The Search Engine three stage process: crawl, index, retrieve.
Building a search engine database is a three-stage process. Search engines must find pages, organize the information for fast retrieval, and serve up the information based on your request. This is an ongoing process because the search engine databases are under constant development.
Stage 1 - Crawling:
Search engines compile huge databases of information. The database is continually updated by robotic 'crawlers or spiders' that automatically copy the contents of millions of pages of web information each day. However, not every page on every site is 'crawled and copied'. Typically a spider program takes a representative sample from a site before moving on. A spider program might copy 100 pages from a 300 page site. Valuable information may be missed. This un-indexed information is sometimes called the 'opaque' or partially visible web.
 |
Crawlers jump from web page to web page by following each site's links. A site that is linked to many others is more likely to be visited frequently. Isolated pages will be 'crawled' less often, or can be missed altogether. This rather haphazard method of collecting information can lead to gaps in a search engine's database. This explains in part why different search engines produce different results. It takes time and money to 'crawl' the net. Economic decisions are made that affect the depth, breadth and currency of a search engine's database. While currency and completeness are touted by each search engine, users should be aware that they are searching copies of information that could be months, even years old. |
Stage 2 - Indexing:
Indexing or cataloging is the process of organizing a search engine's database of web pages to maximize retrieval efficiency. The exact methods of indexing information used by commercial search engines remain closely guarded, proprietary information. Each search engine uses a variety of 'black box' indexing algorithms to sort the contents of a page. In general the occurrence of keywords, the proximity of keywords to each other, and the contents of html elements like meta tags, titles, and headers, are all taken into account as the index is created.
 |
The raw results delivered by the spider software, are'pre-processed' to eliminate duplicate pages, and remove 'spam' pages. Spam pages are designed to fool search engines and achieve a higher ranking by using rigged attributes such as misleading meta-tags. Unlike the collection of human edited pages found in Subject Indexes, there is no human judgment regarding the actual quality of information. Robotic crawling and indexing is an automated process involving millions of pages of information a day. Pages remain unavailable until they are indexed. There is lag time between the 'spidering' of a page and when it is indexed and made available to the users of the search engine. Once the web page copy is in place, it remains unchanged in the database until the next 'visit' by the search engine's crawler. |
Stage 3 - Retrieving:
Finally, a search engine must provide a way for a user to retrieve information from its database. Each search engine uses a proprietary 'retrieval algorithm' to process your query. Responding to a user query is usually a two-step procedure. First the system checks for records that match your query, then it sorts or arranges the results in some kind of hierarchy. Exactly how each search engine matches queries to records is a carefully guarded trade secret.
 |
In general search engines look for the frequency of key word matches, and the distribution of those words in a document to determine which information is relevant to your request. Key word matches found in titles and headings on the first page are given greater weight and considered semantically more relevant. To improve retrieval speed, many search engine's filter out high frequency words such as: or, to, with, and, if, etc. These words are sometimes referred to as 'stop words'. The most likely matches for your query are then displayed in a results list. The order in which matched pages are listed varies. Google lists results based in part on their popularity. Popularity is based on the number of other pages that link to the page in question. Commercial providers might also pay to be ranked at the top of the results list. |
Would you like to see an animated overview of how search engines work? Try Learnthenet.com!
