MapReduce/Hadoop的TopN解决方案之键唯一的情况

TopN问题：上星期访问次数最多的10个URL是哪些？所有猫中体重最大的10只猫是哪些？

本文使用 MapReduce/Hadoop的TopN解决方案，假设所有输入键都是唯一的。也就是说，对于一个给定的输入集合{}，所有K都是唯一的。

例如对于下面的猫，cat1不会再出现第二次

输入：cat.txt

12,cat1,cat1
13,cat2,cat2
14,cat3,cat3
15,cat4,cat4
10,cat5,cat5
100,cat100,cat100
200,cat200,cat200
300,cat300,cat300
1,cat001,cat001
67,cat67,cat67
22,cat22,cat22
23,cat23,cat23
1000,cat1000,cat1000
2000,cat2000,cat2000

期待输出：

2000	cat2000,cat2000
1000	cat1000,cat1000
300	cat300,cat300
200	cat200,cat200
100	cat100,cat100
67	cat67,cat67
23	cat23,cat23
22	cat22,cat22
15	cat4,cat4
14	cat3,cat3

一、用到的核心数据结构：Java中的SortedMap和TreeMap，其中SortedMap可以实现按key值排序。对于如下测试类

package topN_hadoop1;

import java.util.Map.Entry;
import java.util.SortedMap;
import java.util.TreeMap;

public class Test {

	public static void main(String[] args) {
		 SortedMap top = new TreeMap();
		 top.put(1, "chenjie,1");
		 top.put(10, "zhanghan,10");
		 top.put(3 ,"renbo,3");
		for(Entry< Integer, String> entry : top.entrySet())
		{
			System.out.println(entry);
		}
		System.out.println("------------------------------------------------------");
		System.out.println("firstKey:" + top.firstKey());
		System.out.println("first:" + top.get(top.firstKey()));
		System.out.println("lastKey:" + top.lastKey());
		System.out.println("last:" + top.get(top.lastKey()));
		top.remove(top.firstKey());
		System.out.println("remove first ");
		System.out.println("------------------------------------------------------");
		for(Entry< Integer, String> entry : top.entrySet())
		{
			System.out.println(entry);
		}
		top.remove(top.lastKey());
		System.out.println("remove last ");
		System.out.println("------------------------------------------------------");
		for(Entry< Integer, String> entry : top.entrySet())
		{
			System.out.println(entry);
		}
	}

}

输出为：

1=chenjie,1
3=renbo,3
10=zhanghan,10
------------------------------------------------------
firstKey:1
first:chenjie,1
lastKey:10
last:zhanghan,10
remove first 
------------------------------------------------------
3=renbo,3
10=zhanghan,10
remove last 
------------------------------------------------------
3=renbo,3

二、code

package topN_hadoop1;

import java.io.IOException;
import java.util.SortedMap;
import java.util.TreeMap;

import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.NullWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Mapper;

public class TopNMapper extends
   Mapper {

   private int N = 10; // default
   private SortedMap top = new TreeMap();

   @Override
   public void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException {
	   String[] lines = value.toString().split(",");
      String keyAsString =  value.toString();
      int frequency =  Integer.valueOf(lines[0]);
      String compositeValue = keyAsString + "," + frequency;
      top.put(frequency, compositeValue);
      if (top.size() > N) {
         top.remove(top.firstKey());
      }
   }
   
   @Override
   protected void setup(Context context) throws IOException,
         InterruptedException {
      this.N = context.getConfiguration().getInt("N", 10); // default is top 10
   }

   @Override
   protected void cleanup(Context context) throws IOException,
         InterruptedException {
      for (String str : top.values()) {
         context.write(NullWritable.get(), new Text(str));
      }
   }

}

package topN_hadoop1;

import java.io.IOException;
import java.util.SortedMap;
import java.util.TreeMap;
import java.util.List;
import java.util.ArrayList;

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


public class TopNReducer  extends
   Reducer {

   private int N = 10; // default
   private SortedMap top = new TreeMap();

   @Override
   public void reduce(NullWritable key, Iterable values, Context context) 
      throws IOException, InterruptedException {
      for (Text value : values) { //value:	2000,cat2000,cat2000,2000
         String valueAsString = value.toString().trim();
         System.out.println(value);
         String[] tokens = valueAsString.split(",");
         String url = tokens[1] + "," + tokens[2];//,cat2000,cat2000
         int frequency =  Integer.parseInt(tokens[0]);//2000
         top.put(frequency, url);
         if (top.size() > N) {
            top.remove(top.firstKey());
         }
      }
      
      // emit final top N
        List keys = new ArrayList(top.keySet());
        for(int i=keys.size()-1; i>=0; i--){
         context.write(new IntWritable(keys.get(i)), new Text(top.get(keys.get(i))));
      }
   }
   
   @Override
   protected void setup(Context context) 
      throws IOException, InterruptedException {
      this.N = context.getConfiguration().getInt("N", 10); // default is top 10
   }
}

package topN_hadoop1;

import org.apache.log4j.Logger;
import org.apache.hadoop.util.Tool;
import org.apache.hadoop.util.ToolRunner;
import org.apache.hadoop.conf.Configured;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.NullWritable;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.lib.input.TextInputFormat;
import org.apache.hadoop.mapreduce.lib.output.TextOutputFormat;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;



public class TopNDriver  extends Configured implements Tool {

   private static Logger THE_LOGGER = Logger.getLogger(TopNDriver.class);

   public int run(String[] args) throws Exception {
      Job job = new Job(getConf());
      int N = Integer.parseInt(args[0]); // top N
      job.getConfiguration().setInt("N", N);
      job.setJobName("TopNDriver");

      job.setInputFormatClass(TextInputFormat.class);
      job.setOutputFormatClass(TextOutputFormat.class);

      job.setMapperClass(TopNMapper.class);
      job.setReducerClass(TopNReducer.class);
      job.setNumReduceTasks(1);

      // map()'s output (K,V)
      job.setMapOutputKeyClass(NullWritable.class);   
      job.setMapOutputValueClass(Text.class);   
      
      
      // reduce()'s output (K,V)
      job.setOutputKeyClass(IntWritable.class);
      job.setOutputValueClass(Text.class);

       // args[1] = input directory
       // args[2] = output directory
      FileInputFormat.setInputPaths(job, new Path(args[1]));
      FileOutputFormat.setOutputPath(job, new Path(args[2]));
      

      boolean status = job.waitForCompletion(true);
      THE_LOGGER.info("run(): status="+status);
      return status ? 0 : 1;
   }

   private static final String INPATH = "input/cat.txt";// 输入文件路径
	private static final String OUTPATH = "output/cat_out1";// 输出文件路径
   

   public static void main(String[] args) throws Exception {
	   args = new String[3];
	   args[0] = "10";
	   args[1] = INPATH;
	   args[2] = OUTPATH;
	   
      // Make sure there are exactly 3 parameters
      if (args.length != 3) {
         THE_LOGGER.warn("usage TopNDriver   
");
         System.exit(1);
      }

      THE_LOGGER.info("N="+args[0]);
      THE_LOGGER.info("inputDir="+args[1]);
      THE_LOGGER.info("outputDir="+args[2]);
      int returnStatus = ToolRunner.run(new TopNDriver(), args);
      System.exit(returnStatus);
   }

}

四、扩展

1、Top5怎么办？传入另一个参数

2、不求前10个求后10个怎么办？将

if (top.size() > N) {
            top.remove(top.firstKey());
         }

改成top.lastKey()