Search Engines and How They Work
Search Engines are special sites on the Web that are designed to help people find information stored on other sites. There are differences in the ways various Search Engines work, but they all perform three basic tasks:
􀀹 They search the Internet - or select pieces of the Internet - based on important words,
􀀹 They keep an index of the words they find, and where they find them, and
􀀹 They allow users to look for words or combinations of words found in that index.
Early Search Engines held an index of a few hundred thousand pages and documents, and received maybe one or two thousand inquiries each day. Today, a top Search Engine will index hundreds of millions of pages, and respond to tens of millions of queries per day.
Before a Search Engine can tell you where a file or document is, it must be found. To find information on the hundreds of millions of Web pages that exist, a Search Engine employs special software robots, called spiders, to build lists of the words found on Web sites.
When a spider is building its lists, the process is called web crawling.
In order to build and maintain a useful list of words, a Search Engine's spiders have to look at a lot of pages. How does any spider start its travels over the Web? The usual starting points are lists of heavily used servers and very popular pages. The spider will begin with a popular site, indexing the words on its pages and following every link found within the site. In this way, the spidering system quickly begins to travel, spreading out across the most widely used portions of the Web.
Once the spiders have completed the task of finding information on Web pages, the Search Engine must store the information in a way that makes it useful.