Combiner

The combiner would be same as the Reducer in most of the cases and it can be added to the Driver class with the same class as that of the reducer. The advantage of the combiner is that it works as a mini reducer and runs on the same machine as the mapper, thus reducing the amount of data shuffling. The Driver class of the word count application as follows:

import org.apache.Hadoop.conf.Configuration;
import org.apache.Hadoop.conf.Configured;
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.lib.input.FileInputFormat;
import org.apache.Hadoop.mapreduce.lib.input.TextInputFormat;
import org.apache.Hadoop.mapreduce.lib.output.FileOutputFormat;
import org.apache.Hadoop.mapreduce.lib.output.TextOutputFormat;
import org.apache.Hadoop.util.Tool;
import org.apache.Hadoop.util.ToolRunner;

public class Driver extends Configured implements Tool {


    public static void main(String[] args) throws Exception {
        int res = ToolRunner.run(new Configuration(), (Tool) new Driver(), args);
        System.exit(res);
    }

    public int run(String[] args) throws Exception {

        Configuration conf = new Configuration();
        Job job = Job.getInstance(conf, "WordCount");

        job.setJarByClass(Driver.class);


        if (args.length < 2) {
            System.out.println("Jar requires 2 paramaters :  \""
                    + job.getJar()
                    + " input_path output_path");
            return 1;
        }

        job.setMapperClass(WordcountMapper.class);

        job.setReducerClass(WordcountReducer.class);

        job.setCombinerClass(WordcountReducer.class);

        job.setOutputKeyClass(Text.class);
        job.setOutputValueClass(IntWritable.class);
        job.setInputFormatClass(TextInputFormat.class);

        job.setOutputFormatClass(TextOutputFormat.class);

        Path filePath = new Path(args[0]);
        FileInputFormat.setInputPaths(job, filePath);

        Path outputPath = new Path(args[1]);
        FileOutputFormat.setOutputPath(job, outputPath);

        job.waitForCompletion(true);
        return 0;
    }
}