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CHAPTER 1 s DISTRIBUTED ARCHITECTURE
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Figure 1-18. Business logic shared between the UI and Data Access layers
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Local, Anchored, and Mobile Objects
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Normally, one might think of objects as being part of a single application, running on a single machine in a single process. A distributed application requires a broader perspective. Some of the objects might only run in a single process on a single machine. Others may run on one machine, but may be called by code running on another machine. Still others may be mobile objects: moving from machine to machine.
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Local Objects
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By default, .NET objects are local. This means that ordinary .NET objects aren t accessible from outside the process in which they were created. Without taking extra steps in your code, it isn t possible to pass objects to another process or another machine (a procedure known as marshaling), either by value or by reference.
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Anchored Objects
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In many technologies, including COM, objects are always passed by reference. This means that when you pass an object from one machine or process to another, what actually happens is that the object remains in the original process, and the other process or machine merely gets a pointer, or reference, back to the object, as shown in Figure 1-19.
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Figure 1-19. Calling an object by reference
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CHAPTER 1 s DISTRIBUTED ARCHITECTURE
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By using this reference, the other machine can interact with the object. Because the object is still on the original machine, however, any property or method calls are sent across the network, and the results are returned back across the network. This scheme is only useful if the object is designed so that it can be used with very few method calls; just one is ideal! The recommended designs for MTS or COM+ objects call for a single method on the object that does all the work for precisely this reason, thereby sacrificing proper object-oriented design in order to reduce latency. This type of object is stuck, or anchored, on the original machine or process where it was created. An anchored object never moves; it s accessed via references. In .NET, an anchored object is created by having it inherit from MarshalByRefObject: Public Class MyAnchoredClass Inherits MarshalByRefObject End Class From this point on, the .NET Framework takes care of the details. Remoting can be used to pass an object of this type to another process or machine as a parameter to a method call, for example, or to return it as the result of a function.
Mobile Objects
The concept of mobile objects relies on the idea that an object can be passed from one process to another, or from one machine to another, by value. This means that the object is physically copied from the original process or machine to the other process or machine, as shown in Figure 1-20.
Figure 1-20. Passing a physical copy of an object across the network
Because the other machine gets a copy of the object, it can interact with the object locally. This means that there s effectively no performance overhead involved in calling properties or methods on the object the only cost was in copying the object across the network in the first place.
s Note
One caveat here is that transferring a large object across the network can cause a performance problem. Returning a DataSet that contains a great deal of data can take a long time. This is true of all mobile objects, including business objects. You need to be careful in your application design in order to avoid retrieving very large sets of data.
CHAPTER 1 s DISTRIBUTED ARCHITECTURE
Objects that can move from process to process or from machine to machine are mobile objects. Examples of mobile objects include the DataSet and the business objects created in this book. Mobile objects aren t stuck in a single place, but can move to where they re most needed. To create one in .NET, add the <Serializable()> attribute to your class definition. You may also optionally implement the ISerializable interface. I ll discuss this further in 2, but the following illustrates the start of a class that defines a mobile object: <Serializable()> _ Public Class MyMobileClass End Class Again, the .NET Framework takes care of the details, so an object of this type can be simply passed as a parameter to a method call or as the return value from a function. The object will be copied from the original machine to the machine where the method is running. It is important to understand that the code for the object isn t automatically moved across the network. Before an object can move from machine to machine, both machines must have the .NET assembly containing the object s code installed. Only the object s serialized data is moved across the network by .NET. Installing the required assemblies is often handled by ClickOnce or other .NET deployment technologies.
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