Visual Basic.NET How to Program (PDF)

Chapter 20 ASP .NET, Web Forms and Web Controls 945
The server then sends one or more HTTP headers, which provide additional information
about the data that will be sent. In this case, the server is sending an HTML text document,
so the HTTP header for this example reads:
Content-type: text/html
The information provided in this header specifies the Multipurpose Internet Mail Extensions
(MIME) type of the content that the server is transmitting to the browser. MIME is an
Internet standard that specifies the way in which certain data must be formatted so that programs
can interpret the data correctly. For example, the MIME type text/plain indicates
that the sent information is text that can be displayed directly, without any
interpretation of the content as HTML markup. Similarly, the MIME type image/gif indicates
that the content is a GIF image. When the browser receives this MIME type, it attempts
to display the image.
The header or set of headers is followed by a blank line, which indicates to the client
that the server is finished sending HTTP headers. The server then sends the contents of the
requested HTML document (downloads.htm). The server terminates the connection
when the transfer of the resource is complete. At this point, the client-side browser parses
the HTML it has received and renders (or displays) the results.
20.3 System Architecture
Web-based applications are multi-tier applications, which sometimes are referred to as ntier
applications. Multi-tier applications divide functionality into separate tiers (i.e., logical
groupings of functionality). Although tiers can be located on the same computer, the tiers
of Web-based applications typically reside on separate computers. Figure 20.3 presents the
basic structure of a three-tier Web-based application.
The information tier (also called the data tier or the bottom tier) maintains data pertaining
to the application. This tier typically stores data in a relational database management
system (RDBMS). We discussed RDBMSs in Chapter 19. For example, a retail store
might have a database for storing product information, such as descriptions, prices and
quantities in stock. The same database also might contain customer information, such as
user names, billing addresses and credit-card numbers. This tier can be comprised of multiple
databases, which together contain the data needed for our application.
Fig. 20.3 Three-tier architecture.