A focused crawler or topical crawler is a web crawler that attempts to download only web pages that are relevant to a pre-defined topic or set of topics. Topical crawling generally assumes that only the topic is given, while focused crawling also assumes that some labeled examples of relevant and not relevant pages are available. Topical crawling was first introduced by Menczer . Focused crawling was first introduced by Chakrabarti et al..

As an example of use, CiteSeerX, a search engine and digital library primarily for computer science, uses focused crawling to harvest its academic documents.

Strategies

A focused crawler ideally would like to download only web pages that are relevant to a particular topic and avoid downloading all others.

Therefore a focused crawler may predict the probability that a link to a particular page is relevant before actually downloading the page. A possible predictor is the anchor text of links; this was the approach taken by Pinkerton in a crawler developed in the early days of the Web. In a review of topical crawling algorithms, Menczer et al. show that such simple strategies are very effective for short crawls, while more sophisticated techniques such as reinforcement learning and evolutionary adaptation can give the best performance over longer crawls. Diligenti et al. propose to use the complete content of the pages already visited to infer the similarity between the driving query and the pages that have not been visited yet.

In another approach, the relevance of a page is determined after downloading its content. Relevant pages are sent to content indexing and their contained URLs are added to the crawl frontier; pages that fall below a relevance threshold are discarded.

The performance of a focused crawler depends mostly on the richness of links in the specific topic being searched, and focused crawling usually relies on a general web search engine for providing starting points.