code: public  DDF load(DataSourceDescriptor dataSourceDescriptor,
                    Boolean persist) throws DDFException {
      Class sourceClass = dataSourceDescriptor.getClass();
      DDF ddf = null;
      if (sourceClass.equals(S3DataSourceDescriptor.class)) {
        ddf = loadFromS3((S3DataSourceDescriptor)dataSourceDescriptor);
      } else if (sourceClass.equals(HDFSDataSourceDescriptor.class)){
        ddf = loadFromHDFS((HDFSDataSourceDescriptor)dataSourceDescriptor);
      } else if (sourceClass.equals(JDBCDataSourceDescriptor.class)) {
        ddf = loadFromJDBC((JDBCDataSourceDescriptor)dataSourceDescriptor);
      } else if (sourceClass.equals(SQLDataSourceDescriptor.class)) {
        ddf = loadFromSQL((SQLDataSourceDescriptor) dataSourceDescriptor);
      } else {
        throw new DDFException("Cannot find datasource " + sourceClass);
      }
      return ddf;
    }
docs: To override