code: @SuppressWarnings("unchecked")
      public void reduce(BytesWritable key,
                         Iterator<BytesWritable> iterator,
                         OutputCollector<Text, Text> output,
                         Reporter reporter) throws IOException {
  
          writer.write(key, iterator, reporter);
  
      }
docs: Reduce should get sorted MD5 of Voldemort key ( either 16 bytes if saving
  keys is disabled, else 8 bytes ) as key and for value (a) node-id,
  partition-id, value - if saving keys is disabled (b) node-id,
  partition-id, replica-type, [key-size, value-size, key, value]* if saving
  keys is enabled