The most widespread web robots are used by Internet search engines. Internet search engines allow users to find documents about any subject all around the world.
Many of the most popular sites on the Web today are search engines. They serve as a starting point for many web users and provide the invaluable service of helping users find the information in which they are interested.
Web crawlers feed Internet search engines, by retrieving the documents that exist on the Web and allowing the search engines to create indexes of what words appear in what documents, much like the index at the back of this book. Search engines are the leading source of web robots—let's take a quick look at how they work.
When the Web was in its infancy, search engines were relatively simple databases that helped users locate documents on the Web. Today, with the billions of pages accessible on the Web, search engines have become essential in helping Internet users find information. They also have become quite complex, as they have had to evolve to handle the sheer scale of the Web.
With billions of web pages and many millions of users looking for information, search engines have to deploy sophisticated crawlers to retrieve these billions of web pages, as well as sophisticated query engines to handle the query load that millions of users generate.
Think about the task of a production web crawler, having to issue billions of HTTP queries in order to retrieve the pages needed by the ...