Question

Here is my code:

import java.io.DataInput;
import java.io.DataOutput;
import java.io.IOException;
import java.util.Iterator;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.conf.Configured;
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.io.WritableComparable;
import org.apache.hadoop.io.WritableUtils;
import org.apache.hadoop.mapred.*;
import org.apache.hadoop.util.Tool;
import org.apache.hadoop.util.ToolRunner;

public class SecondarySort extends Configured implements Tool{

public static void main(String[] args) {
    try {
        ToolRunner.run(new Configuration(), new SecondarySort(), args);
    } catch (Exception e) {
        e.printStackTrace();
    }
}

static class KeyPartitioner implements Partitioner<StockKey, DoubleWritable> {

    @Override
    public int getPartition(StockKey arg0, DoubleWritable arg1, int arg2) {

        int partition = arg0.name.hashCode() % arg2;

        return partition;
    }

    @Override
    public void configure(JobConf job) {
    }

}


static class StockKey implements WritableComparable<StockKey> {

    String name;
    Long timestamp;

    public StockKey() {

    }

    StockKey(String name, Long timestamp){
        this.name = name;
        this.timestamp = timestamp;
    }

    @Override
    public void readFields(DataInput arg0) throws IOException {
        name = WritableUtils.readString(arg0);
        timestamp = arg0.readLong();
    }

    @Override
    public void write(DataOutput arg0) throws IOException {
        WritableUtils.writeString(arg0, name);
        arg0.writeLong(timestamp);
    }

    @Override
    public int compareTo(StockKey arg0) {
        int result = 0;

        result = name.compareToIgnoreCase(arg0.name);

        if(result == 0)
            result = timestamp.compareTo(arg0.timestamp);   

        return result;
    }

    public String toString() {
        String outputString = name+","+timestamp;
        return outputString;
    }
}


static class StockReducer implements Reducer<StockKey, DoubleWritable, Text, Text>{

    public void reduce(StockKey key, Iterator<DoubleWritable> value, Outp      
OutputCollector<Text, Text> context, Reporter reporter) 
                throws IOException {

        Text k = new Text(key.toString());

        while(value.hasNext()) {

            Double v = value.next().get();
            Text t = new Text(v.toString());

            context.collect(k, t);          
        }
    }

    @Override
    public void configure(JobConf job) {
        // TODO Auto-generated method stub

    }

    @Override
    public void close() throws IOException {
        // TODO Auto-generated method stub

    }
}


static class StockMapper implements Mapper<LongWritable, Text, StockKey, 
DoubleWritable> {

    public void map(LongWritable offset, Text value, OutputCollector<StockKey, 
DoubleWritable> context, Reporter reporter) 
                throws IOException {

        String[] values = value.toString().split(",");
        StockKey key = new StockKey(values[0].trim(), 
Long.parseLong(values[1].trim()));
        DoubleWritable val = new 
DoubleWritable(Double.parseDouble(values[2].trim()));


            context.collect(key, val);

    }

    @Override
    public void configure(JobConf job) {
        // TODO Auto-generated method stub

    }

    @Override
    public void close() throws IOException {
        // TODO Auto-generated method stub

    }

}

@SuppressWarnings("unchecked")
@Override
public int run(String[] arg) throws Exception {

    JobConf conf = new JobConf(getConf(), SecondarySort.class);
    conf.setJobName(SecondarySort.class.getName());

    conf.setJarByClass(SecondarySort.class);

    conf.setInputFormat(TextInputFormat.class);
    conf.setOutputFormat(TextOutputFormat.class);

    conf.setMapOutputKeyClass(StockKey.class);
    conf.setMapOutputValueClass(Text.class);

    conf.setPartitionerClass((Class<? extends Partitioner<StockKey, 
DoubleWritable>>) KeyPartitioner.class);

    conf.setMapperClass((Class<? extends Mapper<LongWritable, Text, StockKey, 
DoubleWritable>>) StockMapper.class);
    conf.setReducerClass((Class<? extends Reducer<StockKey, DoubleWritable, 
Text, Text>>) StockReducer.class);

    FileInputFormat.addInputPath(conf, new Path(arg[0]));
    FileOutputFormat.setOutputPath(conf, new Path(arg[1]));

