Hadoop中Partition深度解析

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旧版 API 的 Partitioner 解析

Partitioner 的作用是对 Mapper 产生的中间结果进行分片，以便将同一分组的数据交给同一个 Reducer 处理，它直接影响 Reduce 阶段的负载均衡。旧版 API 中 Partitioner 的类图如图所示。它继承了JobConfigurable，可通过 configure 方法初始化。它本身只包含一个待实现的方法 getPartition。 该方法包含三个参数， 均由框架自动传入，前面两个参数是key/value，第三个参数 numPartitions 表示每个 Mapper 的分片数，也就是 Reducer 的个数。

MapReduce 提供了两个Partitioner 实 现：HashPartitioner和TotalOrderPartitioner。其中 HashPartitioner 是默认实现，它实现了一种基于哈希值的分片方法，代码如下：

public int getPartition(K2 key, V2 value, int numReduceTasks) {
    return (key.hashCode() & Integer.MAX_VALUE) % numReduceTasks;
}

TotalOrderPartitioner 提供了一种基于区间的分片方法，通常用在数据全排序中。在MapReduce 环境中，容易想到的全排序方案是归并排序，即在 Map 阶段，每个 Map Task进行局部排序；在 Reduce 阶段，启动一个 Reduce Task 进行全局排序。由于作业只能有一个 Reduce Task，因而 Reduce 阶段会成为作业的瓶颈。为了提高全局排序的性能和扩展性，MapReduce 提供了 TotalOrderPartitioner。它能够按照大小将数据分成若干个区间（分片），并保证后一个区间的所有数据均大于前一个区间数据，这使得全排序的步骤如下：
步骤1：数据采样。在 Client 端通过采样获取分片的分割点。Hadoop 自带了几个采样算法，如 IntercalSampler、 RandomSampler、 SplitSampler 等（具体见org.apache.hadoop.mapred.lib 包中的 InputSampler 类）。 下面举例说明。
采样数据为： b， abc， abd， bcd， abcd， efg， hii， afd， rrr， mnk
经排序后得到： abc， abcd， abd， afd， b， bcd， efg， hii， mnk， rrr
如果 Reduce Task 个数为 4，则采样数据的四等分点为 abd、 bcd、 mnk，将这 3 个字符串作为分割点。
步骤2：Map 阶段。本阶段涉及两个组件，分别是 Mapper 和 Partitioner。其中，Mapper 可采用 IdentityMapper，直接将输入数据输出，但 Partitioner 必须选用TotalOrderPartitioner，它将步骤 1 中获取的分割点保存到 trie 树中以便快速定位任意一个记录所在的区间，这样，每个 Map Task 产生 R（Reduce Task 个数）个区间，且区间之间有序。TotalOrderPartitioner 通过 trie 树查找每条记录所对应的 Reduce Task 编号。 如图所示， 我们将分割点 保存在深度为 2 的 trie 树中， 假设输入数据中 有两个字符串“ abg”和“ mnz”， 则字符串“ abg” 对应 partition1， 即第 2 个 Reduce Task， 字符串“ mnz” 对应partition3， 即第 4 个 Reduce Task。

步骤 3：Reduce 阶段。每个 Reducer 对分配到的区间数据进行局部排序，最终得到全排序数据。从以上步骤可以看出，基于 TotalOrderPartitioner 全排序的效率跟 key 分布规律和采样算法有直接关系；key 值分布越均匀且采样越具有代表性，则 Reduce Task 负载越均衡，全排序效率越高。TotalOrderPartitioner 有两个典型的应用实例： TeraSort 和 HBase 批量数据导入。 其中，TeraSort 是 Hadoop 自 带的一个应用程序实例。 它曾在 TB 级数据排序基准评估中 赢得第一名，而 TotalOrderPartitioner正是从该实例中提炼出来的。HBase 是一个构建在 Hadoop之上的 NoSQL 数据仓库。它以 Region为单位划分数据，Region 内部数据有序（按 key 排序），Region 之间也有序。很明显，一个 MapReduce 全排序作业的 R 个输出文件正好可对应 HBase 的 R 个 Region。

