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If you do not find the required information using the tips from the previous
section, then you need to consult an online service to find the piece of
information you were looking for. Three different types of services have been
established over the years: crawlers, directories and meta search engines. The
first crawler was created back in 1993. It was called the World Wide Worm. It
crawled from one site to the next and indexed all pages by saving the content of
the web pages into a huge database. Crawlers or spiders visit a web page, read
it, and then follow links to other pages within the site and even follow links
to other sites. Web crawlers return to each site on a regular basis, such as
every month or two, to look for changes. Everything a crawler finds goes into a
database, which people are able to query. The advantage of the web crawlers is
that they have an extensive database with almost the complete Internet indexed
in it. The disadvantage is that you get thousands of web pages as a response for
almost any request. Web directories work a little differently. First of all they
contain a structured tree of information. All information entered into this tree
is either entered by the webmaster who wants to announce his new web page or by
the directory maintainer who looks at the web pages submitted. The directories
where webmasters can submit both the URL (Uniform Resource Locator) and the
description normally contain more misleading information. Getting into most web
directories is a combination of luck and quality. Although anyone is able to
submit a web page, there is no guarantee that a certain page will be included.
Some directories charge for submission, which is basically a rip-off. Some
crawlers maintain an associated directory. If your site is in the directory, the
ranking of your web site normally will be higher, as it is already
pre-classified. Many directories work with crawlers together, in order to
deliver results, when nothing can be found in the directory. Meta search engines
do not have a database with URLs and descriptions. Instead they have a database
of search engines. If you enter your keywords into a meta search engine it will
send out requests to all the directories and crawlers it has stored in its
database. The meta search engines with a more sophisticated application in the
background are able to detect double URLs which come back from the various
search engines and present only a single URL to the customer.
34. Crawler is a
kind of__________.
A. worm B. search engine C. service D. directory
35. The
disadvantage of Crawler is that it__________.
A. provides a complete database
B.
takes too much time to find the needed information
C. provides too much useless
information
D. gives response to every quest
36. Who can input information to
the web directories?
A. Only webmasters.
B. Both directory maintainers and
webmasters.
C. Any one who wants to announce a web page.
D. Any one who has
found a new web page.
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