@Override
    public int compare(WritableComparable w1, WritableComparable w2) {
      LinkKey k1 = (LinkKey) w1;
      LinkKey k2 = (LinkKey) w2;

      return k1.getChainId().compareTo(k2.getChainId());
    }
Comparator used to figure out if a linkKey should be grouped together. This is based upon the
linkKey's ChainId.
hbase-it/src/test/java/org/apache/hadoop/hbase/mapreduce/IntegrationTestBulkLoad.java