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multiple languages, both human and artiicial. A web server knows who you
are (to be precise, it knows the Internet address of your computer and what
browser is being used), it keeps track of each request you make, and it logs
whether it was able to comply with the request.
he Web has a client/server architecture, as illustrated in Figure 1.3. Most
Internet protocols are client/server, including File Transfer Protocol (FTP),
email, and many online games. A web server is a computer that resides on a
rack somewhere, or is tucked into a back closet, patiently waiting for a client
program to send it a request it can fulill. As far as the web server is concerned,
anything that sends it a request is considered an important client. In Web-
speak, the client programs are called user agents. Web browsers are the most
important user agents. Robots, or “bots” as they are sometimes called, are
another kind.
HTTP Request
User Agent
Web Browser
Search Robot
Web Server
HTTP Response
Figure 1.3: The Web's client/server architecture
Widgets can also be user agents. Loosely deined, a widget is a small com-
puter program. It is packaged so that it can be easily installed as an extension
of a larger computer program, such as a web browser or mobile device, and it
runs in its user interface. A widget can, in response to a mouse click or other
user action, send requests to web servers just like browsers and robots do.
Unlike robots running on large servers, organizing large masses of informa-
tion, a widget typically uses the returned information to update the content in
a speciic page element.
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