本文深入探讨了Hadoop代码测试版中的全局排序原理及其实现方式，通过实例展示了如何在多个Reducer产生的输出文件之间实现全局排序。包括自定义全局排序类的使用、Mapper输出调整、以及如何通过TotalOrderPartitioner达到不同文件间的排序效果。此外，还提供了一个实际应用示例，说明了如何通过特定的Mapper逻辑来解决样本值加入问题，并最终展示全局排序的效果。
Hadoop代码测试版：Hadoop2.4
原理：携带MR该程序随机抽样提取前的输入数据，样本分类，然后，MR该过程的中间Partition此值用于当样品排序分组数据。这使得可以实现全球排名的目的。
难度：假设Hadoop全局排序，那么要求Mapper的输入、输出的key不变才干够，由于在源代码InputSampler中提供的随机抽取的数据是输入数据最原始的key，例如以下代码（line：225）：
for (int i = 0; i < splitsToSample ||
(i < splits.size() && samples.size() < numSamples); ++i) {
TaskAttemptContext samplingContext = new TaskAttemptContextImpl(
job.getConfiguration(), new TaskAttemptID());
RecordReader<K,V> reader = inf.createRecordReader(
splits.get(i), samplingContext);
reader.initialize(splits.get(i), samplingContext);
while (reader.nextKeyValue()) {
if (r.nextDouble() <= freq) {
if (samples.size() < numSamples) {
samples.add(ReflectionUtils.copy(job.getConfiguration(),// here is line 225
reader.getCurrentKey(), null));
} else {
// When exceeding the maximum number of samples, replace a
// random element with this one, then adjust the frequency
// to reflect the possibility of existing elements being
// pushed out
int ind = r.nextInt(numSamples);
if (ind != numSamples) {
samples.set(ind, ReflectionUtils.copy(job.getConfiguration(),
reader.getCurrentKey(), null));
}
freq *= (numSamples - 1) / (double) numSamples;
}
}
}
reader.close();
}举一个非常实际的样例，我的输入一般都是LongWritable（距离文本首的长度），可是我的Mapper输出的key能够是Text的类型的，那么在建立样本值的时候就会有问题。
假设解决呢？
事实上能够把上面的代码中的加入部分改动一下。改为Mapper的map逻辑就可以（參考以下的实例）。
应用场景：当MR程序有多个reducer的时候，就会对应的产生多个输出文件。这些输出文件内部是有按顺序排列的，可是，文件之间却没有依照顺序排列。使用TotalOrderPartitioner就能够达到不同的文件之间也是排序的效果。
实例：
測试数据。採用三个測试数据（这样就能够有三个分片，默认一个文件一个分片）
測试主程序完毕的任务就是把上面的数据依照“_”分隔，然后把“_”后面的数字作为key，前面的字符串作为value进行输出。这样就能够看到输出的key是否是全局排序的了。
測试主程序：
package fz.totalorder.partitioner;
import java.net.URI;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.conf.Configured;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.Reducer;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.input.TextInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
import org.apache.hadoop.mapreduce.lib.output.TextOutputFormat;
import org.apache.hadoop.mapreduce.lib.partition.MyInputSampler;
import org.apache.hadoop.mapreduce.lib.partition.TotalOrderPartitioner;
import org.apache.hadoop.util.Tool;
import org.apache.hadoop.util.ToolRunner;
//import fz.Utils;
public class PartitionerDriver extends Configured implements Tool {
@Override
public int run(String[] arg0) throws Exception {
Configuration conf = getConf();
if(arg0.length!=3){
System.err.println("Usage:\nfz.partitioner.PartitionerDriver <in> <out> <useTotalOrder>");
return -1;
}
// System.out.println(conf.get("fs.defaultFS"));
Path in = new Path(arg0[0]);
Path out= new Path(arg0[1]);
out.getFileSystem(conf).delete(out, true);
Job job = Job.getInstance(conf,"total order partitioner");
job.setJarByClass(getClass());
job.setInputFormatClass(TextInputFormat.class);
job.setOutputFormatClass(TextOutputFormat.class);
job.setMapperClass(PartitionerMapper.class);
job.setMapOutputKeyClass(Text.class);
job.setMapOutputValueClass(Text.class);
job.setOutputKeyClass(Text.class);
job.setOutputValueClass(Text.class);
job.setReducerClass(Reducer.class);
job.setNumReduceTasks(2);
// System.out.println(job.getConfiguration().get("mapreduce.job.reduces"));
// System.out.println(conf.get("mapreduce.job.reduces"));
FileInputFormat.setInputPaths(job, in);
FileOutputFormat.setOutputPath(job, out);
// reducer全局排序
if(arg0[2]!=null&&"true".equals(arg0[2])){
job.setPartitionerClass(TotalOrderPartitioner.class);
// InputSampler.Sampler<Text, Text> sampler = new
// InputSampler.RandomSampler<Text, Text>(0.1,20,3);
// InputSampler.writePartitionFile(job, sampler);
MyInputSampler.Sampler<Text, Text> sampler = new