新版 API 的 Partitioner 解析

新版 API 中的Partitioner类图如图所示。它不再实现JobConfigurable 接口。当用户需要让 Partitioner通过某个JobConf 对象初始化时，可自行实现Configurable 接口，如：

public class TotalOrderPartitioner<K, V> extends Partitioner<K,V> implements Configurable

Partition所处的位置

Partition主要作用就是将map的结果发送到相应的reduce。这就对partition有两个要求：

1）均衡负载，尽量的将工作均匀的分配给不同的reduce。

2）效率，分配速度一定要快。

Mapreduce提供的Partitioner

patition类结构

1. Partitioner<k,v>是partitioner的基类，如果需要定制partitioner也需要继承该类。源代码如下：

package org.apache.hadoop.mapred;
/** 
 * Partitions the key space.
 * 
 * <p><code>Partitioner</code> controls the partitioning of the keys of the 
 * intermediate map-outputs. The key (or a subset of the key) is used to derive
 * the partition, typically by a hash function. The total number of partitions
 * is the same as the number of reduce tasks for the job. Hence this controls
 * which of the <code>m</code> reduce tasks the intermediate key (and hence the 
 * record) is sent for reduction.</p>
 * 
 * @see Reducer
 * @deprecated Use {@link org.apache.hadoop.mapreduce.Partitioner} instead.
 */
@Deprecated
public interface Partitioner<K2, V2> extends JobConfigurable {
  
  /** 
   * Get the paritition number for a given key (hence record) given the total 
   * number of partitions i.e. number of reduce-tasks for the job.
   *   
   * <p>Typically a hash function on a all or a subset of the key.</p>
   *
   * @param key the key to be paritioned.
   * @param value the entry value.
   * @param numPartitions the total number of partitions.
   * @return the partition number for the <code>key</code>.
   */
  int getPartition(K2 key, V2 value, int numPartitions);
}

2. HashPartitioner<k,v>是mapreduce的默认partitioner。源代码如下：

package org.apache.hadoop.mapreduce.lib.partition;
import org.apache.hadoop.mapreduce.Partitioner;
/** Partition keys by their {@link Object#hashCode()}. */
public class HashPartitioner<K, V> extends Partitioner<K, V> {
  /** Use {@link Object#hashCode()} to partition. */
  public int getPartition(K key, V value,
                          int numReduceTasks) {
    return (key.hashCode() & Integer.MAX_VALUE) % numReduceTasks;
  }
}

3. BinaryPatitioner继承于Partitioner<BinaryComparable ,V>，是Partitioner<k,v>的偏特化子类。该类提供leftOffset和rightOffset，在计算which reducer时仅对键值K的[rightOffset，leftOffset]这个区间取hash。

reducer=(hash & Integer.MAX_VALUE) % numReduceTasks

4. KeyFieldBasedPartitioner<k2, v2="">也是基于hash的个partitioner。和BinaryPatitioner不同，它提供了多个区间用于计算hash。当区间数为0时KeyFieldBasedPartitioner退化成HashPartitioner。 源代码如下：

package org.apache.hadoop.mapred.lib;

import java.io.UnsupportedEncodingException;
import java.util.List;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.apache.hadoop.mapred.JobConf;
import org.apache.hadoop.mapred.Partitioner;
import org.apache.hadoop.mapred.lib.KeyFieldHelper.KeyDescription;

