code: protected RequiredFieldResponse pushProjectionHelper(
                                            RequiredFieldList requiredFieldList)
                                            throws FrontendException {
      try {
        getUDFProperties().setProperty(projectionKey,
                                       ObjectSerializer.serialize(requiredFieldList));
      } catch (IOException e) { // not expected
        throw new FrontendException(e);
      }
  
      return new RequiredFieldResponse(true);
    }
docs: A helper method for implementing
  {@link LoadPushDown#pushProjection(RequiredFieldList)}. <p>
  
  Stores requiredFieldList in context. The requiredFields are read from
  context on the backend (in side {@link #setLocation(String, Job)}).