MyInputSampler.RandomSampler<Text, Text>(0.1,20,3);
MyInputSampler.writePartitionFile(job, sampler);
String partitionFile = TotalOrderPartitioner.getPartitionFile(getConf());
URI partitionUri= new URI(partitionFile+"#"+TotalOrderPartitioner.DEFAULT_PATH);
job.addCacheArchive(partitionUri);
}
return job.waitForCompletion(true)?0:-1;
}
public static void main(String[] args) throws Exception {
ToolRunner.run(new Configuration(), new PartitionerDriver(),args);
// String[] arg = new String[]{
// "hdfs://node33:8020/user/root/partition",
// "hdfs://node33:8020/user/Administrator/partition",
// "true"
// };
// ToolRunner.run(Utils.getConf(), new PartitionerDriver(),arg);
}
}
PartitionerMapper类：
package fz.totalorder.partitioner;
import java.io.IOException;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Mapper;
public class PartitionerMapper extends Mapper<LongWritable,Text,Text ,Text>{
private Text newKey= new Text();
private Text newValue = new Text();
public void map(LongWritable key, Text value, Context cxt) throws IOException,InterruptedException{
String [] line =value.toString().split("_");
if(line.length!=2){
return ;
}
newKey.set(line[1]);
newValue.set(line[0]);
cxt.write(newKey, newValue);
}
}
这里能够看到Mapper的输出和输出是不一样的。所以我们须要自己定义InputSampler类，加入经过处理的key。
其详细代码为：
/**
* Licensed to the Apache Software Foundation (ASF) under one
* or more contributor license agreements. See the NOTICE file
* distributed with this work for additional information
* regarding copyright ownership. The ASF licenses this file
* to you under the Apache License, Version 2.0 (the
* "License"); you may not use this file except in compliance
* with the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.hadoop.mapreduce.lib.partition;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Random;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.apache.hadoop.classification.InterfaceAudience;
import org.apache.hadoop.classification.InterfaceStability;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.conf.Configured;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.NullWritable;
import org.apache.hadoop.io.RawComparator;
import org.apache.hadoop.io.SequenceFile;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.io.WritableComparable;
import org.apache.hadoop.mapreduce.InputFormat;
import org.apache.hadoop.mapreduce.InputSplit;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.RecordReader;
import org.apache.hadoop.mapreduce.TaskAttemptContext;
import org.apache.hadoop.mapreduce.TaskAttemptID;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.task.TaskAttemptContextImpl;
import org.apache.hadoop.util.ReflectionUtils;
import org.apache.hadoop.util.Tool;
import org.apache.hadoop.util.ToolRunner;
/**
* Utility for collecting samples and writing a partition file for
* {@link TotalOrderPartitioner}.
*/
@InterfaceAudience.Public
@InterfaceStability.Stable
public class MyInputSampler<K,V> extends Configured implements Tool {
private static final Log LOG = LogFactory.getLog(MyInputSampler.class);
static int printUsage() {
System.out.println("sampler -r <reduces>\n" +
" [-inFormat <input format class>]\n" +
" [-keyClass <map input & output key class>]\n" +
" [-splitRandom <double pcnt> <numSamples> <maxsplits> | " +
" // Sample from random splits at random (general)\n" +
" -splitSample <numSamples> <maxsplits> | " +
" // Sample from first records in splits (random data)\n"+
" -splitInterval <double pcnt> <maxsplits>]" +
" // Sample from splits at intervals (sorted data)");
System.out.println("Default sampler: -splitRandom 0.1 10000 10");
ToolRunner.printGenericCommandUsage(System.out);
return -1;
}
public MyInputSampler(Configuration conf) {
setConf(conf);
}
/**
* Interface to sample using an
* {@link org.apache.hadoop.mapreduce.InputFormat}.
*/
public interface Sampler<K,V> {
/**
* For a given job, collect and return a subset of the keys from the
* input data.
*/
K[] getSample(InputFormat<K,V> inf, Job job)
throws IOException, InterruptedException;
}
/**
* Samples the first n records from s splits.
* Inexpensive way to sample random data.
*/
public static class SplitSampler<K,V> implements Sampler<K,V> {
protected final int numSamples;
protected final int maxSplitsSampled;
/**
* Create a SplitSampler sampling <em>all</em> splits.
* Takes the first numSamples / numSplits records from each split.
* @param numSamples Total number of samples to obtain from all selected
* splits.
*/
public SplitSampler(int numSamples) {
this(numSamples, Integer.MAX_VALUE);
}
/**
* Create a new SplitSampler.
* @param numSamples Total number of samples to obtain from all selected
* splits.
* @param maxSplitsSampled The maximum number of splits to examine.
*/
public SplitSampler(int numSamples, int maxSplitsSampled) {
this.numSamples = numSamples;
this.maxSplitsSampled = maxSplitsSampled;
}
/**
* From each split sampled, take the first numSamples / numSplits records.
*/
@SuppressWarnings("unchecked") // ArrayList::toArray doesn't preserve type
public K[] getSample(InputFormat<K,V> inf, Job job)
throws IOException, InterruptedException {
List<InputSplit> splits = inf.getSplits(job);
ArrayList<K> samples = new ArrayList<K>(numSamples);
int splitsToSample = Math.min(maxSplitsSampled, splits.size());
int samplesPerSplit = numSamples / splitsToSample;
long records = 0;
for (int i = 0; i < splitsToSample; ++i) {
TaskAttemptContext samplingContext = new TaskAttemptContextImpl(
job.getConfiguration(), new TaskAttemptID());
RecordReader<K,V> reader = inf.createRecordReader(
splits.get(i), samplingContext);
reader.initialize(splits.get(i), samplingContext);
while (reader.nextKeyValue()) {
samples.add(ReflectionUtils.copy(job.getConfiguration(),
reader.getCurrentKey(), null));
++records;
if ((i+1) * samplesPerSplit <= records) {
break;
}
}
reader.close();
}
return (K[])samples.toArray();
}
}
/**
* Sample from random points in the input.
* General-purpose sampler. Takes numSamples / maxSplitsSampled inputs from
* each split.
*/
public static class RandomSampler<K,V> implements Sampler<K,V> {
protected double freq;
protected final int numSamples;
protected final int maxSplitsSampled;
/**
* Create a new RandomSampler sampling <em>all</em> splits.
* This will read every split at the client, which is very expensive.
* @param freq Probability with which a key will be chosen.
* @param numSamples Total number of samples to obtain from all selected
* splits.
*/
public RandomSampler(double freq, int numSamples) {
this(freq, numSamples, Integer.MAX_VALUE);
}
/**
* Create a new RandomSampler.
* @param freq Probability with which a key will be chosen.
* @param numSamples Total number of samples to obtain from all selected
* splits.
* @param maxSplitsSampled The maximum number of splits to examine.
*/
public RandomSampler(double freq, int numSamples, int maxSplitsSampled) {
this.freq = freq;
this.numSamples = numSamples;
this.maxSplitsSampled = maxSplitsSampled;
}
/**
* Randomize the split order, then take the specified number of keys from
* each split sampled, where each key is selected with the specified
* probability and possibly replaced by a subsequently selected key when
* the quota of keys from that split is satisfied.
