Maintaining currency of search engine indices by exhaustive crawling is rapidly becoming impossible due to the increasing size and dynamic content of the web. Focused crawlers aim to search only the subset of the web related to a speciﬁc category, and offer a potential solution to the currency problem. The major problem in focused crawling is performing appropriate credit assignment to different documents along a crawl path, such that short-term gains are not pursued at the expense of less-obvious crawl paths that ultimately yield larger sets of valuable pages. To address this problem we present a focused crawling algorithm that builds a model for the context within which topically relevant pages occur on the web. This context model can capture typical link hierarchies within which valuable pages occur, as well as model content on documents that frequently cooccur with relevant pages. Our algorithm further leverages the existing capability of large search engines to provide partial reverse crawling capabilities. Our algorithm shows signiﬁcant performance improvements in crawling efﬁciency over standard focused crawling.
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