Before delving into the details of building a Web-based application, it might be helpful to review the architectural model of the Web, and the roles of the browser and server in that model.
Typically, cooperating applications can be categorized as either a client or a server. The client application requests services and data from the server and the server application responds to client requests. Early two-tier (client/server) applications were developed to access large databases, and combined the rules used to manipulate the data with the user interface into the client application. The server's task was simply to process requests for data storage and retrieval, as many and as fast as possible.
Two-tier applications perform many of the functions of stand-alone systems; they present a user interface, gather and process user input, perform the requested processing, and report the status of the request. This sequence of commands may be repeated as many times as an application runs. Because servers only provide access to the data, the client uses its local resources to perform most of the processing. The client application must contain information about where the data resides and how it is laid out in the database. Once the data has been retrieved, the client is responsible for formatting and displaying it to the user.
One major advantage of the client/server model is that by allowing multiple users to simultaneously access the same application data, updates from one computer were instantly made available to all computers that had access to the server. However, as the number of clients increased, the server was quickly overwhelmed by client requests. Also, since much of the processing logic was tied to a monolithic suite of applications, changes in business rules lead to expensive and time-consuming alterations to source code. Although the ease and flexibility of two-tier products continue to drive many small-scale business applications, an increasingly competitive need for faster data access and more rapid developmental timelines has persuaded systems developers to seek out a new way of creating distributed applications.