Editing Web crawler (section)

{{short description|Software which systematically browses the World Wide Web}}
{{hatnote group|
{{about|the internet bot|the search engine|WebCrawler}}
{{redirect-distinguish|Web spider|Spider web}}
{{redirect|Spiderbot|the video game|Arac (video game)}}
}}
{{Use dmy dates|date=September 2020}}
[[File:WebCrawlerArchitecture.svg|thumb|Architecture of a Web crawler]]

'''Web crawler''', sometimes called a '''spider''' or '''spiderbot''' and often shortened to '''crawler''', is an [[Internet bot]] that systematically browses the [[World Wide Web]] and that is typically operated by search engines for the purpose of [[Web indexing]] (''web spidering'').<ref>{{Cite web |title=Web Crawlers: Browsing the Web |url=https://webbrowsersintroduction.com/ |url-status=dead |archive-url=https://web.archive.org/web/20211206205907/https://webbrowsersintroduction.com/ |archive-date=2021-12-06}}</ref>

Web [[search engine]]s and some other [[website]]s use Web crawling or spidering [[software]] to update their [[web content]] or indices of other sites' web content. Web crawlers copy pages for processing by a search engine, which [[Index (search engine)|indexes]] the downloaded pages so that users can search more efficiently.

Crawlers consume resources on visited systems and often visit sites unprompted. Issues of schedule, load, and "politeness" come into play when large collections of pages are accessed. Mechanisms exist for public sites not wishing to be crawled to make this known to the crawling agent. For example, including a <code>[[robots.txt]]</code> file can request [[Software agent|bots]] to index only parts of a website, or nothing at all.

The number of Internet pages is extremely large; even the largest crawlers fall short of making a complete index. For this reason, search engines struggled to give relevant search results in the early years of the World Wide Web, before 2000. Today, relevant results are given almost instantly.

Crawlers can validate [[hyperlink]]s and [[HTML]] code. They can also be used for [[web scraping]] and [[data-driven programming]].