How do Web applications work? - The Tao of Computing - page 349

5. Relevant Web pages are ordered according to perceived rele
vance, according to some (proprietary) criteria.
6. The results are returned to your browser.
Let's explore each of these points in somewhat more detail.
When you type terms or phrases into a search engine form and
click “Submit,” your request typically is added as a query string
to the URL after a question mark (recall our discussion of ex
tended URLs in Chapter 11). For technical reasons, blank spaces
usually are replaced by plus signs (+); otherwise, you can read
your query just by looking at the last part of the URL, which you
can view in the address or location field of your browser when the
new page loads. The search engine uses the query string as input
to a program, and that program compiles the results of your
search, which are sent to you when it finishes. This queryre
sponse format is common to virtually all Web applications; what
makes search engines special has to do with the middle points
listed earlier: the database, information areas, searching, and or
dering of results.
How do search engines find out about materials on
the Web?
Every search engine maintains a database containing informa
tion about Web pages; that is, when you initiate a search for specific
pages, a search engine does not start scanning information out on
the Web to locate material you might want. Rather, that material al
ready has been collected by the search engine, so the process focuses
on identifying material already found rather than looking for new
material. For example, in April 2004, the search engine Google
stated that it provided “access to more than 4 billion web pages”
( http://www.google.com/help/features.html). The same press release
states,
Google takes a snapshot of each page examined as it
crawls the Web and caches these as a backup in case the
original page is unavailable. If you click on the “Cached”
link, you will see the page as it looked when we indexed
it. The cached content is the content Google uses to judge
whether this page is a relevant match for your query.