@Override
 public void delete(DataRecord record) {
   storage.remove(record);
 }
 @Override
 public void close(boolean dropExistingData) {
   storage.clear();
   isClosed = true;
 }
 @Override
 public void update(Iterable<DataRecord> records) {
   for (DataRecord record : records) {
     storage.add(record);
   }
 }