*/
@SuppressWarnings("unchecked") // ArrayList::toArray doesn't preserve type
public K[] getSample(InputFormat<K,V> inf, Job job)
throws IOException, InterruptedException {
List<InputSplit> splits = inf.getSplits(job);
ArrayList<K> samples = new ArrayList<K>(numSamples);
int splitsToSample = Math.min(maxSplitsSampled, splits.size());
Random r = new Random();
long seed = r.nextLong();
r.setSeed(seed);
LOG.debug("seed: " + seed);
// shuffle splits
for (int i = 0; i < splits.size(); ++i) {
InputSplit tmp = splits.get(i);
int j = r.nextInt(splits.size());
splits.set(i, splits.get(j));
splits.set(j, tmp);
}
// our target rate is in terms of the maximum number of sample splits,
// but we accept the possibility of sampling additional splits to hit
// the target sample keyset
for (int i = 0; i < splitsToSample ||
(i < splits.size() && samples.size() < numSamples); ++i) {
TaskAttemptContext samplingContext = new TaskAttemptContextImpl(
job.getConfiguration(), new TaskAttemptID());
RecordReader<K,V> reader = inf.createRecordReader(
splits.get(i), samplingContext);
reader.initialize(splits.get(i), samplingContext);
while (reader.nextKeyValue()) {
if (r.nextDouble() <= freq) {
if (samples.size() < numSamples) {
samples.add(ReflectionUtils.copy(job.getConfiguration(),
getFixedKey(reader), null)); // add here
} else {
// When exceeding the maximum number of samples, replace a
// random element with this one, then adjust the frequency
// to reflect the possibility of existing elements being
// pushed out
int ind = r.nextInt(numSamples);
if (ind != numSamples) {
samples.set(ind, ReflectionUtils.copy(job.getConfiguration(),
getFixedKey(reader), null)); // add here
}
freq *= (numSamples - 1) / (double) numSamples;
}
}
}
reader.close();
}
return (K[])samples.toArray();
}
/**
* use new key
* @param reader
* @return
*/
private K getFixedKey(RecordReader<K, V> reader) {
K newKey =null;
String[] line;
try {
line = reader.getCurrentValue().toString().split("_");
Text newTmpKey = new Text(line[1]);
newKey =(K) newTmpKey;
} catch (IOException e) {
e.printStackTrace();
} catch (InterruptedException e) {
e.printStackTrace();
}
return newKey;
}
}
/**
* Sample from s splits at regular intervals.
* Useful for sorted data.
*/
public static class IntervalSampler<K,V> implements Sampler<K,V> {
protected final double freq;
protected final int maxSplitsSampled;
/**
* Create a new IntervalSampler sampling <em>all</em> splits.
* @param freq The frequency with which records will be emitted.
*/
public IntervalSampler(double freq) {
this(freq, Integer.MAX_VALUE);
}
/**
* Create a new IntervalSampler.
* @param freq The frequency with which records will be emitted.
* @param maxSplitsSampled The maximum number of splits to examine.
* @see #getSample
*/
public IntervalSampler(double freq, int maxSplitsSampled) {
this.freq = freq;
this.maxSplitsSampled = maxSplitsSampled;
}
/**
* For each split sampled, emit when the ratio of the number of records
* retained to the total record count is less than the specified
* frequency.
*/
@SuppressWarnings("unchecked") // ArrayList::toArray doesn't preserve type
public K[] getSample(InputFormat<K,V> inf, Job job)
throws IOException, InterruptedException {
List<InputSplit> splits = inf.getSplits(job);
ArrayList<K> samples = new ArrayList<K>();
int splitsToSample = Math.min(maxSplitsSampled, splits.size());
long records = 0;
long kept = 0;
for (int i = 0; i < splitsToSample; ++i) {
TaskAttemptContext samplingContext = new TaskAttemptContextImpl(
job.getConfiguration(), new TaskAttemptID());
RecordReader<K,V> reader = inf.createRecordReader(
splits.get(i), samplingContext);
reader.initialize(splits.get(i), samplingContext);
while (reader.nextKeyValue()) {
++records;
if ((double) kept / records < freq) {
samples.add(ReflectionUtils.copy(job.getConfiguration(),
reader.getCurrentKey(), null));
++kept;
}
}
reader.close();
}
return (K[])samples.toArray();
}
}
/**
* Write a partition file for the given job, using the Sampler provided.
* Queries the sampler for a sample keyset, sorts by the output key
* comparator, selects the keys for each rank, and writes to the destination
* returned from {@link TotalOrderPartitioner#getPartitionFile}.
*/
@SuppressWarnings("unchecked") // getInputFormat, getOutputKeyComparator
public static <K,V> void writePartitionFile(Job job, Sampler<K,V> sampler)
throws IOException, ClassNotFoundException, InterruptedException {
Configuration conf = job.getConfiguration();
final InputFormat inf =
ReflectionUtils.newInstance(job.getInputFormatClass(), conf);
int numPartitions = job.getNumReduceTasks();
K[] samples = (K[])sampler.getSample(inf, job);
LOG.info("Using " + samples.length + " samples");
RawComparator<K> comparator =
(RawComparator<K>) job.getSortComparator();
Arrays.sort(samples, comparator);
Path dst = new Path(TotalOrderPartitioner.getPartitionFile(conf));
FileSystem fs = dst.getFileSystem(conf);
if (fs.exists(dst)) {
fs.delete(dst, false);
}
SequenceFile.Writer writer = SequenceFile.createWriter(fs,
conf, dst, job.getMapOutputKeyClass(), NullWritable.class);
NullWritable nullValue = NullWritable.get();
float stepSize = samples.length / (float) numPartitions;
int last = -1;
for(int i = 1; i < numPartitions; ++i) {
int k = Math.round(stepSize * i);
while (last >= k && comparator.compare(samples[last], samples[k]) == 0) {
++k;
}
writer.append(samples[k], nullValue);
last = k;
}
writer.close();
}
/**
* Driver for MyInputSampler from the command line.