 /**   
  *  Defines a way to partition keys based on certain key fields (also see
  *  {@link KeyFieldBasedComparator}.
  *  The key specification supported is of the form -k pos1[,pos2], where,
  *  pos is of the form f[.c][opts], where f is the number
  *  of the key field to use, and c is the number of the first character from
  *  the beginning of the field. Fields and character posns are numbered 
  *  starting with 1; a character position of zero in pos2 indicates the
  *  field's last character. If '.c' is omitted from pos1, it defaults to 1
  *  (the beginning of the field); if omitted from pos2, it defaults to 0 
  *  (the end of the field).
  * 
  */
public class KeyFieldBasedPartitioner<K2, V2> implements Partitioner<K2, V2> {
  private static final Log LOG = LogFactory.getLog(KeyFieldBasedPartitioner.class.getName());
  private int numOfPartitionFields;
  
  private KeyFieldHelper keyFieldHelper = new KeyFieldHelper();

  public void configure(JobConf job) {
    String keyFieldSeparator = job.get("map.output.key.field.separator", "\t");
    keyFieldHelper.setKeyFieldSeparator(keyFieldSeparator);
    if (job.get("num.key.fields.for.partition") != null) {
      LOG.warn("Using deprecated num.key.fields.for.partition. " +
              "Use mapred.text.key.partitioner.options instead");
      this.numOfPartitionFields = job.getInt("num.key.fields.for.partition",0);
      keyFieldHelper.setKeyFieldSpec(1,numOfPartitionFields);
    } else {
      String option = job.getKeyFieldPartitionerOption();
      keyFieldHelper.parseOption(option);
    }
  }

  public int getPartition(K2 key, V2 value,
      int numReduceTasks) {
    byte[] keyBytes;

    List <KeyDescription> allKeySpecs = keyFieldHelper.keySpecs();
    if (allKeySpecs.size() == 0) {
      return getPartition(key.toString().hashCode(), numReduceTasks);
    }

    try {
      keyBytes = key.toString().getBytes("UTF-8");
    } catch (UnsupportedEncodingException e) {
      throw new RuntimeException("The current system does not " +
          "support UTF-8 encoding!", e);
    }
    // return 0 if the key is empty
    if (keyBytes.length == 0) {
      return 0;
    }
    
    int []lengthIndicesFirst = keyFieldHelper.getWordLengths(keyBytes, 0, 
        keyBytes.length);
    int currentHash = 0;
    for (KeyDescription keySpec : allKeySpecs) {
      int startChar = keyFieldHelper.getStartOffset(keyBytes, 0, keyBytes.length, 
          lengthIndicesFirst, keySpec);
       // no key found! continue
      if (startChar < 0) {
        continue;
      }
      int endChar = keyFieldHelper.getEndOffset(keyBytes, 0, keyBytes.length, 
          lengthIndicesFirst, keySpec);
      currentHash = hashCode(keyBytes, startChar, endChar, 
          currentHash);
    }
    return getPartition(currentHash, numReduceTasks);
  }
  
  protected int hashCode(byte[] b, int start, int end, int currentHash) {
    for (int i = start; i <= end; i++) {
      currentHash = 31*currentHash + b[i];
    }
    return currentHash;
  }

  protected int getPartition(int hash, int numReduceTasks) {
    return (hash & Integer.MAX_VALUE) % numReduceTasks;
  }
}

5. TotalOrderPartitioner这个类可以实现输出的全排序。不同于以上3个partitioner，这个类并不是基于hash的。下面详细的介绍TotalOrderPartitioner

TotalOrderPartitioner 类

每一个reducer的输出在默认的情况下都是有顺序的，但是reducer之间在输入是无序的情况下也是无序的。如果要实现输出是全排序的那就会用到TotalOrderPartitioner。

要使用TotalOrderPartitioner，得给TotalOrderPartitioner提供一个partition file。这个文件要求Key（这些key就是所谓的划分）的数量和当前reducer的数量-1相同并且是从小到大排列。对于为什么要用到这样一个文件，以及这个文件的具体细节待会还会提到。

