There is a Map/Reduce section in the user documentation on the wiki:
http://wiki.apache.org/hadoop/Hbase/MapReduce

---
Jim Kellerman, Senior Engineer; Powerset


> -----Original Message-----
> From: Naama Kraus [mailto:naamakraus@gmail.com]
> Sent: Monday, June 23, 2008 9:58 AM
> To: hbase-user@hadoop.apache.org
> Subject: Re: Map Reduce over HBase - sample code
>
> OK. Where would you suggest to put it ? Under 'User
> Documentation' section ?
> Or anywhere else ?
>
> Naama
>
> On Mon, Jun 23, 2008 at 5:21 PM, Jim Kellerman
> <jim@powerset.com> wrote:
>
> > In order to edit the wiki you just need to create an
> account and log in.
> > That is the only restriction on modifying the wiki.
> >
> > ---
> > Jim Kellerman, Senior Engineer; Powerset
> >
> >
> > > -----Original Message-----
> > > From: Naama Kraus [mailto:naamakraus@gmail.com]
> > > Sent: Sunday, June 22, 2008 11:35 PM
> > > To: hbase-user@hadoop.apache.org
> > > Subject: Re: Map Reduce over HBase - sample code
> > >
> > > Oh, didn't know that. I also couldn't find a way to edit
> the Wiki, I
> > > think I am not permitted.
> > > Well, here is the code that does the MapReduce, I'd be glad for
> > > comments.
> > >
> > > - Naama
> > >
> > > /**
> > >  * A map reduce job over {@link GradesTable}
> > >  * The job produces for each student (row) the average
> grade of his
> > > course grades.
> > >  * It puts the average in a separate column in the original
> > > (source) table
> > >  *
> > >  */
> > > public class GradesTableMapReduce  extends Configured implements
> > > Tool {
> > >
> > >   /**
> > >    * Map a row to a {key, value} pairs.
> > >    * Emit a {student, grade} pair for each course grade
> appearing in
> > > the student row.
> > >    * E.g. sara {62, 45, 87} -> {sara, 62}, {sara, 45}, {sara, 87}
> > >    *
> > >    */
> > >   public static class GradesTableMap extends TableMap<Text,
> > > IntWritable> {
> > >
> > >     @Override
> > >     public void map(HStoreKey key, MapWritable value,
> > >         OutputCollector<Text, IntWritable> output, Reporter
> > > reporter) throws IOException {
> > >
> > >       // Raw id is student name
> > >       Text student = key.getRow();
> > >       // Walk through the columns
> > >       for (Map.Entry<Writable, Writable> e: value.entrySet()) {
> > > //        Text course = (Text) e.getKey();
> > >         byte [] gradeInBytes = ((ImmutableBytesWritable)
> > > e.getValue()).get();
> > >         DataInputStream in = new DataInputStream(new
> > > ByteArrayInputStream(gradeInBytes));
> > >         IntWritable grade = new IntWritable();
> > >         grade.readFields(in);
> > > //        System.out.println("map() -- student: " +
> > > //            student.toString() +
> > > //            ", course: " +
> > > //            course.toString() +
> > > //            ", grade: " +
> > > //            grade.toString());
> > >         // Emit student name and a grade
> > >         output.collect(student, grade);
> > >       }
> > >     }
> > >   }
> > >
> > >   /**
> > >    * Reduce - compute an average of key's values which is
> actually
> > > the average grade of each student.
