Hadoop ChainReducer example

Below is example of how to use ChainReducer, you can override data at each stage.

(It is important to note that you use ChainReducer.setReducer only once for a job )

To run jar (pack things in jar it is easy) use:
hadoop jar chainingreducer.jar  /user/hduser/wordcount/input /user/hduser/wordcount/output

Below is the flow:
TokenizerMapper (data read from file) --> TokenizerMapper2 (data modified) --> IntSumReducer (Reduced on key and sum calculated, word count) -> TokenizerMapper3 (again data modified)


ChainingReducerClass.java

import java.io.IOException;
import java.util.StringTokenizer;

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.chain.ChainMapper;
import org.apache.hadoop.mapreduce.lib.chain.ChainReducer;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;

public class ChainingReducerClass {

public static class TokenizerMapper extends
Mapper<Object, Text, TextPair, IntWritable> {

private final static IntWritable one = new IntWritable(1);
private Text word = new Text();
private static TextPair textPair = new TextPair();

public void map(Object key, Text value, Context context)
throws IOException, InterruptedException {
StringTokenizer itr = new StringTokenizer(value.toString());
while (itr.hasMoreTokens()) {
word.set(itr.nextToken());
Text word2 = new Text();
word2.set("mapper1");
Text word3 = new Text();
word3.set("ggg");
textPair.set(word, word2, word3);
context.write(textPair, one);
}
}
}

public static class TokenizerMapper2 extends
Mapper<TextPair, IntWritable, TextPair, IntWritable> {

private static TextPair textPair = new TextPair();

public void map(TextPair key, IntWritable value, Context context)
throws IOException, InterruptedException {

Text text3 = new Text();
text3.set("fff");
textPair.set(key.getFirst(), key.getSecond(), text3);
context.write(textPair, value);

}

}

public static class IntSumReducer extends
Reducer<TextPair, IntWritable, TextPair, IntWritable> {
private IntWritable result = new IntWritable();

public void reduce(TextPair key, Iterable<IntWritable> values,
Context context) throws IOException, InterruptedException {
int sum = 0;
for (IntWritable val : values) {
sum += val.get();
}
result.set(sum);
context.write(key, result);
}
}

public static class TokenizerMapper3 extends
Mapper<TextPair, IntWritable, TextPair, IntWritable> {

private static TextPair textPair = new TextPair();

public void map(TextPair key, IntWritable value, Context context)
throws IOException, InterruptedException {

Text text3 = new Text();
text3.set("333");
textPair.set(key.getFirst(), key.getSecond(), text3);
context.write(textPair, value);

}

}

public static void main(String[] args) throws Exception {
Configuration conf = new Configuration();
Job job = Job.getInstance(conf, "word count");

// //////////////Here add chaining code for mappers
Configuration mapAConf = new Configuration(false);
ChainMapper.addMapper(job, TokenizerMapper.class, Object.class,
Text.class, TextPair.class, IntWritable.class, mapAConf);

Configuration mapBConf = new Configuration(false);
ChainMapper.addMapper(job, TokenizerMapper2.class, TextPair.class,
IntWritable.class, TextPair.class, IntWritable.class, mapBConf);
// //////////////end of chaining for mappers
job.setJarByClass(ChainingReducerClass.class);

job.setCombinerClass(IntSumReducer.class);

// there can only be one setreducer
ChainReducer.setReducer(job, IntSumReducer.class, TextPair.class,
IntWritable.class, TextPair.class, IntWritable.class,
new Configuration(false));
// reducers results go to the mapper
ChainReducer.addMapper(job, TokenizerMapper3.class, TextPair.class,
IntWritable.class, TextPair.class, IntWritable.class, null);

FileInputFormat.addInputPath(job, new Path(args[0]));
FileOutputFormat.setOutputPath(job, new Path(args[1]));
System.exit(job.waitForCompletion(true) ? 0 : 1);
}

}

TextPair.java

import java.io.DataInput;

import java.io.DataOutput;

import java.io.IOException;

import org.apache.hadoop.io.Text;

import org.apache.hadoop.io.WritableComparable;

public class TextPair implements WritableComparable {

private Text first;

private Text second;

private Text third;

public TextPair(Text first, Text second, Text third) {

set(first, second, third);

}

public TextPair() {

set(new Text(), new Text(), new Text());

}

public TextPair(String first, String second, String third) {

set(new Text(first), new Text(second), new Text(third));

}

public Text getFirst() {

return first;

}

public Text getSecond() {

return second;

}

public Text getThird() {

return third;

}

public void set(Text first, Text second, Text third) {

this.first = first;

this.second = second;

this.third = third;

}

@Override

public void readFields(DataInput in) throws IOException {

first.readFields(in);

second.readFields(in);

third.readFields(in);

}

@Override

public void write(DataOutput out) throws IOException {

first.write(out);

second.write(out);

third.write(out);

}

@Override

public String toString() {

return first + " " + second + " " + third;

}

@Override

public int hashCode() {

return first.hashCode() * 163 + second.hashCode() + third.hashCode();

}

@Override

public boolean equals(Object o) {

if (o instanceof TextPair) {

TextPair tp = (TextPair) o;

return first.equals(tp.first) && second.equals(tp.second)

&& third.equals(tp.third);

}

return false;

}

@Override

public int compareTo(Object tp) {

if (tp instanceof TextPair) {

int cmp = first.compareTo(((TextPair) tp).first);

if (cmp != 0) {

return cmp;

}

int cmp1 = second.compareTo(((TextPair) tp).second);

if (cmp1 != 0) {

return cmp1;

}

return third.compareTo(((TextPair) tp).third);

}

return -1;

}

}