How Search Engines Work

Internet search engines are special sites on the Internet 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.

 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.

Spidering

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 websites.

When a spider is building its lists, the process is called 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.

Indexing

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. There are two key components involved in making the gathered data accessible to users:

 The information stored with the data

 The method by which the information is indexed

In the simplest case, a search engine could just store the word and the URL where it was found. In reality, this would make for an engine of limited use, since there would be no way of telling whether the word was used in an important or a trivial way on the page, whether the word was used once or many times or whether the page contained links to other pages containing the word. In other words, there would be no way of building the ranking list that tries to present the most useful pages at the top of the list of search results.

To make for more useful results, most search engines store more than just the word and URL. An engine might store the number of times that the word appears on a page. The engine might assign a weight to each entry, with increasing values assigned to words as they appear near the top of the document, in sub-headings, in links, in the meta tags or in the title of the page. Each commercial search engine has a different formula for assigning weight to the words in its index. This is one of the reasons that a search for the same word on different search engines will produce different lists, with the pages presented in different orders.

An index has a single purpose: It allows information to be found as quickly as possible. There are quite a few ways for an index to be built, but one of the most effective ways is to build a hash table. In hashing, a formula is applied to attach a numerical value to each word. The formula is designed to evenly distribute the entries across a predetermined number of divisions. This numerical distribution is different from the distribution of words across the alphabet, and that is the key to a hash table's effectiveness.

The Search Engine Program

The search engine software or program is the final part. When a person requests a search on a keyword or phrase, the search engine software searches the index for relevant information. The software then provides a report back to the searcher with the most relevant web pages listed first.

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