TotalOrderPartitioner对不同Key的数据类型提供了两种方案：

1） 对于非BinaryComparable 类型的Key，TotalOrderPartitioner采用二分发查找当前的K所在的index。

例如：reducer的数量为5，partition file 提供的4个划分为【2，4，6，8】。如果当前的一个key/value 是<4,”good”>，利用二分法查找到index=1，index+1=2那么这个key/value 将会发送到第二个reducer。如果一个key/value为<4.5, “good”>。那么二分法查找将返回-3，同样对-3加1然后取反就是这个key/value将要去的reducer。

对于一些数值型的数据来说，利用二分法查找复杂度是O(log(reducer count))，速度比较快。

2） 对于BinaryComparable类型的Key（也可以直接理解为字符串）。字符串按照字典顺序也是可以进行排序的。

这样的话也可以给定一些划分，让不同的字符串key分配到不同的reducer里。这里的处理和数值类型的比较相近。

例如：reducer的数量为5，partition file 提供了4个划分为【“abc”, “bce”, “eaa”, ”fhc”】那么“ab”这个字符串将会被分配到第一个reducer里，因为它小于第一个划分“abc”。

但是不同于数值型的数据，字符串的查找和比较不能按照数值型数据的比较方法。mapreducer采用的Tire tree（关于Tire tree可以参考《字典树(Trie Tree)》）的字符串查找方法。查找的时间复杂度o(m)，m为树的深度，空间复杂度o(255^m-1)。是一个典型的空间换时间的案例。

Tire tree的构建

假设树的最大深度为3，划分为【aaad ，aaaf， aaaeh，abbx】

Mapreduce里的Tire tree主要有两种节点组成：

1） Innertirenode
Innertirenode在mapreduce中是包含了255个字符的一个比较长的串。上图中的例子只包含了26个英文字母。
2） 叶子节点{unslipttirenode, singesplittirenode, leaftirenode}
Unslipttirenode 是不包含划分的叶子节点。
Singlesplittirenode 是只包含了一个划分点的叶子节点。
Leafnode是包含了多个划分点的叶子节点。（这种情况比较少见，达到树的最大深度才出现这种情况。在实际操作过程中比较少见）

Tire tree的搜索过程

接上面的例子：
1）假如当前 key value pair <aad, 10="">这时会找到图中的leafnode，在leafnode内部使用二分法继续查找找到返回 aad在划分数组中的索引。找不到会返回一个和它最接近的划分的索引。
2）假如找到singlenode，如果和singlenode的划分相同或小返回他的索引，比singlenode的划分大则返回索引+1。
3）假如找到nosplitnode则返回前面的索引。如<zaa, 20="">将会返回abbx的在划分数组中的索引。

TotalOrderPartitioner的疑问

上面介绍了partitioner有两个要求，一个是速度，另外一个是均衡负载。使用tire tree提高了搜素的速度，但是我们怎么才能找到这样的partition file 呢？让所有的划分刚好就能实现均衡负载。

InputSampler
输入采样类，可以对输入目录下的数据进行采样。提供了3种采样方法。

采样类结构图

采样方式对比表:

类名称	采样方式	构造方法	效率	特点
SplitSampler<K,V>	对前n个记录进行采样	采样总数，划分数	最高
RandomSampler<K,V>	遍历所有数据，随机采样	采样频率，采样总数，划分数	最低
IntervalSampler<K,V>	固定间隔采样	采样频率，划分数	中	对有序的数据十分适用

writePartitionFile这个方法很关键，这个方法就是根据采样类提供的样本，首先进行排序，然后选定（随机的方法）和reducer数目-1的样本写入到partition file。这样经过采样的数据生成的划分，在每个划分区间里的key/value就近似相同了，这样就能完成均衡负载的作用。

