Programmatically reading the output of Hadoop Mapreduce Program

Question

This may be a basic question, but I could not find an answer for it on Google.
I have a map-reduce job that creates multiple output files in its output directory. My Java a

Answer 1

            FSDataInputStream inputStream = fs.open(path);
            BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream));
            String record;
            while((record = reader.readLine()) != null) {
                int blankPos = record.indexOf(" ");
                System.out.println(record+"blankPos"+blankPos);
                String keyString = record.substring(0, blankPos);
                String valueString = record.substring(blankPos + 1);
                System.out.println(keyString + " | " + valueString);
            }

Answer 2

The method you are looking for is called listStatus(Path). It simply returns all files inside of a Path as a FileStatus array. Then you can simply loop over them create a path object and read it.

    FileStatus[] fss = fs.listStatus(new Path("/"));
    for (FileStatus status : fss) {
        Path path = status.getPath();
        SequenceFile.Reader reader = new SequenceFile.Reader(fs, path, conf);
        IntWritable key = new IntWritable();
        IntWritable value = new IntWritable();
        while (reader.next(key, value)) {
            System.out.println(key.get() + " | " + value.get());
        }
        reader.close();
    }

For Hadoop 2.x you can setup the reader like this:

 SequenceFile.Reader reader = 
           new SequenceFile.Reader(conf, SequenceFile.Reader.file(path))

Answer 3

You have a few options: here are two that I sometimes use.

Method #1: Depending on your data size, is to make use of the following HDFS commands (found here, Item 6)

hadoop fs -getmerge hdfs-output-dir local-file
// example 
hadoop fs -getmerge /user/kenny/mrjob/ /tmp/mrjob_output
// another way
hadoop fs -cat /user/kenny/mrjob/part-r-* > /tmp/mrjob_output

"This concatenates the HDFS files hdfs-output-dir/part-* into a single local file."

Then you can just read in the one single file. (note that it is in local storage and not HDFS)

Method #2: Create a helper method: (I have a class called HDFS which contains the Configuration, FileSystem instances as well as other helper methods)

public List<Path> matchFiles(String path, final String filter) {
        List<Path> matches = new LinkedList<Path>();
        try {
            FileStatus[] statuses = fileSystem.listStatus(new Path(path), new PathFilter() {
                       public boolean accept(Path path) {
                          return path.toString().contains(filter);
                       }
                    });  
            for(FileStatus status : statuses) {
                matches.add(status.getPath());
            }
        } catch(IOException e) {
        LOGGER.error(e.getMessage(), e);
        }
        return matches;
    }

You can then call via a command like this: hdfs.matchFiles("/user/kenny/mrjob/", "part-")