Question

This is the job, which joins two relations,

import org.apache.hadoop.mapred.*;
import org.apache.hadoop.conf.*;
import org.apache.hadoop.util.*;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.*;
import org.apache.hadoop.contrib.utils.join.*;
import java.io.*;

public class TwoByTwo extends Configured implements Tool
{
    public static class MapClass extends DataJoinMapperBase
    {
        protected Text generateInputTag( String inputFile)
        {
            String datasource = inputFile.split( "\\.")[ 0];
            return new Text( datasource);
        }//end generateInputTag

        protected Text generateGroupKey( TaggedMapOutput aRecord)
        {
            String line =  ( ( Text) aRecord.getData()).toString();
            //between two relations there may be more than one common attributes
            //so group key has to include all these common attributes. Common
            //attributes begin with an '_'( underscore).
            String[] tokens = line.split(",");
            String groupKey = "";
            for( String s : tokens)
            {
                if( s.charAt( 0) == '_')
                {
                    groupKey = groupKey + s;
                }
            }
            return new Text( groupKey);
        }//end generateGroupKey

        protected TaggedMapOutput generateTaggedMapOutput( Object value)
        {
            TaggedWritable retv = new TaggedWritable( ( Text) value);
            retv.setTag( this.inputTag);
            return retv;
        }//end TaggedMapOutput
    }//end MapClass

    public static class Reduce extends DataJoinReducerBase
    {
        protected TaggedMapOutput combine( Object[] tags, Object[] values)
        {
            if( tags.length < 2)
            {
                return null;
            }
            String joinedStr = "";
            for( int i = 0; i < values.length; i++)
            {
                if( i > 0)
                {
                    joinedStr += ",";
                }
                TaggedWritable tw = ( TaggedWritable) values[ i];
                String line = ( ( Text) tw.getData()).toString();
                String[] tokens = line.split( ",", 2);
                joinedStr += tokens[ 1];
            }
            TaggedWritable retv = new TaggedWritable( new Text( joinedStr));
            retv.setTag( ( Text) tags[ 0]);
            return retv;
        }//end TaggedMapOutput
    }//end Reduce

    public static class TaggedWritable extends TaggedMapOutput
    {
        private Writable data;

        public TaggedWritable( Writable data)
        {
            this.tag = new Text( "");
            this.data = data;
        }//end TaggedWritable

        public Writable getData()
        {
            return data;
        }//end getData

        public void write( DataOutput out) throws IOException
        {
            this.tag.write( out);
            this.data.write( out);
        }//end write

        public void readFields( DataInput in) throws IOException
        {
            this.tag.readFields( in);
            this.data.readFields( in);      
        }//end readFields
    }//end TaggedWritable

    public int run( String[] args) throws Exception
    {
        Configuration conf = getConf();

        JobConf job = new JobConf( conf, TwoByTwo.class);

        Path in  = new Path( "relations/");
        Path out = new Path( "relout/");
        FileInputFormat.setInputPaths( job, in);
        FileOutputFormat.setOutputPath( job, out);

        job.setJobName( "TwoByTwo");
        job.setMapperClass( MapClass.class);
        job.setReducerClass( Reduce.class);

        job.setInputFormat( TextInputFormat.class);
        job.setOutputFormat( TextOutputFormat.class);
        job.setOutputKeyClass( Text.class);
        job.setOutputValueClass( TaggedWritable.class);
        job.set("mapred.textoutputformat.separator", ",");

        JobClient.runJob( job);
        return 0;
    }//end run

    public static void main( String[] args) throws Exception
    {
        int res = ToolRunner.run( new Configuration(), new TwoByTwo(), args);

        System.exit( res);
    }//end main
}

When I run this job,

bin/hadoop jar /home/hduser/TwoByTwo.jar TwoByTwo -libjars /usr/local/hadoop/contrib/datajoin/hadoop-datajoin-1.0.3.jar

MapClass runs appropriately. When Reduce runs after sometime of running I get this NoSuchMethodException

