Mapreduce Combiner

Question

I have a simple mapreduce code with mapper, reducer and combiner. The output from mapper is passed to combiner. But to the reducer, instead of output from combiner,output from mapper is passed.

Kindly help

Code:

package Combiner;
import java.io.IOException;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.DoubleWritable;
import org.apache.hadoop.io.LongWritable;
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.Mapper.Context;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
import org.apache.hadoop.util.GenericOptionsParser;

public class AverageSalary
{
public static class Map extends  Mapper<LongWritable, Text, Text, DoubleWritable> 
{
    public void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException 
    {    
        String[] empDetails= value.toString().split(",");
        Text unit_key = new Text(empDetails[1]);      
        DoubleWritable salary_value = new DoubleWritable(Double.parseDouble(empDetails[2]));
        context.write(unit_key,salary_value);    

    }  
}
public static class Combiner extends Reducer<Text,DoubleWritable, Text,Text> 
{
    public void reduce(final Text key, final Iterable<DoubleWritable> values, final Context context)
    {
        String val;
        double sum=0;
        int len=0;
        while (values.iterator().hasNext())
        {
            sum+=values.iterator().next().get();
            len++;
        }
        val=String.valueOf(sum)+":"+String.valueOf(len);
        try {
            context.write(key,new Text(val));
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}
public static class Reduce extends Reducer<Text,Text, Text,Text> 
{
    public void reduce (final Text key, final Text values, final Context context)
    {
        //String[] sumDetails=values.toString().split(":");
        //double average;
        //average=Double.parseDouble(sumDetails[0]);
        try {
            context.write(key,values);
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}
public static void main(String args[])
{
    Configuration conf = new Configuration();
    try
    {
     String[] otherArgs = new GenericOptionsParser(conf, args).getRemainingArgs();    
     if (otherArgs.length != 2) {      
         System.err.println("Usage: Main <in> <out>");      
         System.exit(-1);    }    
     Job job = new Job(conf, "Average salary");    
     //job.setInputFormatClass(KeyValueTextInputFormat.class);    
     FileInputFormat.addInputPath(job, new Path(otherArgs[0]));    
     FileOutputFormat.setOutputPath(job, new Path(otherArgs[1]));    
     job.setJarByClass(AverageSalary.class);    
     job.setMapperClass(Map.class);    
     job.setCombinerClass(Combiner.class);
     job.setReducerClass(Reduce.class);    
     job.setOutputKeyClass(Text.class);    
     job.setOutputValueClass(Text.class);    

        System.exit(job.waitForCompletion(true) ? 0 : -1);
    } catch (ClassNotFoundException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } catch (InterruptedException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
}

}

Answer 1

The #1 rule of Combiners are: do not assume that the combiner will run . Treat the combiner only as an optimization .

The Combiner is not guaranteed to run over all of your data. In some cases when the data doesn't need to be spilled to disk, MapReduce will skip using the Combiner entirely. Note also that the Combiner may be ran multiple times over subsets of the data! It'll run once per spill.

In your case, you are making this bad assumption. You should be doing the sum in the Combiner AND the Reducer.

Also, you should follow @user987339's answer as well. The input and output of the combiner needs to be identical (Text,Double -> Text,Double) and it needs to match up with the output of the Mapper and the input of the Reducer.

Answer 2

It seems that you forgot about important property of a combiner:

the input types for the key/value and the output types of the key/value need to be the same.

You can't take in a Text/DoubleWritable and return a Text/Text . I suggest you to use Text Instead DoubleWritable , and do proper parsing inside Combiner .

Answer 3

如果使用了combine函数，那么它与reduce函数的形式相同（并且是Reducer的实现），除了它的输出类型是中间键和值类型（K2和V2），因此它们可以提供reduce函数：map：（K1，V1）→list（K2，V2）组合：（K2，list（V2））→list（K2，V2）reduce：（K2，list（V2））→list（K3，V3）经常combine和reduce功能相同，在这种情况下，K3与K2相同，V3与V2相同。

Answer 4

Combiner will not work always when you run mapreduce .

If there is at least three spill files (output of mapper written to local-disk) the combiner will execute so that the size of file can be reduced so that it can be easily transferred to reduce node.

The number of spills for which a combiner need to run can be set through min.num.spills.for.combine property