Database Applications and the Web - Thickening the Client in the Three-Tier Model
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Given that a web database application built with a three-tier architecture doesn’t fit naturally with HTTP, why use that model at all? The answer mostly lies in the popularity and standardization of web browsers: any user who has a web browser can use the web database application, and usually without any restrictions. This means an application can be delivered to any number of diverse, dispersed users who use any platform, operating system, or browser software. This advantage is so significant that our focus in this book is entirely on three-tier solutions that use a web browser as the client tier.
Web browsers are thin clients. This means almost no application logic is included in the client tier. The browser simply sends HTTP requests for resources and then displays the responses, most of which are HTML pages. This thin client model means you don’t have to build, install, or configure the client tier, but that you do need to build almost all of your application to run in the middle tier.
You can thicken the client tier to put more work on the browser. Using popular technologies such as Java, JavaScript, and Macromedia Flash, you can develop application components that process data independently of the web server or preprocess data before sending it to the server.
JavaScript is particularly good for many tasks because it’s easy to use, open source, and built into all popular browsers (although users can turn it off). It’s often used to validate data that’s typed into forms before it’s sent to the server, highlight parts of a page when the mouse passes over, display menus, and perform other simple tasks. However, it’s limited in the information it can store and it can’t communicate with a database server. Therefore, although you shouldn’t depend on JavaScript to do critical tasks, it’s useful for preprocessing and it’s another important technology we discuss in Chapter 7.
The Middle Tier The middle tier has many roles in a web database application. It brings together the other tiers, drives the structure and content of the data displayed to the user, provides security and authentication, and adds state to the application. It’s the tier that integrates the Web with the database server.
Web servers There are essentially two types of request made to a web server: the first asks for a file—often a static HTML web page or an image—to be returned, and the second asks for a program or script to be run and its output to be returned. We’ve shown you a simple example previously in this chapter, and simple requests for files are further discussed in Appendix D. HTTP requests for PHP scripts require a server to run PHP’s Zend scripting engine, process the instructions in the script (which may access a database), and return the script output to the browser to output as plain HTML.
Apache is an open source, fast, and scalable web server. It can handle simultaneous requests from browsers and is designed to run under multitasking operating systems such as Linux, Mac OS X, and Microsoft Windows. It has low resource requirements, can effectively handle changes in request loads, and can run fast on even modest hardware. It is widely used and tested. The current release at the time of writing is 2.0.48.
Conceptually, Apache isn’t complicated. On a Unix platform, the web server is actually several running programs, where one coordinates the others and doesn’t serve requests itself. The other server programs notify their availability to handle requests to the coordinating server. If too few servers are available to handle incoming requests, the coordinating server may start new servers; if too many are free, it may kill spare servers to save resources.
Apache’s configuration file controls how it listens on the network and serves requests. The server administrator controls the behavior of Apache through more than 150 directives that affect resource requirements, response time, flexibility in dealing with request load variability, security, how HTTP requests are handled and logged, how scripting engines are used to run scripts, and most other aspects of its operation.
The configuration of Apache for most web database applications is straightforward. We discuss how to install Apache in Appendixes A through C, how to hide files that you don’t want to serve in Chapter 6, and the features of a secure web server in Chapter 11. We discuss the HTTP protocol and how it’s implemented in Appendix D. More details on Apache configuration can be found in the resources listed in Appendix G.
Next: Web Scripting with PHP >>
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 This article is excerpted from chapter one of the book Web Database Applications with PHP and MySQL, written by Hugh E. Williams & David Lane (O'Reilly, 2004; ISBN: 0596005431). Check it out at your favorite bookstore. Buy this book now.
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