> > >    * E.g. {sara, {62, 45, 87}} -> {sara, 65.6}
> > >    *
> > >    */
> > >   public static class GradesTableReduce extends TableReduce<Text,
> > > IntWritable> {
> > >
> > >     @Override
> > >     // key is student name, values are his grades
> > >     public void reduce(Text key, Iterator<IntWritable> values,
> > >         OutputCollector<Text, MapWritable> output,
> Reporter reporter)
> > >     throws IOException {
> > >       // Compute grades average
> > >       int total = 0;
> > >       int sum = 0;
> > >       while (values.hasNext()) {
> > >         total++;
> > >         sum += values.next().get();
> > >       }
> > >       float average = sum / total;
> > >
> > >       // We put the average as a separate column in the
> source table
> > >       ByteArrayOutputStream baos = new ByteArrayOutputStream();
> > >       DataOutputStream out = new DataOutputStream(baos);
> > >       FloatWritable avgWritable = new FloatWritable(average);
> > >       avgWritable.write(out);
> > >       MapWritable map = new MapWritable();
> > >       map.put(new Text(GradesTable.STATS_FAMILY +
> GradesTable.AVG),
> > >               new ImmutableBytesWritable(baos.toByteArray()));
> > >       output.collect(key, map);
> > >     }
> > >   }
> > >
> > >   /**
> > >    * Run
> > >    */
> > >   public int run(String[] args) throws Exception {
> > >     JobConf jobConf = new JobConf();
> > >     jobConf.setJobName("compute average grades");
> > >     jobConf.setNumReduceTasks(1);
> > >
> > >     // All columns in the course family (i.e. all grades)
> get into
> > > the map
> > >     TableMap.initJob(GradesTable.TABLE_NAME,
> > > GradesTable.COURSE_FAMILY,
> > >         GradesTableMap.class, jobConf);
> > >
> > >     TableReduce.initJob(GradesTable.TABLE_NAME,
> > >         GradesTableReduce.class, jobConf);
> > >
> > >     // Map produces a value which is an IntWritable
> > >     jobConf.setMapOutputValueClass(IntWritable.class);
> > >
> > >     JobClient.runJob(jobConf);
> > >     return 0;
> > >   }
> > >
> > >   public static void main(String [] args) throws Exception {
> > >     ToolRunner.run(new Configuration(), new
> GradesTableMapReduce(),
> > > args);
> > >   }
> > > }
> > >
> > >
> > > ==================================================
> > >
> > > Here is the code that creates the original table (I used hbase
> > > 0.1.1)
> > >
> > >
> > > /**
> > >  * An HBase table of the form -
> > >  * raw id is a student name
> > >  * column name is Course:course_name
> > >  * cell value is the student's grade in the course 'course_name'
> > >  *
> > >  * There is also another column, Stats:Average. This one
> gets filled
> > > by a MapReduce job.
> > >  * The cell contains the average grade of the student in
> all courses.
> > >  *
> > >  * Exmaple:
> > >  *
> > >  *         Course:Math  |  Course:Art  |  Course:Sports  |
> > > Stats:Average
> > >  *
> > > --------------------------------------------------------------
> > > ---------------------------
> > >  * Dan        87                97
> > > 99                      94.3
> > >  * Dana      100              100                   80
> > > 93.3
> > >  *
> > >  * @see GradesTableMapReduce
> > >  *
> > >  *
> > >  */
> > > public class GradesTable {
> > >
> > >   public static final String TABLE_NAME = "grades";
> > >   public static final String COURSE_FAMILY = "Course:";
> > >   // A column family holding grades statistics
> > >   public static final String STATS_FAMILY = "Stats:";
> > >   // A column member holding average grade (per student)
> > >   public static final String AVG = "Average";
> > >
> > >   private static final String [] STUDENT_NAMES = {
> > >     "Dan", "Dana", "Sara", "David"
