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The Magic of the Three-Tier Architecture
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Generally, the architecture refers to splitting each piece of the application s functionality into separate components based on what they do and grouping each kind of component into a single logical tier. Almost every module that you ll create for your site will have components in these three tiers from the application server: The presentation tier The business tier The data tier The presentation tier contains the user interface elements of the site and includes all the logic that manages the interaction between the visitor and the client s business. This tier makes the whole site feel alive, and the way you design it has a crucial importance for the site s success. Because your application is a web site, its presentation tier is composed of dynamic web pages. The business tier (also called the middle tier) receives requests from the presentation tier and returns a result to the presentation tier depending on the business logic it contains. Almost any event that happens in the presentation tier usually results in the business tier
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CHAPTER 2 LAYING OUT THE FOUNDATIONS
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being called (except events that can be handled locally by the presentation tier, such as simple input data validation, and so on). For example, if the visitor is doing a product search, the presentation tier calls the business tier and says, Please send me back the products that match this search criterion. Almost always, the business tier needs to call the data tier for information to be able to respond to the presentation tier s request. The data tier (sometimes referred to as the database tier) is responsible for managing the application s data and sending it to the business tier when requested. For the HatShop e-commerce site, you ll need to store data about products (including their categories and their departments), users, shopping carts, and so on. Almost every client request finally results in the data tier being interrogated for information (except when previously retrieved data has been cached at the business tier or presentation tier levels), so it s important to have a fast database system. In s 3 and 4, you ll learn how to design the database for optimum performance. These tiers are purely logical there is no constraint on the physical location of each tier. In theory, you are free to place all of the application, and implicitly all of its tiers, on a single server machine, or you can place each tier on a separate machine if the application permits this. 17 explains how to integrate functionality from other web sites using XML Web Services. XML Web Services permit easy integration of functionality across multiple servers. An important constraint in the three-layered architecture model is that information must flow in sequential order between tiers. The presentation tier is only allowed to access the business tier, and it can never directly access the data tier. The business tier is the brain in the middle that communicates with the other tiers and processes and coordinates all the information flow. If the presentation tier directly accessed the data tier, the rules of three-tier architecture programming would be broken. When you implement a three-tier architecture, you must be consistent and obey its rules to reap the benefits. Figure 2-1 is a simple representation of the way data is passed in an application that implements the three-tier architecture.
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Data Tier
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Figure 2-1. Simple representation of the three-tier architecture
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CHAPTER 2 LAYING OUT THE FOUNDATIONS
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It s easier to understand how data is passed and transformed between tiers if you take a closer look at a simple example. To make the example even more relevant to our project, let s analyze a situation that will actually happen in HatShop. This scenario is typical for three-tier applications. Like most e-commerce sites, HatShop will have a shopping cart, which we will discuss later in the book. For now, it s enough to know that the visitor will add products to the shopping cart by clicking an Add to Cart button. Figure 2-2 shows how the information flows through the application when that button is clicked. When the user clicks on the Add to Cart button for a specific product (step 1), the presentation tier (which contains the button) forwards the request to the business tier Hey, I want this product added to my shopping cart! (step 2). The business tier receives the request, understands that the user wants a specific product added to the shopping cart, and handles the request by telling the data tier to update the visitor s shopping cart by adding the selected product (step 3). The data tier needs to be called because it stores and manages the entire web site s data, including users shopping cart information. The data tier updates the database (step 4) and eventually returns a success code to the business tier. The business tier (step 5) handles the return code and any errors that might have occurred in the data tier while updating the database and then returns the output to the presentation tier.
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