Question

I have a problem with mapreduce. Giving as input a list of song (“Songname”#”UserID”#”boolean”) i must have as result a song list in which is specified how many time different users listen them… so a ” output (“Songname”,”timelistening”).
I used hashtable to allow only one couple .
With short files it works well but when I put as input a list about 1000000 of records it returns me the same value (20) for all records.

This is my mapper:

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

    private IntWritable userID = new IntWritable(0);
    private Text song = new Text();

    public void map(Object key, Text value, Context context) throws IOException, InterruptedException {
        String[] caratteri = value.toString().split("#");
        if(caratteri[2].equals("1")){
            song.set(caratteri[0]);
            userID.set(Integer.parseInt(caratteri[1]));
            context.write(song,userID);
        }
    }
  }

This is my reducer:

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

    public void reduce(Text key, Iterable<IntWritable> values, Context context) throws IOException, InterruptedException {
      Hashtable<IntWritable,Text> doppioni = new Hashtable<IntWritable,Text>();
      for (IntWritable val : values) {
        doppioni.put(val,key);
      }
      result.set(doppioni.size());
      doppioni.clear();
      context.write(key,result);
    }
  }

and main:

Configuration conf = new Configuration();

    Job job = new Job(conf, "word count");
    job.setJarByClass(Canzoni.class);
    job.setMapperClass(CanzoniMapper.class);
    //job.setCombinerClass(CanzoniReducer.class);
    //job.setNumReduceTasks(2);
    job.setReducerClass(CanzoniReducer.class);
    job.setOutputKeyClass(Text.class);
    job.setOutputValueClass(IntWritable.class);
    FileInputFormat.addInputPath(job, new Path(args[0]));
    FileOutputFormat.setOutputPath(job, new Path(args[1]));
    System.exit(job.waitForCompletion(true) ? 0 : 1);

Any idea???

Answer 1

Maybe I solved it. It’s an input problem. There were too many records compared to the number of songs, so in these records’ list each song was listed at least once by each user.
In my test I had 20 different users, so naturally the result gives me 20 for each song.
I must increase the number of different songs.