mapreduce中的全局文件使用方法：以k-means为例

【转自：http://www.linuxidc.com/Linux/2012-10/71540p3.htm】　　

以前有做过在Hadoop编 写程序时使用全局变量的想法，但是最后却没有实现，上网查才看到说Hadoop不支持全局变量。但是有时候编程的时候又会用到，比如编写k-means算 法的时候，如果可以有个全局变量存储中心点该多好呀。其实在hadoop中确实是有相关的实现的，比如可以在mapper中的setup函数中读取一个小 文件，然后从这个文件中取出全局变量的值。

那具体如何实现呢？首先提出一个问题，然后利用这种思想去解决会比较好。首先说下我要实现的问题：我现在有输入数据如下：

0.0 0.2 0.4
0.3 0.2 0.4
0.4 0.2 0.4
0.5 0.2 0.4
5.0 5.2 5.4
6.0 5.2 6.4
4.0 5.2 4.4
10.3    10.4    10.5
10.3    10.4    10.5
10.3    10.4    10.5

而且还有一个小数据文件（中心点）如下：

0   0   0
5   5   5
10  10  10

我想做的事情就是把输入数据按照中心点求平均值，即首先我把输入数据分类，比如倒数三行应该都是属于（10，10，10）这个中心点的，那么我的 map就把倒数三行的key都赋值为2，然后value值还是保持这三行不变。在reduce阶段，我求出相同key的sum值，同时求出一共的行数 count，最后我用sum/count得到我想要的按照中心点求出的平均值了。

下面贴代码：

KmeansDriver:

package org.fansy.date927;

import java.io.IOException;

//import org.apache.commons.logging.Log;
//import org.apache.commons.logging.LogFactory;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.filecache.DistributedCache;
//import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.NullWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
import org.apache.hadoop.util.GenericOptionsParser;

public class KmeansDriver {
    /**
     *   k-means algorithm program
     */
    public static void main(String[] args) throws IOException, ClassNotFoundException, InterruptedException {
        // TODO Auto-generated method stub
        Configuration conf=new Configuration();
        // set the centers data file
        <SPAN style="COLOR: #ff0000">Path centersFile=new Path("hdfs://fansyPC:9000/user/fansy/input/centers");
        DistributedCache.addCacheFile(centersFile.toUri(), conf);</SPAN>
        String[] otherArgs = new GenericOptionsParser(conf, args).getRemainingArgs();
        if (otherArgs.length != 2) {
          System.err.println("Usage: KmeansDriver <in> <out>");
          System.exit(2);
        }
        Job job = new Job(conf, "kmeans job");
        job.setJarByClass(KmeansDriver.class);
        job.setMapperClass(KmeansM.class);
        job.setMapOutputKeyClass(IntWritable.class);
        job.setMapOutputValueClass(DataPro.class);
        job.setNumReduceTasks(2);
                //    job.setCombinerClass(KmeansC.class);
        job.setReducerClass(KmeansR.class);
        job.setOutputKeyClass(NullWritable.class);
        job.setOutputValueClass(Text.class);

        FileInputFormat.addInputPath(job, new Path(otherArgs[0]));
        FileOutputFormat.setOutputPath(job, new Path(otherArgs[1]));

        if(!job.waitForCompletion(true)){
            System.exit(1); // run error then exit
        }
    }

}

Mapper:
 
    package org.fansy.date927;

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.apache.Hadoop.filecache.DistributedCache;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Mapper;

public class KmeansM extends Mapper<LongWritable,Text,IntWritable,DataPro>{
    private static Log log=LogFactory.getLog(KmeansM.class);

    private double[][] centers;
    private int dimention_m;  //  this is the k
    private int dimention_n;   //  this is the features


    static enum Counter{Fansy_Miss_Records};
    @Override
    public void setup(Context context) throws IOException,InterruptedException{
        <SPAN style="COLOR: #ff0000">Path[] caches=DistributedCache.getLocalCacheFiles(context.getConfiguration());</SPAN>
        if(caches==null||caches.length<=0){
            log.error("center file does not exist");
            System.exit(1);
        }
        @SuppressWarnings("resource")
        BufferedReader br=new BufferedReader(new FileReader(caches[0].toString()));
        String line;
        List<ArrayList<Double>> temp_centers=new ArrayList<ArrayList<Double>>();
        ArrayList<Double> center=null;
        //  get the file data
        <SPAN style="COLOR: #ff0000">while((line=br.readLine())!=null){
            center=new ArrayList<Double>();
            String[] str=line.split("\t");
            for(int i=0;i<str.length;i++){
                center.add(Double.parseDouble(str[i]));
            }
            temp_centers.add(center);
        }</SPAN>
        //  fill the centers
        @SuppressWarnings("unchecked")
        ArrayList<Double>[] newcenters=temp_centers.toArray(new ArrayList[]{});
         dimention_m=temp_centers.size();
         dimention_n=newcenters[0].size();
        centers=new double[dimention_m][dimention_n];
        for(int i=0;i<dimention_m;i++){
            Double[] temp_double=newcenters[i].toArray(new Double[]{});
            for(int j=0;j<dimention_n;j++){
                centers[i][j]=temp_double[j];
        //      System.out.print(temp_double[j]+",");
            }
    //      System.out.println();
        }
    }

