HBase - Hadoop: The Definitive Guide - page 713

MapReduce
HBase classes and utilities in the org.apache.hadoop.hbase.mapreduce pack-
age facilitate using HBase as a source and/or sink in MapReduce jobs. The TableIn-
putFormat class makes splits on region boundaries so maps are handed a single region
to work on. The TableOutputFormat will write the result of the reduce into HBase.
The SimpleRowCounter class in Example 20-2 (which is a simplified version of
RowCounter in the HBase mapreduce package) runs a map task to count rows using
TableInputFormat .
Example 20-2. A MapReduce application to count the number of rows in an HBase table
public class SimpleRowCounter extends Configured implements Tool {
static class RowCounterMapper extends
TableMapper < ImmutableBytesWritable , Result > {
public static enum Counters { ROWS }
@Override
public void map ( ImmutableBytesWritable row , Result value , Context
context ) {
context . getCounter ( Counters . ROWS ). increment ( 1 );
}
}
@Override
public int run ( String [] args ) throws Exception {
if ( args . length != 1 ) {
System . err . println ( "Usage: SimpleRowCounter <tablename>" );
return - 1 ;
}
String tableName = args [ 0 ];
Scan scan = new Scan ();
scan . setFilter ( new FirstKeyOnlyFilter ());
Job job = new Job ( getConf (), getClass (). getSimpleName ());
job . setJarByClass ( getClass ());
TableMapReduceUtil . initTableMapperJob ( tableName , scan ,
RowCounterMapper . class , ImmutableBytesWritable . class ,
Result . class , job );
job . setNumReduceTasks ( 0 );
job . setOutputFormatClass ( NullOutputFormat . class );
return job . waitForCompletion ( true ) ? 0 : 1 ;
}