As we saw in Chapter 1, Internet applications are extremely demanding. Applications must be able to handle large numbers of users connected via slow or intermittent network connections, and the number of users at any particular time is difficult to predict. The three-tier application architecture is promoted by Windows DNA largely to meet the demands of Internet applications.
In an Internet application, a Web browser displays an HTML page_based presentation layer. Requests from the presentation layer are transmitted via HTTP to the Web server. The Web server handles these requests using business services that may in turn talk to data services.
ASP straddles the line between the presentation layer and the business layer. ASP pages are executed on the Web server, in response to requests from the Web browser. In this sense, they are part of the business layer. ASP pages contain server-side script code that uses middle-tier business objects to do much of the work. The business objects call data objects to access data sources, as described by the three-tier architecture. On the other hand, ASP pages are used to generate the presentation layer, HTML pages and the HTML and client-side script code is located within the ASP pages. So ASP pages look a bit like presentation layer code as well.