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This chapter is from the book

Basic Definitions

In this book, Web application has a very general definition—client/server software that is connected by Internet technologies to route the data it processes. By "Internet technologies," I mean the collection of hardware and software that comprises the network infrastructure between consumers and providers of information. Web applications can be made accessible by specific client software or by one or more related Web pages that are logically grouped for a specific productive purpose. That purpose can be one of any number of things, for example, to buy books, to process stock orders, or to simply exist as content to be read by the consumer.

Notice that our discussion is about Web applications, not just "Web sites." In truth, the difference between the two is essential to understanding one of the key themes of this book. Most nonengineers do not make a distinction between a Web site and a Web application. Regardless of the term, it's the thing that allows them to buy their books online, to make plane reservations, to purchase tickets, and so forth.

If you're an engineer, however, there is a difference. For you, it's likely that when someone talks about, say, the performance of a Web site, you start thinking of back-end details. And so do I. Your mind begins to consider if it's running an Apache or IIS and whether it works using Java servlets, PHP, or CGI-bin Perl scripts. This difference in thinking between engineers and nonengineers could be confusing. Engineers, by habit, tend to associate "Web site" with the server side. However, as we all know, there is more to a Web application than just the server side; there's the network and the client. So, based on just that, a Web site (server) is not the same thing as a Web application (the client, network, and server).

While this book emphasizes server-side solutions, it is also concerned with client-side and networking topics because they have a fundamental impact on how end users perceive Web applications. That is, we will be concerned with the end-to-end interaction with a Web site, which simply means from client to server and back to client. This is a reasonable focus. After all, most people who use the Web are concerned with its end-to-end behavior. If it takes them a while to buy concert tickets online, it doesn't matter if the problem is caused by a slow modem, an overtaxed server, or network congestion. Whatever the reason(s), the effect is the same—a slow application that's eating up time. As engineers, we are concerned not only that such applications might be slow for one user, but also that the system becomes slower as more users access it.

Now that we have a better fix on the scope of a Web application, let us review its core components. These are the major pieces of any online application and each represents an opportunity—a problem or a challenge, depending on how you look at it. Although you're probably familiar with the components, it doesn't hurt to make sure everyone is on the same page, especially since these terms appear throughout the book. Let's start with the client side.

We will say that Web applications are used by consumers via client software (i.e., Web browsers or applications that use the Web to retrieve or process data) running on client hardware (i.e., PCs, PDAs). Application data is provided and processing is handled by producers via server software (i.e., Web server, server-side component software, database) running on server hardware (i.e., high-end multiprocessor systems, clusters, etc.). Connecting the client to the server (from the modem or network port on the client device to the networking equipment on the server side) is the networking infrastructure. Figure 1–1 shows the client/server relationship graphically. Notice that the server side is bigger; in general, we assume that the server side has more resources at its disposal.

Figure 1–1 Client, network, and server

At this point, it is important to distinguish one piece of server software, the Web server, because it nearly always plays a central role in brokering communication (HTTP traffic) between client and server. In this book, when I refer to the "server side," I am nearly always including the Web server. When it is necessary to distinguish it from the rest of the software on the server side, I will do so explicitly.

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