So,using one of those crawlers presented in that article would be enough? I read something about Nutch,Lucene,Sphinx but I'm not sure how the process of building an agregator works: go crawl some websites,get the data in the database or disk, analyze the data, display the data according to some specified criterias. Are those crawlers spidering RSS feeds or a special crawler is needed for that? A diagram would be helpul...