Web Crawler Primer

Search Engines! - For most people, this term immediatly translates to Google! 
But almost 60% of the people don’t know how it works. So I thought of making this document about the basics of a search engine…. A web crawler!

A Search Engine Spider (also known as a crawler, Robot, SearchBot or simply a Bot) is a program that most search engines use to find what’s new on the Internet. Google’s web crawler is known as GoogleBot. There are many types of web spiders in use, but for now, we’re only interested in the Bot that actually “crawls” the web and collects documents to build a searchable index for the different search engines. 

The program starts at a website and follows every hyperlink on each page. So we can say that everything on the web will eventually be found and spidered, as the so called “spider” crawls from one website to another. Search engines may run thousands of instances of their web crawling programs simultaneously, on multiple servers (like the one shown above). When a web crawler visits one of your pages, it loads the site’s content into a database. Once a page has been fetched, the text of your page is loaded into the search engine’s index, which is a massive database of words, and where they occur on different web pages. All of this may sound too technical for most people, but it’s important to understand the basics of how a Web Crawler works.

So there are basically three steps that are involved in the web crawling procedure. First, the search bot starts by crawling the pages of your site. Then it continues indexing the words and content of the site, and finally it visit the links (web page addresses or URLs) that are found in your site. When the spider doesn’t find a page, it will eventually be deleted from the index. However, some of the spiders will check again for a second time to verify that the page really is offline.

The first thing a spider is supposed to do when it visits your website is look for a file called “robots.txt”. This file contains instructions for the spider on which parts of the website to index, and which parts to ignore. The only way to control what a spider sees on your site is by using a robots.txt file. All spiders are supposed to follow some rules, and the major search engines do follow these rules for the most part. Fortunately, the major search engines like Google or Bing are finally working together on standards.

Here's a short video by Google explaining their search process