我试图在AWS上运行KMeans，并且尝试从DistributedCache读取更新的集群质心时遇到以下异常:
java.io.IOException: The distributed cache object s3://mybucket/centroids_6/part-r-00009 changed during the job from 4/8/13 2:20 PM to 4/8/13 2:20 PM
at org.apache.hadoop.filecache.TrackerDistributedCacheManager.downloadCacheObject(TrackerDistributedCacheManager.java:401)
at org.apache.hadoop.filecache.TrackerDistributedCacheManager.localizePublicCacheObject(TrackerDistributedCacheManager.java:475)
at org.apache.hadoop.filecache.TrackerDistributedCacheManager.getLocalCache(TrackerDistributedCacheManager.java:191)
at org.apache.hadoop.filecache.TaskDistributedCacheManager.setupCache(TaskDistributedCacheManager.java:182)
at org.apache.hadoop.mapred.TaskTracker$4.run(TaskTracker.java:1246)
at java.security.AccessController.doPrivileged(Native Method)
at javax.security.auth.Subject.doAs(Subject.java:396)
at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1132)
at org.apache.hadoop.mapred.TaskTracker.initializeJob(TaskTracker.java:1237)
at org.apache.hadoop.mapred.TaskTracker.localizeJob(TaskTracker.java:1152)
at org.apache.hadoop.mapred.TaskTracker$5.run(TaskTracker.java:2541)
at java.lang.Thread.run(Thread.java:662)
设置此问题apart from this one的原因是该错误间歇出现。我已经在较小的数据集上成功运行了相同的代码。此外，当我将质心的数量从12(在代码中如上所示)更改为8时，它在迭代5而不是6(在上面的
centroids_6名称中可以看到)上失败。这是运行KMeans循环的主驱动程序中的相关DistributedCache代码:
int iteration = 1;
long changes = 0;
do {
// First, write the previous iteration's centroids to the dist cache.
Configuration iterConf = new Configuration();
Path prevIter = new Path(centroidsPath.getParent(),
String.format("centroids_%s", iteration - 1));
FileSystem fs = prevIter.getFileSystem(iterConf);
Path pathPattern = new Path(prevIter, "part-*");
FileStatus [] list = fs.globStatus(pathPattern);
for (FileStatus status : list) {
DistributedCache.addCacheFile(status.getPath().toUri(), iterConf);
}
// Now, set up the job.
Job iterJob = new Job(iterConf);
iterJob.setJobName("KMeans " + iteration);
iterJob.setJarByClass(KMeansDriver.class);
Path nextIter = new Path(centroidsPath.getParent(),
String.format("centroids_%s", iteration));
KMeansDriver.delete(iterConf, nextIter);
// Set input/output formats.
iterJob.setInputFormatClass(SequenceFileInputFormat.class);
iterJob.setOutputFormatClass(SequenceFileOutputFormat.class);
// Set Mapper, Reducer, Combiner
iterJob.setMapperClass(KMeansMapper.class);
iterJob.setCombinerClass(KMeansCombiner.class);
iterJob.setReducerClass(KMeansReducer.class);
// Set MR formats.
iterJob.setMapOutputKeyClass(IntWritable.class);
iterJob.setMapOutputValueClass(VectorWritable.class);
iterJob.setOutputKeyClass(IntWritable.class);
iterJob.setOutputValueClass(VectorWritable.class);
// Set input/output paths.
FileInputFormat.addInputPath(iterJob, data);
FileOutputFormat.setOutputPath(iterJob, nextIter);
iterJob.setNumReduceTasks(nReducers);
if (!iterJob.waitForCompletion(true)) {
System.err.println("ERROR: Iteration " + iteration + " failed!");
System.exit(1);
}
iteration++;
changes = iterJob.getCounters().findCounter(KMeansDriver.Counter.CONVERGED).getValue();
iterJob.getCounters().findCounter(KMeansDriver.Counter.CONVERGED).setValue(0);
} while (changes > 0);
文件将如何进行其他修改？我能想到的唯一可能性是，在一次迭代完成时，循环在上一个作业的质心完成编写之前再次开始。但是在注释中，我使用
waitForCompletion(true)调用了作业，因此在循环重新开始时，不应有任何剩余的作业在运行。有任何想法吗？ 最佳答案
这并不是一个真正的答案，但是我确实意识到，以这种方式使用DistributedCache是愚蠢的，而不是直接从HDFS中读取上一次迭代的结果。相反，我在主驱动程序中编写了此方法:
public static HashMap<Integer, VectorWritable> readCentroids(Configuration conf, Path path)
throws IOException {
HashMap<Integer, VectorWritable> centroids = new HashMap<Integer, VectorWritable>();
FileSystem fs = FileSystem.get(path.toUri(), conf);
FileStatus [] list = fs.globStatus(new Path(path, "part-*"));
for (FileStatus status : list) {
SequenceFile.Reader reader = new SequenceFile.Reader(fs, status.getPath(), conf);
IntWritable key = null;
VectorWritable value = null;
try {
key = (IntWritable)reader.getKeyClass().newInstance();
value = (VectorWritable)reader.getValueClass().newInstance();
} catch (InstantiationException e) {
e.printStackTrace();
} catch (IllegalAccessException e) {
e.printStackTrace();
}
while (reader.next(key, value)) {
centroids.put(new Integer(key.get()),
new VectorWritable(value.get(), value.getClusterId(), value.getNumInstances()));
}
reader.close();
}
return centroids;
}
在每次迭代期间，都会在Mapper和Reducer的
setup()方法中调用此方法，以读取前一次迭代的质心。protected void setup(Context context) throws IOException {
Configuration conf = context.getConfiguration();
Path centroidsPath = new Path(conf.get(KMeansDriver.CENTROIDS));
centroids = KMeansDriver.readCentroids(conf, centroidsPath);
}
这使我可以在我的原始问题中删除循环中的代码块，该代码块将质心写入DistributedCache。我对其进行了测试，现在它可以在大型和小型数据集上使用。
我仍然不知道为什么会收到我发布的错误(如何更改只读DistributedCache中的某些内容？特别是当我在每次迭代中更改HDFS路径时？)，但这似乎既有效，又是一个错误。更少的读取质心的方法。