* Configures a JobConf instance and calls {@link #writePartitionFile}.
*/
public int run(String[] args) throws Exception {
Job job = new Job(getConf());
ArrayList<String> otherArgs = new ArrayList<String>();
Sampler<K,V> sampler = null;
for(int i=0; i < args.length; ++i) {
try {
if ("-r".equals(args[i])) {
job.setNumReduceTasks(Integer.parseInt(args[++i]));
} else if ("-inFormat".equals(args[i])) {
job.setInputFormatClass(
Class.forName(args[++i]).asSubclass(InputFormat.class));
} else if ("-keyClass".equals(args[i])) {
job.setMapOutputKeyClass(
Class.forName(args[++i]).asSubclass(WritableComparable.class));
} else if ("-splitSample".equals(args[i])) {
int numSamples = Integer.parseInt(args[++i]);
int maxSplits = Integer.parseInt(args[++i]);
if (0 >= maxSplits) maxSplits = Integer.MAX_VALUE;
sampler = new SplitSampler<K,V>(numSamples, maxSplits);
} else if ("-splitRandom".equals(args[i])) {
double pcnt = Double.parseDouble(args[++i]);
int numSamples = Integer.parseInt(args[++i]);
int maxSplits = Integer.parseInt(args[++i]);
if (0 >= maxSplits) maxSplits = Integer.MAX_VALUE;
sampler = new RandomSampler<K,V>(pcnt, numSamples, maxSplits);
} else if ("-splitInterval".equals(args[i])) {
double pcnt = Double.parseDouble(args[++i]);
int maxSplits = Integer.parseInt(args[++i]);
if (0 >= maxSplits) maxSplits = Integer.MAX_VALUE;
sampler = new IntervalSampler<K,V>(pcnt, maxSplits);
} else {
otherArgs.add(args[i]);
}
} catch (NumberFormatException except) {
System.out.println("ERROR: Integer expected instead of " + args[i]);
return printUsage();
} catch (ArrayIndexOutOfBoundsException except) {
System.out.println("ERROR: Required parameter missing from " +
args[i-1]);
return printUsage();
}
}
if (job.getNumReduceTasks() <= 1) {
System.err.println("Sampler requires more than one reducer");
return printUsage();
}
if (otherArgs.size() < 2) {
System.out.println("ERROR: Wrong number of parameters: ");
return printUsage();
}
if (null == sampler) {
sampler = new RandomSampler<K,V>(0.1, 10000, 10);
}
Path outf = new Path(otherArgs.remove(otherArgs.size() - 1));
TotalOrderPartitioner.setPartitionFile(getConf(), outf);
for (String s : otherArgs) {
FileInputFormat.addInputPath(job, new Path(s));
}
MyInputSampler.<K,V>writePartitionFile(job, sampler);
return 0;
}
public static void main(String[] args) throws Exception {
MyInputSampler<?,?> sampler = new MyInputSampler(new Configuration()); int res = ToolRunner.run(sampler, args); System.exit(res); } }
其主要代码是：
/**
* use new key
* @param reader
* @return
*/
private K getFixedKey(RecordReader<K, V> reader) {
K newKey =null;
String[] line;
try {
line = reader.getCurrentValue().toString().split("_");
Text newTmpKey = new Text(line[1]);
newKey =(K) newTmpKey;
} catch (IOException e) {
e.printStackTrace();
} catch (InterruptedException e) {
e.printStackTrace();
}
return newKey;
}
}首先不使用全局分类的代码：
能够看到文件之间是没有排序的；
全局排序的文件输出为：
能够看到文件之间是有全局排序的。
总结：使用Hadoop默认提供的全局排序功能有限，能够自己定义全局排序的类，可是这样针对每一个MR可能都须要提供一个自己定义的类。这样也比較麻烦。
整体来说。全局排序的应用场景比較少见。
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