SplitSampler类的源代码如下：

  /**
   * Samples the first n records from s splits.
   * Inexpensive way to sample random data.
   */
  public static class SplitSampler<K,V> implements Sampler<K,V> {
    private final int numSamples;
    private 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, JobConf job) throws IOException {
      InputSplit[] splits = inf.getSplits(job, job.getNumMapTasks());
      ArrayList<K> samples = new ArrayList<K>(numSamples);
      int splitsToSample = Math.min(maxSplitsSampled, splits.length);
      int splitStep = splits.length / splitsToSample;
      int samplesPerSplit = numSamples / splitsToSample;
      long records = 0;
      for (int i = 0; i < splitsToSample; ++i) {
        RecordReader<K,V> reader = inf.getRecordReader(splits[i * splitStep],
            job, Reporter.NULL);
        K key = reader.createKey();
        V value = reader.createValue();
        while (reader.next(key, value)) {
          samples.add(key);
          key = reader.createKey();
          ++records;
          if ((i+1) * samplesPerSplit <= records) {
            break;
          }
        }
        reader.close();
      }
      return (K[])samples.toArray();
    }
  }

RandomSampler类的源代码如下：

  /**
   * 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> {
    private double freq;
    private final int numSamples;
    private 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, JobConf job) throws IOException {
      InputSplit[] splits = inf.getSplits(job, job.getNumMapTasks());
      ArrayList<K> samples = new ArrayList<K>(numSamples);
      int splitsToSample = Math.min(maxSplitsSampled, splits.length);

      Random r = new Random();
      long seed = r.nextLong();
      r.setSeed(seed);
      LOG.debug("seed: " + seed);
      // shuffle splits
      for (int i = 0; i < splits.length; ++i) {
        InputSplit tmp = splits[i];
        int j = r.nextInt(splits.length);
        splits[i] = splits[j];
        splits[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.length && samples.size() < numSamples); ++i) {
        RecordReader<K,V> reader = inf.getRecordReader(splits[i], job,
            Reporter.NULL);
        K key = reader.createKey();
        V value = reader.createValue();
        while (reader.next(key, value)) {
          if (r.nextDouble() <= freq) {
            if (samples.size() < numSamples) {
              samples.add(key);
            } 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, key);
              }
              freq *= (numSamples - 1) / (double) numSamples;
            }
            key = reader.createKey();
          }
        }
        reader.close();
      }
      return (K[])samples.toArray();
    }
  }

IntervalSampler类的源代码为：

  /**
   * Sample from s splits at regular intervals.
   * Useful for sorted data.
   */
  public static class IntervalSampler<K,V> implements Sampler<K,V> {
    private final double freq;
    private 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, JobConf job) throws IOException {
      InputSplit[] splits = inf.getSplits(job, job.getNumMapTasks());
      ArrayList<K> samples = new ArrayList<K>();
      int splitsToSample = Math.min(maxSplitsSampled, splits.length);
      int splitStep = splits.length / splitsToSample;
      long records = 0;
      long kept = 0;
      for (int i = 0; i < splitsToSample; ++i) {
        RecordReader<K,V> reader = inf.getRecordReader(splits[i * splitStep],
            job, Reporter.NULL);
        K key = reader.createKey();
        V value = reader.createValue();
        while (reader.next(key, value)) {
          ++records;
          if ((double) kept / records < freq) {
            ++kept;
            samples.add(key);
            key = reader.createKey();
          }
        }
        reader.close();
      }
      return (K[])samples.toArray();
    }
  }

InputSampler类完整源代码如下：

package org.apache.hadoop.mapred.lib;

import java.io.IOException;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Random;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

import org.apache.hadoop.conf.Configuration;
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.WritableComparable;
import org.apache.hadoop.mapred.FileInputFormat;
import org.apache.hadoop.mapred.InputFormat;
import org.apache.hadoop.mapred.InputSplit;
import org.apache.hadoop.mapred.JobConf;
import org.apache.hadoop.mapred.RecordReader;
import org.apache.hadoop.mapred.Reporter;
import org.apache.hadoop.util.Tool;
import org.apache.hadoop.util.ToolRunner;

