@Override
    public KV<Object, Object> convertToSamzaMessage(SamzaSqlRelMessage relMessage) {
      String jsonValue = convertToSamzaMessage(relMessage.getSamzaSqlRelRecord());
      return new KV<>(relMessage.getKey(), jsonValue.getBytes());
    }
SamzaRelConverter that can convert {@link SamzaSqlRelMessage} to json string byte array.
samza-tools/src/main/java/org/apache/samza/tools/json/JsonRelConverterFactory.java