AJAX In Action [11]
Finished? I am. While typing a few simple equations into my spreadsheet, I found that I could interact with it in a number of ways, editing data in situ, navigating the data with keyboard and mouse, and reorganizing data using drag and drop.
As I did these things, the program gave me feedback. The cursor changed shape, buttons lit up as I hovered over them, selected text changed color, highlighted windows and dialogs were represented differently, and so on (figure 1.1). That’s what passes for rich interactivity these days. Arguably there’s still some way to go, but it’s a start.
So is the spreadsheet application a rich client? I would say that it isn’t. In a spreadsheet or similar desktop application, the logic and the data model are both executed in a closed environment, in which they can see each other very clearly but shut the rest of the world out (figure 1.2). My definition of a client is a program that communicates to a different, independent process, typically running on a server. Traditionally, the server is bigger, stronger, and better than the client, and it stores monstrously huge amounts of information. The client allows end users to view and modify this information, and if several clients are connected to the same server, it allows them to share that data. Figure 1.3 shows a simple schematic of a client/server architecture.
Licensed to jonathan zheng 6 CHAPTER 1 A new design for the Web Figure 1.1 This desktop spreadsheet application illustrates a variety of possibilities for user interaction. The headers for the selected rows and columns are highlighted; buttons offer tooltips on mouseover; toolbars contain a variety of rich widget types; and the cells can be interactively inspected and edited. Process 1 Process 2 Logic Logic Data Data model model Filesystem Figure 1.2 Schematic architectures for a standalone desktop application. The application runs in a process of its own, within which the data model and the program logic can “see” one another. A second running instance of the application on the same computer has no access to the data model of the first, except via the filesystem. Typically, the entire program state is stored in a single file, which is locked while the application is running, preventing any simultaneous exchange of information. Licensed to jonathan zheng Why Ajax rich clients? 7 Process 1 Process 2 Process 1 Process 2 Logic Logic Data Data Logic Logic Data Data model model model model "Server" Middleware layer(s) Server (e.g. database) Server farm Internet services Figure 1.3 Schematic architectures for client/server systems and n-tier architectures. The server offers a shared data model, with which clients can interact. The clients still maintain their own partial data models, for rapid access, but these defer to the server model as the definitive representation of the business domain objects. Several clients can interact with the same server, with locking of resources handled at a fine-grain level of individual objects or database rows. The server may be a single process, as in the traditional client/server model of the early-to mid-1990s, or consist of several middleware tiers, external web services, and so on. In any case, from the client’s perspective, the server has a single entry point and can be considered a black box. In a modern n-tier architecture, of course, the server will communicate to further back-end servers such as databases, giving rise to middleware layers that act as both client and server. Our Ajax applications typically sit at the end of this chain, acting as client only, so we can treat the entire n-tier system as a single black box labeled “server” for the purposes of our current discussion. My spreadsheet sits on its own little pile of data, stored locally in memory and on the local filesystem. If it is well architected, the coupling