Figure 21 Graphic architecture for a standalone desktop application.
The application runs in its own process-where the data model and program logic are clearly visible to each other. The second instance of the application running on the same computer has no access to the data model of the first instance, except through the file system. Typically, all program states are stored in a single file-when the application is run it is locked to prevent synchronous exchange of any information.
Figure 3 Client/server systems and N-tier schema diagrams.
The server provides a shareable data model that customers can interact with. The client still maintains some of its own data models to achieve fast access. Multiple customers can interact with the same server, while resources that are controlled at a good granularity level on a single object or database row are locked. The server can be a single process, like a traditional client/server model earlier in the 90 or a modern model consisting of several middleware layers, external Web services, and so on. In any case, from the customer's point of view, the server has a single entry point and can be considered a black box.
Of course, in a modern n-tier architecture, servers will be able to communicate with back-end servers such as databases-which leads to the emergence of middleware layers-both as clients and as server-side. Typically, our AJAX application is located at one end of the chain-only as a client, so we may think of the entire N-tier system as a single black box-we mark it as a server for our current discussion.
My worksheet focuses only on its own data stored in local memory and on local file systems. If it is well structured, the coupling between the data layer and the description layer may be fairly loose, but I can't break it down and share it over the network. So, from our description level goal, it's not a client.
Of course, a Web browser is a client that connects to a Web server and makes page requests from it. These browsers have a wealth of features to manage users ' web browsing, such as Back buttons, history lists, and multiple-page storage documents. But if we treat a Web page of a particular site as an application, then these generic browser controls can no longer be associated with the application, just as the Windows Start menu or window list is related to my worksheet.
Let's look at a modern web application. Mainly because everyone may have heard of it, we will select amazon-online booksellers For example (Figure 4). Now I'm pointing my browser to the Amazon site, because the site remembers who I was from my last visit, so it shows me a friendly greeting, a list of referrals, and historical information about the books I've purchased.
Figure 4 Amazon.com home page. The system remembers my previous visit to the site, where navigable links are a mixture of generic and private information.
Clicking a title from the list of suggestions will lead me to a separate page (i.e., the screen flashes, so I lose the list that I can see a few minutes ago). The new page will then be full of contextual information (see Figure 5).
Figure 5 Amazon.com site book details page.
Again, a large number of hyperlinks appear with generic and private information. However, a great deal of detail is the same as shown in Figure 4-this, due to the document-based operation of the Web browser, must be forwarded back to each page.
In short, I have shown you a very rich and closely related information. And the only way I can interact with this information is by clicking the hyperlink and filling out the text form. If I fall asleep while browsing the site and wake up the next day, I won't know that the new Harry Potter book has been released until I refresh all the pages. I can't go from one page to another with my list, and I can't zoom in on a part of the document to see multiple content at once.
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