What is a Spider ?
Spider or crawler is a program that visits Web sites and reads their pages and other information in order to create entries for a search engine index. The major search engines on the Web all have such a program, which is also known as a “spider” or a “bot.” Crawlers are typically programmed to visit sites that have been submitted by their owners as new or updated. Entire sites or specific pages can be selectively visited and indexed. Spiders apparently gained the name because they crawl through a site a page at a time, following the links to other pages on the site until all pages have been read.
Web crawlers collect information such the URL of the website, the meta tag information, the Web page content, the links in the webpage and the destinations leading from those links, the web page title and any other relevant information. They keep track of the URLs which have already been downloaded to avoid downloading the same page again. A combination of policies such as re-visit policy, selection policy, palatalization policy and politeness policy determine the behavior of the Web crawler. There are many challenges for web crawlers, namely the large and continuously evolving World Wide Web, content selection trade offs, social obligations and dealing with adversaries.
Web crawlers are the key components of Web search engines and systems that investigate web pages. They help in indexing the Web entries and allow users to send queries against the index and provide the web pages that match the queries. Another use of Web crawler is in Web archiving, which involves large sets of web pages to be periodically collected and archived. Web spiders are also used in data mining, wherein pages are analyzed for different properties like statistics, and data analytics are then performed on them.
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