Web crawlers and spiders, what they are and how they work?

  • 15/05/2021

They are the brains of modern search engines. They allow you to archive web pages and index them in the database of various Google, Bing, and Yahoo!

Google, Bing, Yahoo!, etc. adopt highly refined computer technologies to offer - almost instantly - thousands and thousands of results for every single search made by users. But what makes search engines work possible? Their brain, aka the web crawler spider.

What is a web crawler?

The web crawler spider (sometimes abbreviated simply to “spider” or “web crawler ") is an Internet bot that periodically scans the World Wide Web in order to create an index or, better still, a map. Search engines - and some other Internet services - use such software to update their content or to update the web indexes in their databases. The spiders can copy the content of all the pages they visit and keep it to allow the search engine to analyze and index it, or catalog it by identifying keywords and topics covered, at a later time. By doing this, it is possible to return search results quickly and accurately.

How the web crawler works?

A spider begins its work with the so-called seeds ("seeds" translated into Italian). The seeds are nothing more than a list of URLs, corresponding to as many websites that the program will have to visit systematically. The content of these addresses will be analyzed and saved in memory to be indexed by the cataloging software associated with the search engine. In particular, the web crawler will search for hypertext links within the pages, adding them to the list of URLs to visit later. The URLs of this list, called crawl frontier ("Indexing frontier" in Italian), are recursively visited by the spider, so as to be able to record any changes or updates.

Of course, the URLs and hyperlinks on the border pages will also be added to the general list and visited later. Thus is created a real network of Internet pages linked to each other through hypertext links (hyperlinks). Hence the explanation of the name spider ("spider" in English) and why Tim Berners-Lee decided to call the World Wide Web, more or less, his Internet-based service.

In the event that the crawler acts in "archiving" mode, it copies and stores the contents of every single page it visits. To speed up the process, the pages are saved as a snapshot, while remaining stay legible and navigable.

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