192 Chapter 5 Basic Web Application Strategies Traditional
192 Chapter 5 Basic Web Application Strategies Traditional Client/Server In the past, a traditional, non-Web application was responsible for handling everything from user input to application logic to data storage.These three entities were interwoven with each other, making it difficult or impossible to change one of them without affecting the others. If you wanted to provide such an application to multiple users,you d have a problem:What if the data format changes? What happens if you re required to change the application logic? You d have to provide all users with a new local copy of the application, an unfeasible approach for larger systems. With the advent of object-oriented analysis, design, and programming principles, business applications became component-based. User interfaces were commonly deployed on workstations, and data and application logic was hosted on a mainframe server.The term client/server has been used to describe this separation of layers or tiers. A two-tier client/server architecture provides a basic separation of tasks.The client,or first tier, is responsible for the presentation of data to the user (user interface), and the server, or second tier, is responsible for supplying data services to the client and handling the application logic. So far, so good, but you see that the two-tier model still combines two distinct concepts in the second tier (the server): data services and application logic.The application logic is the heart of the program,responsible for data validation, processing rules, etc. The data services manage the application s raw data and consist often of a relational database management system (RDBMS) or a mainframe legacy system, in which a company has already invested considerable time and money.The interweaving of these two concepts in a two-tier application introduces problems of scalability, reuse, and maintenance.Take the recent introduction of the Euro as the official currency throughout the European Community: If an application hadn t cleanly separated application logic from data, both tiers would need to be changed. Hard-coded currency values in the data validation were to be altered as well as the data-storage format of currency amounts. Using a multi-tier approach, all three concepts can be cleanly separated. PHP and Multi-Tier Applications The concept of multi-tier systems became popular in the early 1990s and is now establishing itself as the enterprise application software architecture for the late 90s and early 21st century.A multi-tier (often referred to as three-tier) architecture provides greater application scalability, lower maintenance, and increased reuse of components. Three-tier architecture offers a technology-neutral method of building client/server applications with vendors who employ standard interfaces, providing services for each logical layer. Look at the three-tier model in Figure 5.5 doesn t it look familiar to you as a Web application developer? In the first tier is a thin client translated to the world of Web applications, this would be the browser.The middle tier (application server) is obviously PHP (and the Web server as host application), while the third tier consists of an RDBMS.
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