Web crawlers, also known as spiders or spiderbots, are computer programs that systematically browse web pages to index content and other information over the internet. They are typically operated by search engines for the purpose of web indexing or web spidering. Web crawlers are used to fulfill two main functions:
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Web indexing: Web crawlers systematically browse the web to index content for search engines. They index all the pages on the internet by using automated scripts or programs so that users can find information quickly.
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Web scraping: Web crawlers are also used by companies other than search engines to retrieve web information. This is known as web scraping or web data extraction. Web scraping involves using web crawlers to scan and store content from a targeted webpage to create a dataset, be it product prices for e-commerce or finance news for investment analysis.
Web crawlers work by starting at a seed, or list of known URLs, reviewing and then categorizing the webpages. Before each page is reviewed, the web crawler looks at the webpages robots.txt file, which specifies the rules for bots that access the website. These rules define which pages can be crawled and the links that can be followed. Then, they start to crawl by using a set of known URLs. The spiders follow the hyperlinks contained in those pages and repeat the cycle on the newly found websites. The crawlers goal is to download and index as much content as possible from the websites it visits.
Web crawlers are a central part of search engines, and details on their algorithms and architecture are kept as business secrets. When crawler designs are published, there is often an important lack of detail that prevents others from reproducing the work. There are also emerging concerns about "search engine spamming," which prevent major search engines from publishing their ranking algorithms.