12/10/18 16:38:17 INFO mapred.JobClient:  map 100% reduce 27%
12/10/18 16:38:19 INFO mapred.JobClient: Task Id : attempt_201210181416_0013_r_000000_0, Status : FAILED
java.lang.RuntimeException: java.lang.NoSuchMethodException: TwoByTwo$TaggedWritable.<init>()
    at org.apache.hadoop.util.ReflectionUtils.newInstance(ReflectionUtils.java:115)
    at org.apache.hadoop.io.serializer.WritableSerialization$WritableDeserializer.deserialize(WritableSerialization.java:62)
    at org.apache.hadoop.io.serializer.WritableSerialization$WritableDeserializer.deserialize(WritableSerialization.java:40)
    at org.apache.hadoop.mapred.Task$ValuesIterator.readNextValue(Task.java:1271)
    at org.apache.hadoop.mapred.Task$ValuesIterator.next(Task.java:1211)
    at org.apache.hadoop.mapred.ReduceTask$ReduceValuesIterator.moveToNext(ReduceTask.java:249)
    at org.apache.hadoop.mapred.ReduceTask$ReduceValuesIterator.next(ReduceTask.java:245)
    at org.apache.hadoop.contrib.utils.join.DataJoinReducerBase.regroup(DataJoinReducerBase.java:106)
    at org.apache.hadoop.contrib.utils.join.DataJoinReducerBase.reduce(DataJoinReducerBase.java:129)
    at org.apache.hadoop.mapred.ReduceTask.runOldReducer(ReduceTask.java:519)
    at org.apache.hadoop.mapred.ReduceTask.run(ReduceTask.java:420)
    at org.apache.hadoop.mapred.Child$4.run(Child.java:255)
    at java.security.AccessController.doPrivileged(Native Method)
    at javax.security.auth.Subject.doAs(Subject.java:415)
    at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1121)
    at org.apache.hadoop.mapred.Child.main(Child.java:249)
Caused by: java.lang.NoSuchMethodException: TwoByTwo$TaggedWritable.<init>()
    at java.lang.Class.getConstructor0(Class.java:2721)
    at java.lang.Class.getDeclaredConstructor(Class.java:2002)
    at org.apache.hadoop.util.ReflectionUtils.newInstance(ReflectionUtils.java:109)
    ... 15 more

I have problem with nested class TaggedWritable. Why do I have problem with this class on reduce side and not on map side? How can I resolve this error? The constraint of two relations plays any role to the error? Thanks for any help.

Answer 1

This code solves the problem and gives the expected result. It is from here,

public static class TaggedWritable extends TaggedMapOutput
{
    private Writable data;

    public TaggedWritable()
    {
        this.tag = new Text();
    }//end empty( taking no parameters) constructor TaggedWritable

    public TaggedWritable( Writable data)
    {
        this.tag = new Text( "");
        this.data = data;
    }//end constructor TaggedWritable

    public Writable getData()
    {
        return data;
    }//end getData

    public void setData( Writable data)
    {
        this.data = data;
    }//end setData

    public void write( DataOutput out) throws IOException
    {
        this.tag.write( out);
        out.writeUTF( this.data.getClass().getName());
        this.data.write( out);
    }//end write

    public void readFields( DataInput in) throws IOException
    {
        this.tag.readFields( in);
        String dataClz = in.readUTF();
        try
        {
            //try - catch is needed because the " error: unreported exception
            //ClassNotFoundException; must be caught or declared to be thrown"
            //is "raised" from compiler
            if( this.data == null || !this.data.getClass().getName().equals( dataClz))
            {
                //this line of code "raises" the compile error mentioned above
                this.data = (Writable) ReflectionUtils.newInstance( Class.forName( dataClz), null);
            }
            this.data.readFields( in);
        }
        catch( ClassNotFoundException cnfe)
        {
            System.out.println( "Problem in TaggedWritable class, method readFields.");
        }
    }//end readFields
}//end TaggedWritable

As rystsov says above, an empty( without parameters) constructor is needed. This solves the problem,

NoSuchMethodException.

Then another error occurs,

NullPointerException,

in readFields, at line

this.data.readFields( in);

This is solved, writing readFields the way above, where null values are handled properly.