Hadoop代码测试版本：Hadoop2.4
原理：在进行MR程序之前对输入数据进行随机提取样本，把样本排序，然后在MR的中间过程Partition的时候使用这个样本排序的值进行分组数据，这样就可以达到全局排序的目的了。
难点：如果使用Hadoop提供的方法来实现全局排序，那么要求Mapper的输入、输出的key不变才可以，因为在源码InputSampler中提供的随机抽取的数据是输入数据最原始的key，如下代码（line：225）：
for (int i = 0; i < splitsToSample ||
(i < splits.size() && samples.size() < numSamples); ++i) {
TaskAttemptContext samplingContext = new TaskAttemptContextImpl(
job.getConfiguration(), new TaskAttemptID());
RecordReader 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();
} 其中的samples.add( ... 就是添加的样本，这样其实应该是需要调整的。
举一个很实际的例子，我的输入一般都是LongWritable（距离文本首的长度），但是我的Mapper输出的key可以是Text的类型的，那么在建立样本值的时候就会有问题。
如果解决呢？
其实可以把上面的代码中的添加部分修改一下，改为Mapper的map逻辑即可（参考下面的实例）。
应用场景：当MR程序有多个reducer的时候，就会相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，但是，文件之间却没有按照顺序排列，使用TotalOrderPartitioner就可以达到不同的文件之间也是排序的效果。
实例：
测试数据，采用三个测试数据（这样就可以有三个分片，默认一个文件一个分片）
测试主程序完成的任务就是把上面的数据按照“_”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，这样就可以看到输出的key是否是全局排序的了。
测试主程序：
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 ");
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 sampler = new
// InputSampler.RandomSampler(0.1,20,3);
// InputSampler.writePartitionFile(job, sampler);
MyInputSampler.Sampler sampler = new
MyInputSampler.RandomSampler(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{
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);
}
}
这里可以看到Mapper的输出和输出是不一样的，所以我们需要自定义InputSampler类，添加经过处理的key。其具体代码为：
/**
* 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
*
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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 extends Configured implements Tool {
private static final Log LOG = LogFactory.getLog(MyInputSampler.class);
static int printUsage() {
System.out.println("sampler -r \n" +
" [-inFormat ]\n" +
" [-splitRandom | " +
" // Sample from random splits at random (general)\n" +
" -splitSample | " +
" // Sample from first records in splits (random data)\n"+
" -splitInterval ]" +
" // 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 {
/**
* For a given job, collect and return a subset of the keys from the
* input data.
*/
K[] getSample(InputFormat inf, Job job)
throws IOException, InterruptedException;
}
/**
* Samples the first n records from s splits.
* Inexpensive way to sample random data.
*/
public static class SplitSampler implements Sampler {
protected final int numSamples;
protected final int maxSplitsSampled;
/**
* Create a SplitSampler sampling all 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 inf, Job job)
throws IOException, InterruptedException {
List splits = inf.getSplits(job);
ArrayList samples = new ArrayList(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 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 implements Sampler {
protected double freq;
protected final int numSamples;
protected final int maxSplitsSampled;
/**
* Create a new RandomSampler sampling all 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 inf, Job job)
throws IOException, InterruptedException {
List splits = inf.getSplits(job);
ArrayList samples = new ArrayList(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 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 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 implements Sampler {
protected final double freq;
protected final int maxSplitsSampled;
/**
* Create a new IntervalSampler sampling all 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 inf, Job job)
throws IOException, InterruptedException {
List splits = inf.getSplits(job);
ArrayList samples = new ArrayList();
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 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 void writePartitionFile(Job job, Sampler 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 comparator =
(RawComparator) 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 otherArgs = new ArrayList();
Sampler 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(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(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(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(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.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 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;
}
} 其实就是把Mapper的逻辑拿过来而已。
查看输出：
首先不使用全局分类的代码：
全局排序的文件输出为：
可以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，可以自定义全局排序的类，但是这样针对每个MR可能都需要提供一个自定义的类，这样也比较麻烦。总体来说，全局排序的应用场景比较少见。
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