Overwhelming number of search-engines in the WWW like Google, AltaVista, Lycos, InfoSeek etc. are spider-based. An understanding of how they work can greatly help you make the best out of them.

Though the term "search engine" is often used to describe all kinds of retrieval tools, spider-based search engines differ considerably from human-powered directories. We discussed human-powered directories in last issue, this week we take a close look at spider-based search engines.

Unlike directory-type search engines, spider-based search engines (also called crawlers, robots, worms) seek out webpages by 'crawling' through the WWW and automatically index sites using its own indexing rules or algorithm.

By simply telling the search engine what your URL is, its software robot will go there automatically and index everything they need. How much it will index and to what degree depends upon its algorithm - a closely guarded secret in many cases.

Parts of Spider-Based Search Engine

Spider-based search engines have three major elements:

1. Spider
2. Index
3. Search

The spider or crawler, as its name implies, crawls through the WWW, finds web page, reads it, and then follows links to other pages within the site. It repeats this process at regular intervals to check for new information/changes in the page.

Information collected by the spider goes into the second part of the search engine - the index. The index is like a giant book containing a copy of every web page that the spider finds. If a web page changes, then this book is updated with new information.

The above two parts work in the background, we only get to see the third part of a search engine - the search software. This is a computer program that sifts through the millions of pages recorded in the index to find matches to a search and rank them in an order of relevance. The order of relevance is entirely decided by its own algorithm.

Features of Spider-based Search Engine and Implication in Search Result

The ability of a spider to crawl through millions of web-pages and creating index without human intervention makes it very powerful search tool with extremely broad coverage. The second ability of checking for changes/new information in indexed pages by re-visiting them at regular intervals and keeping the index up-to-date, again without human intervention - is really awesome.

However, the greatest strength of spider-based search engine is also its greatest weakness. Great coverage and absence of human editing ensures significant amount of junk or useless information in search result. This is particularly so when search query is loosely worded.

The key to get the best out of a spider-based search engine is to understand some basics of searching. We shall discuss a few tips that can get you significantly better search result in next issues.