final CrawlerClientFactory crawlerClientFactory = crawlingConfig.initializeClientFactory(ComponentUtil::getCrawlerClientFactory);
        final CrawlerClient client = crawlerClientFactory.getClient(url);
        if (client == null) {
            throw new CrawlingAccessException(
                    "CrawlerClient is null for URL: " + url + ". Unable to access the document without a crawler client.");
        }

└── fess-crawler-opensearch/   # OpenSearch backend
```

### Key Design Patterns

- **Factory**: `CrawlerClientFactory`, `ExtractorFactory` - protocol/format-specific component selection
- **Strategy**: `CrawlerClient`, `Extractor`, `Transformer` - pluggable implementations
- **Builder**: `RequestDataBuilder`, `ExtractorBuilder` - fluent construction
- **Template Method**: `AbstractCrawlerClient`, `AbstractExtractor` - common logic with overrides