} else {
            if ((value instanceof final Collection<?> coll) && !coll.isEmpty()) {
                // care performance for List that the most frequent pattern
                final Object first = coll instanceof List<?> ? ((List<?>) coll).get(0) : coll.iterator().next();
                if (first instanceof Entity) {
                    data.register(key, coll.stream().map(BeanUtil::copyBeanToNewMap).collect(Collectors.toList()));

    }

    // ===================================================================================
    //                                                                           Roll-back
    //                                                                           =========
    @Override
    public void rollback() {
        for (final MultipartFormFile formFile : elementsFile.values()) {

            final List<Tuple3<String, String, String>> taskList = new ArrayList<>();
            while (generating) {
                try {
                    final Tuple3<String, String, String> task = thumbnailTaskQueue.poll(thumbnailTaskQueueTimeout, TimeUnit.MILLISECONDS);
                    if (task == null) {
                        if (!taskList.isEmpty()) {
                            storeQueue(taskList);
                        }

                final Map<String, Object> localDataMap =
                        dataMap.entrySet().stream().collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
                String processingUrl = urlQueue.poll();
                processedUrls.add(processingUrl);
                if (deleteUrlList.contains(processingUrl)) {
                    deleteDocuments(); // delete before indexing
                }
                try {