||A Crawler accesses the physical source represented by a DataSource and delivers a stream of DataObjects
representing the resources in that source.
||CrawlerFactories created Crawlers capable of crawling specific types of DataSources.
||CrawlerHandlers are notified by a Crawler about additions, changes and deletions or resources in a
||An CrawlerRegistry serves as a central registry for registering and obtaining CrawlerFactories.
||A CrawlReport instance contains statistics about the last performed or currently active crawl
procedure of a Crawler.