    JobClient.runJob(conf);

    return 0;
}

}

Here is the exception:

java.io.IOException: Type mismatch in value from map: expected 
org.apache.hadoop.io.Text, recieved org.apache.hadoop.io.DoubleWritable
    at org.apache.hadoop.mapred.MapTask$MapOutputBuffer.collect(MapTask.java:876)
    at org.apache.hadoop.mapred.MapTask$OldOutputCollector.collect(MapTask.java:499)
    at SecondarySort$StockMapper.map(SecondarySort.java:135)
    at SecondarySort$StockMapper.map(SecondarySort.java:1)
    at org.apache.hadoop.mapred.MapRunner.run(MapRunner.java:50)
    at org.apache.hadoop.mapred.MapTask.runOldMapper(MapTask.java:391)
    at org.apache.hadoop.mapred.MapTask.run(MapTask.java:325)
    at org.apache.hadoop.mapred.Child$4.run(Child.java:270)
    at java.security.AccessController.doPrivileged(Native Method)
    at javax.security.auth.Subject.doAs(Subject.java:396)
    at     
org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1127)
    at org.apache.hadoop.mapred.Child.main(Child.java:264)


12/07/13 03:22:32 INFO mapred.JobClient: Task Id :   
attempt_201207130314_0002_m_000001_2, Status : FAILED

java.io.IOException: Type mismatch in value from map: expected 
org.apache.hadoop.io.Text, recieved org.apache.hadoop.io.DoubleWritable
    at org.apache.hadoop.mapred.MapTask$MapOutputBuffer.collect(MapTask.java:876)
    at org.apache.hadoop.mapred.MapTask$OldOutputCollector.collect(MapTask.java:499)
    at SecondarySort$StockMapper.map(SecondarySort.java:135)
    at SecondarySort$StockMapper.map(SecondarySort.java:1)
    at org.apache.hadoop.mapred.MapRunner.run(MapRunner.java:50)
    at org.apache.hadoop.mapred.MapTask.runOldMapper(MapTask.java:391)
    at org.apache.hadoop.mapred.MapTask.run(MapTask.java:325)
    at org.apache.hadoop.mapred.Child$4.run(Child.java:270)
    at java.security.AccessController.doPrivileged(Native Method)
    at javax.security.auth.Subject.doAs(Subject.java:396)
    at 
org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1127)
    at org.apache.hadoop.mapred.Child.main(Child.java:264)

Answer 1

There are so many potential problems with this code that could be causing it:

• StockKey – you should override the default hashCode() method – at the moment two StockKey‘s with the same contents will have different hashCode values (as if you don’t override the JVM default, then its going to return a number which is to all extent and purposes is the address in memory of the two objects). I know in your partitioner you only use the name field (which is a String and will have a valid implementation of hashCode(), but this is good practice in case in future you use the entire Stock object’s hashCode() and wonder why two identical Stock objects end up at different reducers

• KeyPartitioner – You need to Math.abs(..) the result of the arg0.name.hashCode(). At the moment, this value could come back negative, which when you modulo with the number of reducers, will return a negative number. The knock-on effect is the MR framework will thrown an exception because it’s expecting a number between 0 (inclusive) and the number of reducers (exclusive). This is probably where you problem lies as i’ll explain in the next point

• Mapper.map method – You are swallowing any potential output exceptions when you call context.collect. Continuing from my previous point about the partitioner – if it returns a negative number, an exception will be thrown, which you need to deal with. In some cases catching and swallowing exceptions may be ok (data validation for input records for example), but any exception that occurs when outputting should be thrown up to the MR framework to flag that something went wrong and the output of this mapper is wrong / incomplete:

  try {
      context.collect(key, val);
  } catch (IOException e) {
      e.printStackTrace();
  }
  

• Finally you need to explicitly declare your map and reduce output types (which is causing the exception as you currently declare the map value output type as Text, when in fact the mapper is outputting a DoubleWritable):

job.setMapOutputKeyClass(StockKey.class);
job.setMapOutputValueClass(DoubleWritable.class);

job.setOutputKeyClass(Text.class);
job.setOutputValueClass(Text.class);

I suggest that you remove the try/catch block around context.collect call and rerun your job (or just check the logs for the map tasks and see if you see a stack trace).