Database Reference
In-Depth Information
User enters the
URL for a Web
page using a
Web browser
Web page request
is transmitted over the Internet from
the Web client to the Web server
using HTTP and TCP/IP
HTTP and
TCP/IP
HTTP and
TCP/IP
Internet
Web server
transmits the Web page
over the Internet to the
Web client using
HTTP and TCP/IP
Web
server
Web browser
displays the Web
page on the
Web client
290
Web server locates
the requested Web
page stored
on disk
Web pages
FIGURE 9-7
Retrieving a Web page on the Internet
Each Web page is a text document that contains the necessary codes, called tags, that the Web browser
interprets to position and format the text in the Web page. A Web page can also contain tags for links to
audio files to be played, to graphics and animations to be displayed on the screen, and to other files, which
are all sent along with the Web page by the Web server. A Web page can also contain tags for hyperlinks,
which link one Web page to another or link to another location in the same Web page. Web pages (such as
index.html in the www.irs.gov/individuals/index.html URL) are usually created using a language called
Hypertext Markup Language (HTML). You can use a program such as ColdFusion or Adobe Dreamweaver to
create the HTML code for Web pages without needing to learn HTML. Many programs, including Microsoft
Access, have built-in tools that convert and export objects such as tables and queries to HTML documents.
Web pages that display the same content for all Web clients are called static Web pages. At the heart of
most Web processing today are activities
—
such as paying bills, ordering merchandise, buying and selling
stocks, and bidding in online auctions
—
for which the Web pages need to change depending on the Web
client
s input and responses; these business activities are called electronic commerce (e-commerce). For
e-commerce activities, Web servers can
'
t use static Web pages. Instead, Web servers use dynamic Web pages,
which are pages whose content changes in response to the different inputs and choices made using Web
clients. A dynamic Web page includes, or triggers, instructions to tell the Web server how to process the page
(server-side extensions or server-side scripts) and possibly other instructions for the Web browser to process
(client-side extensions or client-side scripts). Client-side extensions can be embedded in HTML documents or
contained in separate files that are referenced within the HTML documents, while server-side extensions are
usually separately executed programs. Client-side extensions can change the user interface in response to
user input actions; JavaScript and VBScript are examples of client-side extension languages. Because of the
processing complexities of server-side extensions and the difficulty of creating them, most server-side exten-
sions are created using programming development frameworks, such as ASP.NET and ColdFusion, although
the PHP scripting language is frequently used with the Apache HTTP Server.
Web servers must have a mechanism for communicating with server-side extensions; Common Gateway
Interface (CGI) and Application Program Interface (API) are standard interfaces that provide this capability. In
addition, server-side extensions usually include interaction with databases to send Web clients requested data
from databases and to update databases with data supplied by Web clients. Several standard software interfaces
have been developed to interact with DBMSs; Open Database Connectivity (ODBC), Java Database Connectivity
(JDBC), and ADO.NET are examples of these standard interfaces. These standard software interfaces include
many DBMS-specific drivers so that a given Web server can work with many different DBMSs.
One common Web-based architecture for dealing with dynamic Web pages, shown in Figure 9-8, uses a
three-tier architecture, with the Web clients, a Web server, and a database server as the three tiers. A user
on a Web client sends a request for a Web page to the Web server over the Internet using TCP/IP and HTTP.
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