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30 Gerard Salton The phrase vector space model, which search algorithms still heavily rely upon today, goes back to the 1970s. Gerard Salton was a well-known expert in the field of information retrieval who pioneered many of today's modern methods. If you are interested in learning more about early information retrieval systems, you may want to read A Theory of Indexing, which is a short book by Salton that describes many of the common terms and concepts in the information retrieval field. Mike Grehan's book, Search Engine Marketing: The Essential Best Practices Guide, also discusses some of the technical bits to information retrieval in more detail than this book. My book was created to be a current how-to guide, while his is geared more toward giving information about how information retrieval works. Parts of a Search Engine While there are different ways to organize web content, every crawling search engine has the same basic parts: • a crawler • an index (or catalog) • a search interface Crawler (or Spider) The crawler does just what its name implies. It scours the web following links, updating pages, and adding new pages when it comes across them. Each search engine has periods of deep crawling and periods of shallow crawling. There is also a scheduler mechanism to prevent a spider from overloading servers and to tell the spider what documents to crawl next and how frequently to crawl them. Rapidly changing or highly important documents are more likely to get crawled frequently. The frequency of crawl should typically have little effect on search relevancy; it simply helps the search engines keep fresh content in their index. The home page of CNN.com might get crawled once every ten minutes. A popular, rapidly growing forum might get crawled a few dozen times each day. A static site with little link popularity and rarely changing content might only get crawled once or twice a month. The best benefit of having a frequently crawled page is that you can get your new sites, pages, or projects crawled quickly by linking to them from a powerful or frequently changing page.