + numLocations + " maxsize: " + maxLocations);
}
JobSplit.TaskSplitIndex splitIndex = new JobSplit.TaskSplitIndex(
JobSubmissionFiles.getJobSplitFile(jobSubmitDir).toString(),
splitMetaInfo.getStartOffset());
allSplitMetaInfo[i] = new JobSplit.TaskSplitMetaInfo(splitIndex,
splitMetaInfo.getLocations(),
splitMetaInfo.getInputDataLength());
}
in.close();
return allSplitMetaInfo;
}
涉及读取文件的代码有：
FSDataInputStream in = fs.open(metaSplitFile);
byte[] header = new byte[JobSplit.META_SPLIT_FILE_HEADER.length];
in.readFully(header);
这一部分先读取job.splitmetainfo文件的头部，头部实际上是字符串”META-SPL“，该信息由下面的类指定：
public class JobSplit {
static final int META_SPLIT_VERSION = 1;
static final byte[] META_SPLIT_FILE_HEADER;
static {
try {
META_SPLIT_FILE_HEADER = "META-SPL".getBytes("UTF-8");
} catch (UnsupportedEncodingException u) {
throw new RuntimeException(u);
}
}
.......
读取了文件头之后，剩下的是读取版本信息：
int vers = WritableUtils.readVInt(in);
if (vers != JobSplit.META_SPLIT_VERSION) {
in.close();
throw new IOException("Unsupported split version " + vers);
}
检查了版本（1）后，接下来就是读取Split的数量：
int numSplits = WritableUtils.readVInt(in); //TODO: check for insane values
JobSplit.TaskSplitMetaInfo[] allSplitMetaInfo =
new JobSplit.TaskSplitMetaInfo[numSplits];
并根据Split数量创建JobSplit.TaskSplitMetaInfo数组。接下来对于每个Split，循环读取位置等信息：
for (int i = 0; i < numSplits; i++) {
JobSplit.SplitMetaInfo splitMetaInfo = new JobSplit.SplitMetaInfo();
splitMetaInfo.readFields(in);
final int numLocations = splitMetaInfo.getLocations().length;
if (numLocations > maxLocations) {
throw new IOException("Max block location exceeded for split: #" + i +
" splitsize: " + numLocations + " maxsize: " + maxLocations);
}
JobSplit.TaskSplitIndex splitIndex = new JobSplit.TaskSplitIndex(
JobSubmissionFiles.getJobSplitFile(jobSubmitDir).toString(),
splitMetaInfo.getStartOffset());
allSplitMetaInfo[i] = new JobSplit.TaskSplitMetaInfo(splitIndex,
splitMetaInfo.getLocations(),
splitMetaInfo.getInputDataLength());
}
在上面的代码中，splitMetaInfo.readFields(in)可以获得位置信息：
public void readFields(DataInput in) throws IOException {
int len = WritableUtils.readVInt(in);
locations = new String[len];
for (int i = 0; i < locations.length; i++) {
locations[i] = Text.readString(in);
}
startOffset = WritableUtils.readVLong(in);
inputDataLength = WritableUtils.readVLong(in);
}
所谓的位置，实际上就是指这个Split在j哪些服务器的信息。获取到位置、Split数据长度等信息后，全部纪录在对象JobSplit.TaskSplitMetaInfo中：
JobSplit.TaskSplitIndex splitIndex = new JobSplit.TaskSplitIndex(
JobSubmissionFiles.getJobSplitFile(jobSubmitDir).toString(),
splitMetaInfo.getStartOffset());
allSplitMetaInfo[i] = new JobSplit.TaskSplitMetaInfo(splitIndex,
splitMetaInfo.getLocations(),
splitMetaInfo.getInputDataLength());
返回allSplitMetaInfo数组。
2、根据Map任务数量创建相同数量的TaskInProgress对象：
上面返回的数组大小即纪录了Split的个数，也决定了Map的数量，验证这些服务器的合法性：
numMapTasks = splits.length; // Sanity check the locations so we don't create/initialize unnecessary tasks
for (TaskSplitMetaInfo split : splits) {
NetUtils.verifyHostnames(split.getLocations());
}
在监控相关类中设置相应信息：
jobtracker.getInstrumentation().addWaitingMaps(getJobID(), numMapTasks);
jobtracker.getInstrumentation().addWaitingReduces(getJobID(), numReduceTasks);
this.queueMetrics.addWaitingMaps(getJobID(), numMapTasks);
this.queueMetrics.addWaitingReduces(getJobID(), numReduceTasks);
接下来创建TaskInProgress对象，每个Map都对应于一个TaskInProgress对象：
maps = new TaskInProgress[numMapTasks];
for(int i=0; i < numMapTasks; ++i) {
inputLength += splits[i].getInputDataLength();
maps[i] = new TaskInProgress(jobId, jobFile,
splits[i],
jobtracker, conf, this, i, numSlotsPerMap);
}
TaskInProgress纪录了一个Map Task或Reduce Task运行相关的所有信息，类似于JobInProgress，TaskInProgress的构造函数有两个，分别针对Map和Reduce的，对于Map的：
/**
* Constructor for MapTask
*/
public TaskInProgress(JobID jobid, String jobFile,
TaskSplitMetaInfo split,
JobTracker jobtracker, JobConf conf,
JobInProgress job, int partition,
int numSlotsRequired) {
this.jobFile = jobFile;
this.splitInfo = split;
this.jobtracker = jobtracker;
this.job = job;
this.conf = conf;
this.partition = partition;
this.maxSkipRecords = SkipBadRecords.getMapperMaxSkipRecords(conf);
this.numSlotsRequired = numSlotsRequired;
setMaxTaskAttempts();
init(jobid);
}
splitInfo纪录了当前Split的信息，partition即表示这是第几个Map Task，numSlotsRequired为1.
