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How Do Search Engines Work?

August 19, 2011

By Carol Amato

Search Engines are special sites on the World Wide 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 3 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.

How Do Search Engines Work?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.  There are two key components involved in making the gathered data accessible to users:

  • The information stored with the data, and
  • 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.
How Do Search Engines Work?To make for more useful results, most search engines store more than just the word and URL.  A search engine might store the number of times that the word appears on a page.

How do search engines work?  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.

When a person requests a search on a keyword or phrase, the search engine software searches the index for relevant information.  The software How Do Search Engines Work?then provides a report back to the searcher with the most relevant web pages listed first.

I highly recommend Google XML Site Map plugin which aids in organizing your site’s information, making it easier to search over by the spiders.  This plugin is completely free.

Check out an interesting post I did called, What is Scroogle?

 

 

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  • Mark Zalusky

    The search engine is Scroogle

    • http://www.CarolAmato.com/ Carol Amato

      YOU ARE A WINNER!!

      You are the first one to answer the question in my email….yay! Thanks for participating in this fun little challenge.

      Let me know (contact page) what your PayPal email address is, and if you don’t have a PayPal account, please send me your shipping address so I can send a prize.

      Thanks for commenting.
      Carol

  • JJ Mallrich

    Is the answer Scroogle for today?

    Great article as usual Carol, very informative.

    Cheers,
    JJ

    • http://www.CarolAmato.com/ Carol Amato

      Thanks, JJ,

      You are correct in your answer – appreciate your participation!

      Have a great weekend. :)

      Carol

  • http://sites.google.com/site/impoboxes/ Paulo Teller

    Hi Carol,
    Always taking the stress out of the people, isn’t it? Subject for today – spiders.
    Seriously, now, I couldn’t (and wouldn’t) explain it in a better way to readers from many walks. Nice job – again!
    The post about Scroogle? Delicious. I had forgotten that “bodyguard”, completely.
    Congratulations on this post of yours and best wishes to all,
    PT

    • http://www.CarolAmato.com/ Carol Amato

      Hello Paulo,

      Yep, Scroogle was the answer, but Mark beat everyone to the punch! :)

      Have a great weekend.
      Carol

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