code: public void map(K key,
                      V value,
                      OutputCollector<BytesWritable, BytesWritable> output,
                      Reporter reporter) throws IOException {
  
          byte[] keyBytes = keySerializer.toBytes(makeKey(key, value));
          byte[] valBytes = valueSerializer.toBytes(makeValue(key, value));
  
          this.collectorWrapper.setCollector(output);
          this.mapper.map(keyBytes, valBytes, this.collectorWrapper);
      }
docs: Create the voldemort key and value from the input key and value and map
  it out for each of the responsible voldemort nodes
  
  The output key is the md5 of the serialized key returned by makeKey().
  The output value is the node_id & partition_id of the responsible node
  followed by serialized value returned by makeValue() OR if we have
  setKeys flag on the serialized key and serialized value