1. Crawling the Web
Search engines run automated programs, called “bots” or “spiders”, that use the hyperlink structure of the web to “crawl” the pages and documents that make up the World Wide Web. Estimates are that of the approximately 20 billion existing pages, search engines have crawled between 8 and 10 billion.
2. Indexing Documents
Once a page has been crawled, its contents can be “indexed” - stored in a giant database of documents that makes up a search engine’s “index”. This index needs to be tightly managed so that requests which must search and sort billions of documents can be completed in fractions of a second.
3. Processing Queries
When a request for information comes into the search engine (hundreds of millions do each day), the engine retrieves from its index all the document that match the query. A match is determined if the terms or phrase is found on the page in the manner specified by the user.
4. Ranking Results
Once the search engine has determined which results are a match for the query, the engine’s algorithm (a mathematical equation commonly used for sorting) runs calculations on each of the results to determine which is most relevant to the given query. They sort these on the results pages in order from most relevant to least so that users can make a choice about which to select.
Although a search engine’s operations are not particularly lengthy, systems like Google, Yahoo, MSN are among the most complex, processing-intensive computers in the world, managing millions of calculations each second and funneling demands for information to an enormous group of users.
Watch out
Certain types of navigation may hinder or entirely prevent search engines from reaching your website’s content. As search engine spiders crawl the web, they rely on the architecture of hyperlinks to find new documents and revisit those that may have changed. In the analogy of speed bumps and walls, complex links and deep site structures with little unique content may serve as “bumps.” Data that cannot be accessed by spiderable links qualify as “walls.”
Possible “watch outs” for Search engines Spiders:
* URLs with 2+ dynamic parameters; i.e. http://www.url.com/page.php?id=4&CK=34rr&User=%Tom% (spiders may be reluctant to crawl complex URLs like this because they often result in errors with non-human visitors)
* Pages with more than 100 unique links to other pages on the site (spiders may not follow each one)
* Pages buried more than 3 clicks/links from the home page of a website (unless there are many other external links pointing to the site, spiders will often ignore deep pages)
* Pages requiring a “Session ID” or Cookie to enable navigation (spiders may not be able to retain these elements as a browser user can)
* Pages that are split into “frames” can hinder crawling and cause confusion about which pages to rank in the results.
Possible “Walls” for Search engines Spiders:
* Pages accessible only via a select form and submit button
* Pages requiring a drop down menu (HTML attribute) to access them
* Documents accessible only via a search box
* Documents blocked purposefully (via a robots meta tag or robots.txt file - see more on these here)
* Pages requiring a login
* Pages that re-direct before showing content (search engines call this cloaking or bait-and-switch and may actually ban sites that use this tactic)
The key to ensuring that a site’s contents are fully crawlable is to provide direct, HTML links to each page you want the search engine spiders to index. Remember that if a page cannot be accessed from the home page (where most spiders are likely to start their crawl), it is likely that it will not be indexed by the search engines. A sitemap (which is discussed later in this guide) can be of tremendous help for this purpose.
Measuring Relevance and Popularity
Modern commercial search engines rely on the science of information retrieval (IR). That science has existed since the middle of the 20th century, when retrieval systems powered computers in libraries, research facilities, and government labs. Early in the development of search systems, IR scientists realized that two critical components made up the majority of search functionality:
Relevance - the degree to which the content of the documents returned in a search matched the user’s query intention and terms. The relevance of a document increases if the terms or phrase queried by the user occurs multiple times and shows up in the title of the work or in important headlines or sub headers.
Popularity - the relative importance, measured via citation (the act of one work referencing another, as often occurs in academic and business documents) of a given document that matches the user’s query. The popularity of a given document increases with every other document that references it.
These two items were translated to web search 40 years later and manifest themselves in the form of document analysis and link analysis.
In document analysis, search engines look at whether the search terms are found in important areas of the document - the title, the meta data, the heading tags, and the body of text content. They also attempt to automatically measure the quality of the document (through complex systems beyond the scope of this guide).
In link analysis, search engines measure not only who is linking to a site or page, but what they are saying about that page/site. They also have a good grasp on who is affiliated with whom (through historical link data, the site’s registration records, and other sources), who is worthy of being trusted (links from .edu and .gov pages are generally more valuable for this reason), and contextual data about the site the page is hosted on (who links to that site, what they say about the site, etc.).
Link and document analysis combine and overlap hundreds of factors that can be individually measured and filtered through the search engine algorithms (the set of instructions that tells the engines what importance to assign to each factor). The algorithm then determines scoring for the documents and (ideally) lists results in decreasing order of importance (rankings).
Information Search Engines Can Trust
The theory goes that if hundreds or thousands of other websites link to you, your site must be popular, and thus, have value. If those links come from very popular and important (and thus, trustworthy) websites, their power is multiplied to even greater degrees. Links from sites like NYTimes.com, Yale.edu, Whitehouse.gov, and others carry with them inherent trust that search engines then use to boost your ranking position. If, on the other hand, the links that point to you are from low-quality, interlinked sites or automated garbage domains (aka link farms), search engines have systems in place to discount the value of those links.