创建好的TaskInProgress将会放入缓存中：
if (numMapTasks > 0) {
nonRunningMapCache = createCache(splits, maxLevel);
}
nonRunningMapCache是一个未运行起来的Map任务的关于主机信息等等的缓存，其索引为Node，即服务器；而其值为TaskInProgress对象，其声明为，因此，实际上就是解析Split所在的服务器，缓存下来，供后续调度使用：
Map<Node, List<TaskInProgress>> nonRunningMapCache;
其方法代码为：
private Map<Node, List<TaskInProgress>> createCache(
TaskSplitMetaInfo[] splits, int maxLevel)
throws UnknownHostException {
Map<Node, List<TaskInProgress>> cache =
new IdentityHashMap<Node, List<TaskInProgress>>(maxLevel); Set<String> uniqueHosts = new TreeSet<String>();
for (int i = 0; i < splits.length; i++) {
String[] splitLocations = splits[i].getLocations();
if (splitLocations == null || splitLocations.length == 0) {
nonLocalMaps.add(maps[i]);
continue;
} for(String host: splitLocations) {
Node node = jobtracker.resolveAndAddToTopology(host);
uniqueHosts.add(host);
LOG.info("tip:" + maps[i].getTIPId() + " has split on node:" + node);
for (int j = 0; j < maxLevel; j++) {
List<TaskInProgress> hostMaps = cache.get(node);
if (hostMaps == null) {
hostMaps = new ArrayList<TaskInProgress>();
cache.put(node, hostMaps);
hostMaps.add(maps[i]);
}
//check whether the hostMaps already contains an entry for a TIP
//This will be true for nodes that are racks and multiple nodes in
//the rack contain the input for a tip. Note that if it already
//exists in the hostMaps, it must be the last element there since
//we process one TIP at a time sequentially in the split-size order
if (hostMaps.get(hostMaps.size() - 1) != maps[i]) {
hostMaps.add(maps[i]);
}
node = node.getParent();
}
}
} // Calibrate the localityWaitFactor - Do not override user intent!
if (localityWaitFactor == DEFAULT_LOCALITY_WAIT_FACTOR) {
int jobNodes = uniqueHosts.size();
int clusterNodes = jobtracker.getNumberOfUniqueHosts(); if (clusterNodes > 0) {
localityWaitFactor =
Math.min((float)jobNodes/clusterNodes, localityWaitFactor);
}
LOG.info(jobId + " LOCALITY_WAIT_FACTOR=" + localityWaitFactor);
} return cache;
}
3、根据Reduce任务数量创建相同数量的TaskInProgress对象：
代码和Map基本相同：
//
// Create reduce tasks
//
this.reduces = new TaskInProgress[numReduceTasks];
for (int i = 0; i < numReduceTasks; i++) {
reduces[i] = new TaskInProgress(jobId, jobFile,
numMapTasks, i,
jobtracker, conf, this, numSlotsPerReduce);
nonRunningReduces.add(reduces[i]);
}
4、计算Reduce任务启动前Map最少应该启动的数量：
根据MapReduce原理，先进行Map计算，之后中间结果再传递至Reduce计算，因此，Map要先进行计算，Reduce如果和Map一起启动，那么，Reduce必然先一直处于等待中。这会消耗机器资源，且Shuffle时间比较长。所以，这个值默认是Map所有任务数量的5%：
// Calculate the minimum number of maps to be complete before
// we should start scheduling reduces
completedMapsForReduceSlowstart =
(int)Math.ceil(
(conf.getFloat("mapred.reduce.slowstart.completed.maps",
DEFAULT_COMPLETED_MAPS_PERCENT_FOR_REDUCE_SLOWSTART) *
numMapTasks)); // ... use the same for estimating the total output of all maps
resourceEstimator.setThreshhold(completedMapsForReduceSlowstart);
从DEFAULT_COMPLETED_MAPS_PERCENT_FOR_REDUCE_SLOWSTART可以看出，是5%:
private static float DEFAULT_COMPLETED_MAPS_PERCENT_FOR_REDUCE_SLOWSTART = 0.05f;
5、创建Map和Reduce任务的清理任务，各一个：
// create cleanup two cleanup tips, one map and one reduce.