    public void map(LongWritable key,Text value,Context context)throws IOException,InterruptedException{
        String[] values=value.toString().split("\t");

        if(values.length!=dimention_n){
            context.getCounter(Counter.Fansy_Miss_Records).increment(1);
            return;
        }
        double[] temp_double=new double[values.length];
        for(int i=0;i<values.length;i++){
            temp_double[i]=Double.parseDouble(values[i]);
        }
        //  set the index
        double distance=Double.MAX_VALUE;
        double temp_distance=0.0;
        int index=0;
        for(int i=0;i<dimention_m;i++){
            double[] temp_center=centers[i];
            temp_distance=getEnumDistance(temp_double,temp_center);
            if(temp_distance<distance){
                 index=i;
                distance=temp_distance;
            }
        }
        DataPro newvalue=new DataPro();
        newvalue.set(value, new IntWritable(1));
        context.write(new IntWritable(index), newvalue);

    }
    public static double getEnumDistance(double[] source,double[] other){  //  get the distance
        double distance=0.0;
        if(source.length!=other.length){
            return Double.MAX_VALUE;
        }
        for(int i=0;i<source.length;i++){
            distance+=(source[i]-other[i])*(source[i]-other[i]);
        }
        distance=Math.sqrt(distance);
        return distance;
    }
} Reducer: 
    package org.fansy.date927;

import java.io.IOException;
import org.apache.Hadoop.io.IntWritable;
import org.apache.hadoop.io.NullWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Reducer;

public class KmeansR extends Reducer<IntWritable,DataPro,NullWritable,Text> {

    public void reduce(IntWritable key,Iterable<DataPro> values,Context context)throws InterruptedException, IOException{
        // get dimension first
        int dimension=0;
        for(DataPro val:values){
            String[] datastr=val.getCenter().toString().split("\t");
            dimension=datastr.length;
            break;
        }
        double[] sum=new double[dimension];
        int sumCount=0;
        for(DataPro val:values){
            String[] datastr=val.getCenter().toString().split("\t");
            sumCount+=val.getCount().get();
            for(int i=0;i<dimension;i++){
                sum[i]+=Double.parseDouble(datastr[i]);
            }
        }
        //  calculate the new centers
//      double[] newcenter=new double[dimension];
        StringBuffer sb=new StringBuffer();
        for(int i=0;i<dimension;i++){
            sb.append(sum[i]/sumCount+"\t");
        }
        context.write(null, new Text(sb.toString()));
    }
} DataPro: 
    package org.fansy.date927;

import java.io.DataInput;
import java.io.DataOutput;
import java.io.IOException;

import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.io.WritableComparable;

public class DataPro implements WritableComparable<DataPro>{

    private Text center;
    private IntWritable count;

    public DataPro(){
        set(new Text(),new IntWritable());
    }
    public void set(Text text, IntWritable intWritable) {
        // TODO Auto-generated method stub
        this.center=text;
        this.count=intWritable;
    }

    public Text getCenter(){
        return center;
    }
    public IntWritable getCount(){
        return count;
    }


    @Override
    public void readFields(DataInput arg0) throws IOException {
        // TODO Auto-generated method stub
        center.readFields(arg0);
        count.readFields(arg0);
    }

    @Override
    public void write(DataOutput arg0) throws IOException {
        // TODO Auto-generated method stub
        center.write(arg0);
        count.write(arg0);
    }

    @Override
    public int compareTo(DataPro o) {
        // TODO Auto-generated method stub
        int cmp=count.compareTo(o.count);
        if(cmp!=0){
            return cmp;
        }
        return center.compareTo(o.center);
    }

} 这里自定义了一个DataPro数据类型，主要是为了为以后编写真正的k-means算法时使用combiner做准备，具体思想可以参考上篇combine操作。 输出文件如下： 
     
     0.39999999999999997 0.20000000000000004 0.4000000000000001     
     5.0 5.2 5.4    
     10.3    10.4    10.5    
     这篇文章参考了 http://www.linuxidc.com/Linux/2012-10/71538.htm 部分实现，在那篇文章中的k-means思想的主要思想是：使用map读入centers文件值，然后把数据文件data作为一个全局量，然后reduce在进行求中心点的操作。（或许我理解错了也说不定） 做完这一步后，如果要编写K-means算法就可以说是已经实现了大半了，剩下的就是设置下输入和输出路径，然后进行迭代了。 【转自：http://www.linuxidc.com/Linux/2012-10/71540p3.htm】
0.39999999999999997 0.20000000000000004 0.4000000000000001
5.0 5.2 5.4
10.3    10.4    10.5