Question

I have a table called User with two columns, one called visitorId and the other called friend which is a list of strings. I want to check whether the VisitorId is in the friendlist. Can anyone direct me as to how to access the table columns in a map function?
I’m not able to picture how data is output from a map function in hbase.
My code is as follows:

ublic class MapReduce {

static class Mapper1 extends TableMapper<ImmutableBytesWritable, Text> {

    private int numRecords = 0;
    private static final IntWritable one = new IntWritable(1);       
    private final IntWritable ONE = new IntWritable(1);
    private Text text = new Text();

    @Override
    public void map(ImmutableBytesWritable row, Result values, Context context) throws IOException {

        //What should i do here??
        ImmutableBytesWritable userKey = new ImmutableBytesWritable(row.get(), 0, Bytes.SIZEOF_INT);

        context.write(userkey,One);     
    }

            //context.write(text, ONE);
        } catch (InterruptedException e) {
            throw new IOException(e);
        }

    }
}



public static void main(String[] args) throws Exception {
    Configuration conf = HBaseConfiguration.create();
    Job job = new Job(conf, "CheckVisitor");
    job.setJarByClass(MapReduce.class);
    Scan scan = new Scan();
    Filter f = new RowFilter(CompareOp.EQUAL,new SubstringComparator("mId2"));
    scan.setFilter(f);
    scan.addFamily(Bytes.toBytes("visitor"));
    scan.addFamily(Bytes.toBytes("friend"));
    TableMapReduceUtil.initTableMapperJob("User", scan, Mapper1.class, ImmutableBytesWritable.class,Text.class, job);

}

}

Answer 1

So Result values instance would contain the full row from the scanner.
To get the appropriate columns from the Result I would do something like :-

VisitorIdVal = value.getColumnLatest(Bytes.toBytes(columnFamily1), Bytes.toBytes(“VisitorId”))

friendlistVal = value.getColumnLatest(Bytes.toBytes(columnFamily2), Bytes.toBytes(“friendlist”))

Here VisitorIdVal and friendlistVal are of the type keyValue http://archive.cloudera.com/cdh/3/hbase/apidocs/org/apache/hadoop/hbase/KeyValue.html, to get their values out you can do a Bytes.toString(VisitorIdVal.getValue())
Once you have extracted the values from columns you can check for “VisitorId” in “friendlist”