/**
 * Utility for collecting samples and writing a partition file for
 * {@link org.apache.hadoop.mapred.lib.TotalOrderPartitioner}.
 */
public class InputSampler<K,V> implements Tool {

  private static final Log LOG = LogFactory.getLog(InputSampler.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;
  }

  private JobConf conf;

  public InputSampler(JobConf conf) {
    this.conf = conf;
  }

  public Configuration getConf() {
    return conf;
  }

  public void setConf(Configuration conf) {
    if (!(conf instanceof JobConf)) {
      this.conf = new JobConf(conf);
    } else {
      this.conf = (JobConf) conf;
    }
  }

  /**
   * Interface to sample using an {@link org.apache.hadoop.mapred.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, JobConf job) throws IOException;
  }

  /**
   * Samples the first n records from s splits.
   * Inexpensive way to sample random data.
   */
  public static class SplitSampler<K,V> implements Sampler<K,V> {

    private final int numSamples;
    private 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, JobConf job) throws IOException {
      InputSplit[] splits = inf.getSplits(job, job.getNumMapTasks());
      ArrayList<K> samples = new ArrayList<K>(numSamples);
      int splitsToSample = Math.min(maxSplitsSampled, splits.length);
      int splitStep = splits.length / splitsToSample;
      int samplesPerSplit = numSamples / splitsToSample;
      long records = 0;
      for (int i = 0; i < splitsToSample; ++i) {
        RecordReader<K,V> reader = inf.getRecordReader(splits[i * splitStep],
            job, Reporter.NULL);
        K key = reader.createKey();
        V value = reader.createValue();
        while (reader.next(key, value)) {
          samples.add(key);
          key = reader.createKey();
          ++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> {
    private double freq;
    private final int numSamples;
    private 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, JobConf job) throws IOException {
      InputSplit[] splits = inf.getSplits(job, job.getNumMapTasks());
      ArrayList<K> samples = new ArrayList<K>(numSamples);
      int splitsToSample = Math.min(maxSplitsSampled, splits.length);

      Random r = new Random();
      long seed = r.nextLong();
      r.setSeed(seed);
      LOG.debug("seed: " + seed);
      // shuffle splits
      for (int i = 0; i < splits.length; ++i) {
        InputSplit tmp = splits[i];
        int j = r.nextInt(splits.length);
        splits[i] = splits[j];
        splits[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.length && samples.size() < numSamples); ++i) {
        RecordReader<K,V> reader = inf.getRecordReader(splits[i], job,
            Reporter.NULL);
        K key = reader.createKey();
        V value = reader.createValue();
        while (reader.next(key, value)) {
          if (r.nextDouble() <= freq) {
            if (samples.size() < numSamples) {
              samples.add(key);
            } 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, key);
              }
              freq *= (numSamples - 1) / (double) numSamples;
            }
            key = reader.createKey();
          }
        }
        reader.close();
      }
      return (K[])samples.toArray();
    }
  }

