Mapfile as a input to a MapReduce job

Question

I recently started to use Hadoop and I have a problem while using a Mapfile as a input to a MapReduce job.

The following working code, writes a simple MapFile called "TestMap" in hdfs where there are three keys of type Text and three values of type BytesWritable.

Here the contents of TestMap:

$ hadoop fs  -text /user/hadoop/TestMap/data
11/01/20 11:17:58 INFO util.NativeCodeLoader: Loaded the native-hadoop library
11/01/20 11:17:58 INFO zlib.ZlibFactory: Successfully loaded & initialized native-zlib library
11/01/20 11:17:58 INFO compress.CodecPool: Got brand-new decompressor
A    01
B    02
C    03

Here is the program that creates the TestMap Mapfile:

import java.io.IOException;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.io.MapFile;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.io.BytesWritable;
import org.apache.hadoop.io.SequenceFile;
import org.apache.hadoop.io.IOUtils;

public class CreateMap {

    public static void main(String[] args) throws IOException{

        Configuration conf = new Configuration();
        FileSystem hdfs  = FileSystem.get(conf);

        Text key = new Text();
        BytesWritable value = new BytesWritable();
        byte[] data = {1, 2, 3};
        String[] strs = {"A", "B", "C"};
        int bytesRead;
        MapFile.Writer writer = null;

        writer = new MapFile.Writer(conf, hdfs, "TestMap", key.getClass(), value.getClass());
        try {
            for (int i = 0; i < 3; i++) {
                key.set(strs[i]);
                value.set(data, i, 1);
                writer.append(key, value);
                System.out.println(strs[i] + ":" + data[i] + " added.");
            }
        }
        catch (IOException e) {
            e.printStackTrace();
        }
        finally {
             IOUtils.closeStream(writer);
        }
    }
}

The simple MapReduce job that follows tries to increment by one the values of the mapfile:

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.Text;
import org.apache.hadoop.mapred.FileInputFormat;
import org.apache.hadoop.mapred.FileOutputFormat;
import org.apache.hadoop.mapred.JobClient;
import org.apache.hadoop.mapred.JobConf;
import org.apache.hadoop.mapred.SequenceFileInputFormat;
import org.apache.hadoop.mapred.MapReduceBase;
import org.apache.hadoop.mapred.Mapper;
import org.apache.hadoop.mapred.OutputCollector;
import org.apache.hadoop.mapred.Reducer;
import org.apache.hadoop.mapred.Reporter;
import org.apache.hadoop.mapred.TextOutputFormat;
import org.apache.hadoop.util.Tool;
import org.apache.hadoop.util.ToolRunner;
import org.apache.hadoop.io.BytesWritable;


public class AddOne extends Configured implements Tool {

    public static class MapClass extends MapReduceBase

        implements Mapper<Text, BytesWritable, Text, Text> {

        public void map(Text key, BytesWritable value,
                        OutputCollector<Text, Text> output,
                        Reporter reporter) throws IOException {


            byte[] data = value.getBytes();
            data[0] += 1;
            value.set(data, 0, 1);
            output.collect(key, new Text(value.toString()));
        }
    }

    public static class Reduce extends MapReduceBase
        implements Reducer<Text, Text, Text, Text> {

        public void reduce(Text key, Iterator<Text> values,
                           OutputCollector<Text, Text> output,
                           Reporter reporter) throws IOException {

            output.collect(key, values.next());
        }
    }

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

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

        Path in = new Path("TestMap");
        Path out = new Path("output");
        FileInputFormat.setInputPaths(job, in);
        FileOutputFormat.setOutputPath(job, out);

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

        job.setInputFormat(SequenceFileInputFormat.class);
        job.setMapOutputKeyClass(Text.class);
        job.setMapOutputValueClass(Text.class);

        job.setOutputFormat(TextOutputFormat.class);
        job.setOutputKeyClass(Text.class);
        job.setOutputValueClass(Text.class);
        job.set("key.value.separator.in.input.line", ":");


        JobClient.runJob(job);

        return 0;
    }

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

        System.exit(res);
    }
}

The runtime exception that I get is:

java.lang.ClassCastException: org.apache.hadoop.io.LongWritable cannot be cast to org.apache.hadoop.io.BytesWritable
    at AddOne$MapClass.map(AddOne.java:32)
    at org.apache.hadoop.mapred.MapRunner.run(MapRunner.java:50)
    at org.apache.hadoop.mapred.MapTask.runOldMapper(MapTask.java:358)
    at org.apache.hadoop.mapred.MapTask.run(MapTask.java:307)
    at org.apache.hadoop.mapred.Child.main(Child.java:170)

I don't understand why hadoop is trying to cast a LongWritable, since in my code I define the Mapper interface correctly( Mapper<Text, BytesWritable, Text, Text> ).

Could somebody help me?

Thank you very much

Luca

Answer 1

Your problem comes from the fact that, despite what the name tells you, a MapFile is not a file.

A MapFile is actually a directory that consists of two files: there's a "data" file, which is a SequenceFile containing the keys and values you write into it; however, there is also an "index" file, which is a different SequenceFile containing a subsequence of the keys along with their offsets as LongWritables; this index is loaded into memory by MapFile.Reader to let you quickly binary search to find the offset in the data file that will have the data you want when you do random access.

You're using the old "org.apache.hadoop.mapred" version of SequenceFileInputFormat . It's not smart enough to know to only look at the data file when you tell it to look at a MapFile as input; instead, it actually tries to use the data file and the index file as regular input files. The data file will work correctly because the classes agree with what you specify, but the index file will throw the ClassCastException, because the index file values are all LongWritables.

You have two options: you can start using the "org.apache.hadoop.mapreduce" version of SequenceFileInputFormat (thus changing other parts of your code), which does know enough about MapFiles to just look at the data file; or, instead, you can explicitly give the data file as the file you want as input.

Answer 2

I resolved the same issue using KeyValueTextInputFormat.class

Have mentioned the whole approach in

http://sanketraut.blogspot.in/2012/06/hadoop-example-setting-up-hadoop-on.html

Answer 3

其中一种方法可能是使用自定义InputFormat，其中一个记录用于整个MapFile块，并通过map（）中的键进行查找