What Are Robots, Spiders, and Crawlers?
You should already have a general understanding that a robot, spider, or crawler is a piece of software
that is programmed to “crawl” from one web page to another based on the links on those pages. As this
crawler makes it way around the Internet, it collects content (such as text and links) from web sites
and saves those in a database that is indexed and ranked according to the search engine algorithm.
When a crawler is first released on the Web, it’s usually seeded with a few web sites and it begins on
one of those sites. The first thing it does on that first site is to take note of the links on the page. Then
it “reads” the text and begins to follow the links that it collected previously. This network of links is
called the crawl frontier; it’s the territory that the crawler is exploring in a very systematic way.
The links in a crawl frontier will sometimes take the crawler to other pages on the same web site,
and sometimes they will take it away from the site completely. The crawler will follow the links
until it hits a dead end and then backtrack and begin the process again until every link on a page
has been followed. Figure 16-1 illustrates the path that a crawler might take.
The crawler starts with a seed URL and works it way outward on the Web.
As to what actually happens when a crawler begins reviewing a site, it’s a little more complicated
than simply saying that it “reads” the site. The crawler sends a request to the web server where the
web site resides, requesting pages to be delivered to it in the same manner that your web browser
requests pages that you review. The difference between what your browser sees and what the
crawler sees is that the crawler is viewing the pages in a completely text interface. No graphics or
other types of media files are displayed. It’s all text, and it’s encoded in HTML. So to you it might
look like gibberish.
The crawler can request as many or as few pages as it’s programmed to request at any given time.
This can sometimes cause problems with web sites that aren’t prepared to serve up dozens of pages
of content at a time. The requests will overload the site and cause it to crash, or it can slow down
Optimizing Search Strategies
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