cleanup = new TaskInProgress[2]; // cleanup map tip. This map doesn't use any splits. Just assign an empty
// split.
TaskSplitMetaInfo emptySplit = JobSplit.EMPTY_TASK_SPLIT;
cleanup[0] = new TaskInProgress(jobId, jobFile, emptySplit,
jobtracker, conf, this, numMapTasks, 1);
cleanup[0].setJobCleanupTask(); // cleanup reduce tip.
cleanup[1] = new TaskInProgress(jobId, jobFile, numMapTasks,
numReduceTasks, jobtracker, conf, this, 1);
cleanup[1].setJobCleanupTask();
6、创建Map和Reduce任务的启动任务，各一个：
// create two setup tips, one map and one reduce.
setup = new TaskInProgress[2]; // setup map tip. This map doesn't use any split. Just assign an empty
// split.
setup[0] = new TaskInProgress(jobId, jobFile, emptySplit,
jobtracker, conf, this, numMapTasks + 1, 1);
setup[0].setJobSetupTask(); // setup reduce tip.
setup[1] = new TaskInProgress(jobId, jobFile, numMapTasks,
numReduceTasks + 1, jobtracker, conf, this, 1);
setup[1].setJobSetupTask();
7、Map/Reduce Task初始化完毕：
synchronized(jobInitKillStatus){
jobInitKillStatus.initDone = true;
// set this before the throw to make sure cleanup works properly
tasksInited = true;
if(jobInitKillStatus.killed) {
throw new KillInterruptedException("Job " + jobId + " killed in init");
}
}
初始化完毕后，会通过jobUpdated进行通知。Job更新的事件主要有三种：
static enum EventType {RUN_STATE_CHANGED, START_TIME_CHANGED, PRIORITY_CHANGED}
此时初始化完毕属于RUN_STATE_CHANGED。从其代码来看，如果是运行状态改变，并不执行什么操作：
public synchronized void jobUpdated(JobChangeEvent event) {
JobInProgress job = event.getJobInProgress();
if (event instanceof JobStatusChangeEvent) {
// Check if the ordering of the job has changed
// For now priority and start-time can change the job ordering
JobStatusChangeEvent statusEvent = (JobStatusChangeEvent)event;
JobSchedulingInfo oldInfo =
new JobSchedulingInfo(statusEvent.getOldStatus());
if (statusEvent.getEventType() == EventType.PRIORITY_CHANGED
|| statusEvent.getEventType() == EventType.START_TIME_CHANGED) {
// Make a priority change
reorderJobs(job, oldInfo);
} else if (statusEvent.getEventType() == EventType.RUN_STATE_CHANGED) {
// Check if the job is complete
int runState = statusEvent.getNewStatus().getRunState();
if (runState == JobStatus.SUCCEEDED
|| runState == JobStatus.FAILED
|| runState == JobStatus.KILLED) {
jobCompleted(oldInfo);
}
}
}
}
因为此时Job并未结束。从此可以看出，Job在初始化完毕后，线程池又去执行其他Job的初始化等操作，等待TaskTracker来取。
关于TaskTracker与JobTracker之间的心跳，以及任务的获取等操作，比较复杂，留作后续博文分析。
后记
由流程图来看：
本博文在上一节分析了1、2、3、4的基础上，分析了5、6两个步骤，即Job的初始化、到HDFS中获取资源数据，获得Map和Reduce数量等过程。关于7、8、9、10等后续操作，在后续博文中分析。