code: def _write_log(self, version_key, meta_data, index_fields):
          
          meta_data = meta_data or {}
          meta_data.update({
              'version_key': version_key,
              'timestamp': time.time(),
          })
          obj = log_bucket.new(data=meta_data)
          obj.add_index('version_key_bin', version_key)
          obj.add_index('timestamp_int', int(meta_data['timestamp']))
          for field, index_type in index_fields:
              obj.add_index('%s_%s' % (field, index_type), meta_data.get(field, ""))
          obj.store()
docs: Creates a log entry for current object,
          Args:
              version_key(str): Version_bucket key from _write_version().
              meta_data (dict): JSON serializable meta data for logging of save operation.
                  {'lorem': 'ipsum', 'dolar': 5}
              index_fields (list): Tuple list for secondary indexing keys in riak (with 'bin' or 'int').
                  [('lorem','bin'),('dolar','int')]
  
          Returns: