一、Combiner作用
1、combiner最基本是实现本地key的聚合,对map输出的key排序,value进行迭代。如下所示:
map: (K1, V1) → list(K2,
V2)
combine: (K2, list(V2)) →
list(K2, V2)
reduce: (K2,
list(V2)) → list(K3, V3)
2、combiner还具有类似本地的reduce功能.
例如hadoop自带的wordcount的例子和找出value的最大值的程序,combiner和reduce完全一致。如下所示:
map: (K1, V1) → list(K2, V2)
combine: (K2, list(V2)) → list(K3,
V3)
reduce: (K3, list(V3)) →
list(K4, V4)
3、如果不用combiner,那么,所有的结果都是reduce完成,效率会相对低下。使用combiner,先完成的map会在本地聚合,提升速度。
4、对于hadoop自带的wordcount的例子,value就是一个叠加的数字,所以map一结束就可以进行reduce的value叠加,而不必要等到所有的map结束再去进行reduce的value叠加。
二、总结
1、combiner使用的合适,可以在满足业务的情况下提升job的速度,如果不合适,则将导致输出的结果不正确。
本程序不能是用combiner,不然出错。
import java.io.IOException;import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.Mapper;
import org.apache.hadoop.mapreduce.Reducer;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
import org.apache.hadoop.util.GenericOptionsParser;public class Sort {
public static class Map extends Mapper<Object,Text,IntWritable,IntWritable>{
private static IntWritable num = new IntWritable();
public void map(Object key,Text value,Context context) throws IOException, InterruptedException{
String line = value.toString();
num.set(Integer.parseInt(line));
context.write(num, new IntWritable(1));
}
}public static class Reduce extends Reducer<IntWritable,IntWritable,IntWritable,IntWritable>{
private static IntWritable count = new IntWritable(0);
public void reduce(IntWritable key,Iterable<IntWritable> value,Context context) throws IOException, InterruptedException{
for(IntWritable val : value){
count = new IntWritable(count.get()+1);
context.write(count,key);
}
}
}public static void main(String[] args) throws IOException, ClassNotFoundException, InterruptedException{
Configuration conf = new Configuration();
conf.addResource(new Path("/usr/hadoop-1.0.3/conf/core-site.xml"));String[] arg = new GenericOptionsParser(conf,args).getRemainingArgs();
Job job = new Job(conf,"Sort");
job.setJarByClass(Sort.class);
job.setMapperClass(Map.class);
job.setReducerClass(Reduce.class);job.setOutputKeyClass(IntWritable.class);
job.setOutputValueClass(IntWritable.class);FileInputFormat.addInputPath(job, new Path(arg[0]));
FileOutputFormat.setOutputPath(job, new Path(arg[1]));System.exit(job.waitForCompletion(true) ? 0 : 1);
}
}
File1
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1 2 3 4 5 6 7 |
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File2
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1 2 3 4 |
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File3
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1 2 3 |
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结果:
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 |
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