Automated search engine robots, sometimes called "spiders" or "crawlers", are the seekers of web pages. How do they work? What is it they really do? Why are they important?

You'd think with all the fuss about indexing web pages to add to search engine databases, that robots would be great and powerful beings. Wrong. Search engine robots have only basic functionality like that of early browsers in terms of what they can understand in a web page. Like early browsers, robots just can't do certain things. Robots don't understand frames, Flash movies, images or JavaScript. They can't enter password protected areas and they can't click all those buttons you have on your website. They can be stopped cold while indexing a dynamically generated URL and slowed to a stop with JavaScript navigation.

How Do Search Engine Robots Work?

Think of search engine robots as automated data retrieval programs, traveling the web to find information and links.

When you submit a web page to a search engine at the "Submit a URL" page, the new URL is added to the robot's queue of websites to visit on its next foray out onto the web. Even if you don't directly submit a page, many robots will find your site because of links from other sites that point back to yours. This is one of the reasons why it is important to build your link popularity and to get links from other topical sites back to yours.

When arriving at your website, the automated robots first check to see if you have a robots.txt file. This file is used to tell robots which areas of your site are off-limits to them. Typically these may be directories containing only binaries or other files the robot doesn't need to concern itself with.

Robots collect links from each page they visit, and later follow those links through to other pages. In this way, they essentially follow the links from one page to another. The entire World Wide Web is made up of links, the original idea being that you could follow links from one place to another. This is how robots get around.

The "smarts" about indexing pages online comes from the search engine engineers, who devise the methods used to evaluate the information the search engine robots retrieve. When introduced into the search engine database, the information is available for searchers querying the search engine. When a search engine user enters their query into the search engine, there are a number of quick calculations done to make sure that the search engine presents just the right set of results to give their visitor the most relevant response to their query.

You can see which pages on your site the search engine robots have visited by looking at your server logs or the results from your log statistics program. Identifying the robots will show you when they visited your website, which pages they visited and how often they visit. Some robots are readily identifiable by their user agent names, like Google's "Googlebot"; others are bit more obscure, like Inktomi's "Slurp". Still other robots may be listed in your logs that you cannot readily identify; some of them may even appear to be human-powered browsers.

Along with identifying individual robots and counting the number of their visits, the statistics can also show you aggressive bandwidth-grabbing robots or robots you may not want visiting your website. In the resources section of the end of this article, you will find sites that list names and IP addresses of search engine robots to help you identify them.

How Do They Read The Pages On Your Website?

When the search engine robot visits your page, it looks at the visible text on the page, the content of the various tags in your page's source code (title tag, meta tags, etc.), and the hyperlinks on your page. From the words and the links that the robot finds, the search engine decides what your page is about. There are many factors used to figure out what "matters" and each search engine has its own algorithm in order to evaluate and process the information. Depending on how the robot is set up through the search engine, the information is indexed and then delivered to the search engine's database.

The information delivered to the databases then becomes part of the search engine and directory ranking process. When the search engine visitor submits their query, the search engine digs through its database to give the final listing that is displayed on the results page.

The search engine databases update at varying times. Once you are in the search engine databases, the robots keep visiting you periodically, to pick up any changes to your pages, and to make sure they have the latest info. The number of times you are visited depends on how the search engine sets up its visits, which can vary per search engine.

Sometimes visiting robots are unable to access the website they are visiting. If your site is down, or you are experiencing huge amounts of traffic, the robot may not be able to access your site. When this happens, the website may not be re-indexed, depending on the frequency of the robot visits to your website. In most cases, robots that cannot access your pages will try again later, hoping that your site will be accessible then.

List of robots and protocols for setting up a robots.txt file.
April 14, 2003

Daria Goetsch is the founder and Search Engine Marketing Consultant for Search Innovation, a Search Engine Promotion company serving small businesses. Besides running her own company, Daria is an associate of, an Internet web marketing strategies company. She has specialized in search engine optimization since 1998, including three years as the Search Engine Specialist for O'Reilly & Associates, a technical book publishing company.

Search Engine Guide > Daria Goetsch > Search Engine Robots - How They Work, What They Do (Part I)