> > >   };
> > >
> > >   private static final String [] COURSE_NAMES = {
> > >     "Math", "Art", "Sports"
> > >   };
> > >
> > >   private HBaseConfiguration conf;
> > >   private HBaseAdmin admin;
> > >   private HTableDescriptor desc;
> > >   // Randomly generate a grade
> > >   private Random rand;
> > >
> > >   public GradesTable() throws IOException {
> > >     conf = new HBaseConfiguration();
> > >     admin = new HBaseAdmin(conf);
> > >     desc = new HTableDescriptor(TABLE_NAME);
> > >     rand = new Random();
> > >   }
> > >
> > >   /**
> > >    * Create table and populate with content
> > >    */
> > >   public void create() throws IOException {
> > >     desc.addFamily(new HColumnDescriptor(COURSE_FAMILY));
> > >     desc.addFamily(new HColumnDescriptor(STATS_FAMILY));
> > >     admin.createTable(desc);
> > >     System.out.println("Grades Table created");
> > >
> > >     HTable table = new HTable(conf, new Text(TABLE_NAME));
> > >
> > >     // Start an update transaction, student name is row id
> > >     for (int i = 0; i < STUDENT_NAMES.length; i++) {
> > >       System.out.println("<<< Row " + i + ", student: " +
> > > STUDENT_NAMES[i] + " >>>");
> > >       Text stuName = new Text(STUDENT_NAMES[i]);
> > >       long writeid = table.startUpdate(stuName);
> > >       for (int j = 0; j < COURSE_NAMES.length; j++) {
> > >         Text courseColumn = new Text(COURSE_FAMILY +
> COURSE_NAMES[j]);
> > >         // Put a cell with a grade of the student in this course
> > >         int grade = Math.abs(rand.nextInt()) % 101;
> > >         table.put(writeid, courseColumn, new IntWritable(grade));
> > >         System.out.println("Course: " + COURSE_NAMES[j] + ",
> > > grade: " + grade);
> > >       }
> > >       table.commit(writeid);
> > >     }
> > >     System.out.println("Table created");
> > >   }
> > >
> > >   }
> > >
> > >
> > >
> > > On Sun, Jun 22, 2008 at 5:39 PM, Daniel Blaisdell
> > > <lunk.djedi@gmail.com>
> > > wrote:
> > >
> > > > More than likely, the mailing list does not allow attachments.
> > > >
> > > > On Sun, Jun 22, 2008 at 8:49 AM, Naama Kraus
> > > <naamakraus@gmail.com> wrote:
> > > >
> > > > > Trying to send over files again ...
> > > > >
> > > > > Naama
> > > > >
> > > > > --
> > > > > oo 00 oo 00 oo 00 oo 00 oo 00 oo 00 oo 00 oo 00 oo 00 oo
> > > 00 oo 00 oo
> > > > > 00
> > > > oo
> > > > > 00 oo 00 oo
> > > > > "If you want your children to be intelligent, read them
> > > fairy tales.
> > > > > If
> > > > you
> > > > > want them to be more intelligent, read them more fairy tales."
> > > > > (Albert
> > > > > Einstein)
> > > > >
> > > >
> > >
> > >
> > >
> > > --
> > > oo 00 oo 00 oo 00 oo 00 oo 00 oo 00 oo 00 oo 00 oo 00 oo
> 00 oo 00 oo
> > > 00 oo 00 oo 00 oo "If you want your children to be
> intelligent, read
> > > them fairy tales. If you want them to be more
> intelligent, read them
> > > more fairy tales." (Albert
> > > Einstein)
> > >
> > > No virus found in this incoming message.
> > > Checked by AVG.
> > > Version: 8.0.100 / Virus Database: 270.4.1/1514 - Release
> > > Date: 6/23/2008 7:17 AM
> > >
> > No virus found in this outgoing message.
> > Checked by AVG.
> > Version: 8.0.100 / Virus Database: 270.4.1/1514 - Release Date:
> > 6/23/2008
> > 7:17 AM
> >
>
>
>
> --
> oo 00 oo 00 oo 00 oo 00 oo 00 oo 00 oo 00 oo 00 oo 00 oo 00
> oo 00 oo 00 oo 00 oo 00 oo "If you want your children to be
> intelligent, read them fairy tales. If you want them to be
> more intelligent, read them more fairy tales." (Albert
> Einstein)
>
No virus found in this outgoing message.
Checked by AVG.
Version: 8.0.100 / Virus Database: 270.4.1/1514 - Release Date: 6/23/2008 7:17 AM