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Earlier in this chapter, I discussed various physical architectures in which different parts of an application might run on different machines. With a high-scalability smart client architecture, for example, there will be a client, an application server, and a data server. With a high-security web client architecture, there will be a client, a web server, an application server, and a data server. Parts of the application will run on each of these machines, interacting with each other as needed. In these distributed architectures, you can t use a straightforward object-oriented design, because any communication between classic fine-grained objects on one machine and similar objects on another machine will incur network latency and overhead. This translates into a performance problem that simply can t be ignored. To overcome this problem, most distributed applications haven t used object-oriented designs. Instead, they consist of a set of procedural code running on each machine, with the data kept in a DataSet, an array, or an XML document that s passed around from machine to machine. This isn t to say that object-oriented design and programming are irrelevant in distributed environments just that it becomes complicated. To minimize the complexity, most distributed applications are object-oriented within a tier, but between tiers they follow a procedural or service-based model. The end result is that the application as a whole is neither object-oriented nor procedural, but a blend of both. Perhaps the most common architecture for such applications is to have the Data Access layer retrieve the data from the database into a DataSet. The DataSet is then returned to the client (or the web server). The code in the forms or pages then interacts with the DataSet directly, as shown in Figure 1-15.
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Figure 1-15. Passing a DataSet between the Business and Data Access layers This approach has the maintenance and code-reuse flaws that I ve talked about, but the fact is that it gives pretty good performance in most cases. Also, it doesn t hurt that most programmers are pretty familiar with the idea of writing code to manipulate a DataSet, so the techniques involved are well understood, thus speeding up development. A decision to stick with an object-oriented approach should be undertaken carefully. It s all too easy to compromise the object-oriented design by taking the data out of the objects running on one machine, sending the raw data across the network, and allowing other objects to use that data outside the context of the objects and business logic. Such an approach would break the encapsulation provided by the logical Business layer.
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Mobile objects are all about sending smart data (objects) from one machine to another, rather than sending raw data. Through its WCF, serialization, and deployment technologies, the .NET Framework contains direct support for the concept of mobile objects. Given this ability, you can have your Data Access layer (running on an application server) create a business object and load it with data from the database. You can then send that business object to the client machine (or web server), where the interface control code can use the object (as shown in Figure 1-16).
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Figure 1-16. Using a business object to centralize business logic In this architecture, smart data (in the form of a business object), rather than raw data, is sent to the client. Then the interface control code can use the same business logic as the data access code. This reduces maintenance, because you re not writing some business logic in the Data Access layer, and some other business logic in the Interface Control layer. Instead, all of the business logic is consolidated into a real, separate layer composed of business objects. These business objects will move across the network just like the DataSet did earlier, but they ll include the data and its related business logic something the DataSet can t easily offer.
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