  /**
   * Sample from s splits at regular intervals.
   * Useful for sorted data.
   */
  public static class IntervalSampler<K,V> implements Sampler<K,V> {
    private final double freq;
    private 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, JobConf job) throws IOException {
      InputSplit[] splits = inf.getSplits(job, job.getNumMapTasks());
      ArrayList<K> samples = new ArrayList<K>();
      int splitsToSample = Math.min(maxSplitsSampled, splits.length);
      int splitStep = splits.length / splitsToSample;
      long records = 0;
      long kept = 0;
      for (int i = 0; i < splitsToSample; ++i) {
        RecordReader<K,V> reader = inf.getRecordReader(splits[i * splitStep],
            job, Reporter.NULL);
        K key = reader.createKey();
        V value = reader.createValue();
        while (reader.next(key, value)) {
          ++records;
          if ((double) kept / records < freq) {
            ++kept;
            samples.add(key);
            key = reader.createKey();
          }
        }
        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
     org.apache.hadoop.mapred.lib.TotalOrderPartitioner#getPartitionFile}.
   */
  @SuppressWarnings("unchecked") // getInputFormat, getOutputKeyComparator
  public static <K,V> void writePartitionFile(JobConf job,
      Sampler<K,V> sampler) throws IOException {
    final InputFormat<K,V> inf = (InputFormat<K,V>) job.getInputFormat();
    int numPartitions = job.getNumReduceTasks();
    K[] samples = sampler.getSample(inf, job);
    LOG.info("Using " + samples.length + " samples");
    RawComparator<K> comparator =
      (RawComparator<K>) job.getOutputKeyComparator();
    Arrays.sort(samples, comparator);
    Path dst = new Path(TotalOrderPartitioner.getPartitionFile(job));
    FileSystem fs = dst.getFileSystem(job);
    if (fs.exists(dst)) {
      fs.delete(dst, false);
    }
    SequenceFile.Writer writer = SequenceFile.createWriter(fs, job, 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 InputSampler from the command line.
   * Configures a JobConf instance and calls {@link #writePartitionFile}.
   */
  public int run(String[] args) throws Exception {
    JobConf job = (JobConf) 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.setInputFormat(
              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(job, outf);
    for (String s : otherArgs) {
      FileInputFormat.addInputPath(job, new Path(s));
    }
    InputSampler.<K,V>writePartitionFile(job, sampler);

    return 0;
  }

  public static void main(String[] args) throws Exception {
    JobConf job = new JobConf(InputSampler.class);
    InputSampler<?,?> sampler = new InputSampler(job);
    int res = ToolRunner.run(sampler, args);
    System.exit(res);
  }
}

TotalOrderPartitioner实例

public class SortByTemperatureUsingTotalOrderPartitioner extends Configured
        implements Tool
{
    @Override
    public int run(String[] args) throws Exception
    {
        JobConf conf = JobBuilder.parseInputAndOutput(this, getConf(), args);
        if (conf == null) {
            return -1;
        }
        conf.setInputFormat(SequenceFileInputFormat.class);
        conf.setOutputKeyClass(IntWritable.class);
        conf.setOutputFormat(SequenceFileOutputFormat.class);
        SequenceFileOutputFormat.setCompressOutput(conf, true);
        SequenceFileOutputFormat
                .setOutputCompressorClass(conf, GzipCodec.class);
        SequenceFileOutputFormat.setOutputCompressionType(conf,
                CompressionType.BLOCK);
        conf.setPartitionerClass(TotalOrderPartitioner.class);
        InputSampler.Sampler<IntWritable, Text> sampler = new InputSampler.RandomSampler<IntWritable, Text>(
                0.1, 10000, 10);
        Path input = FileInputFormat.getInputPaths(conf)[0];
        input = input.makeQualified(input.getFileSystem(conf));
        Path partitionFile = new Path(input, "_partitions");
        TotalOrderPartitioner.setPartitionFile(conf, partitionFile);
        InputSampler.writePartitionFile(conf, sampler);
        // Add to DistributedCache
        URI partitionUri = new URI(partitionFile.toString() + "#_partitions");
        DistributedCache.addCacheFile(partitionUri, conf);
        DistributedCache.createSymlink(conf);
        JobClient.runJob(conf);
        return 0;
    }
 
    public static void main(String[] args) throws Exception {
        int exitCode = ToolRunner.run(
                new SortByTemperatureUsingTotalOrderPartitioner(), args);
        System.exit(exitCode);
    }
}

参考资料

1.《Hadoop技术内幕 深入理解MapReduce架构设计与实现原理》

2.http://www.cnblogs.com/xwdreamer/archive/2011/10/27/2296943.html

3.http://blog.oddfoo.net/2011/04/17/mapreduce-partition%E5%88%86%E6%9E%90-2/