Supporting Physical N-Tier Models in VB.NET

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Supporting Physical N-Tier Models
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The question that remains, then, is how to support physical n-tier models if the UI-oriented and data-oriented behaviors reside in one object UI-oriented behaviors almost always involve a lot of properties and methods a very finegrained interface with which the UI can interact in order to set, retrieve, and manipulate the values of an object. Almost by definition, this type of object must run in the same process as the UI code itself, either on the Windows client machine with Windows Forms, or on the web server with Web Forms. Conversely, data-oriented behaviors typically involve very few methods: create, fetch, update, and delete. They must run on a machine where they can establish a physical connection to the database server. Sometimes, this is the client workstation or web server, but often it means running on a physically separate application server. This point of apparent conflict is where the concept of mobile objects enters the picture. It s possible to pass a business object from an application server to the client machine, work with the object, and then pass the object back to the application server so that it can store its data in the database. To do this, there needs to be some black-box component running as a service on the application server with which the client can interact. This black-box component does little more than accept the object from the client, and then call methods on the object to retrieve or update data as required. But the object itself does all the real work. Figure 2-9 illustrates this concept, showing how the same physical business object can be passed from application server to client, and vice versa, via a generic router object that s running on the application server. In 1, I discussed anchored and mobile objects. In this model, the business object is mobile, meaning that it can be passed around the network by value. The router object is anchored, meaning that it will always run on the machine where it s created.
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CHAPTER 2 s FRAMEWORK DESIGN
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In the framework, I ll refer to this router object as a data portal. It will act as a portal for all data access for all the objects. The objects will interact with this portal in order to retrieve default values (create), fetch data (read), update or insert data (update), and remove data (delete). This means that the data portal will provide a standardized mechanism by which objects can perform all CRUD operations. The end result will be that each business class will include a factory method that the UI can call in order to load an object based on data from the database, as follows: Public Shared Function GetCustomer(ByVal customerId As String) As Customer Return DataPortal.Fetch(Of Customer)(New Criteria(customerId)) End Function
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Figure 2-9. Passing a business object to and from the application server
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The actual data access code will be contained within each of the business objects. The data portal will simply provide an anchored object on a machine with access to the database server, and will invoke the appropriate CRUD methods on the business objects themselves. This means that the business object will also implement a method that will be called by the data portal to actually load the data. That method will look something like this: Private Sub DataPortal_Fetch(ByVal criteria As Criteria) ' Code to load the object's fields with data goes here End Sub The UI won t know (or need to know) how any of this works, so in order to create a Customer object, the UI will simply write code along these lines: Dim cust As Customer = Customer.GetCustomer("ABC") The framework, and specifically the data portal, will take care of all the rest of the work, including figuring out whether the data access code should run on the client workstation or on an application server. Using the data portal means that all the logic remains encapsulated within the business objects, while physical n-tier configurations are easily supported. Better still, by implementing the data portal correctly, you can switch between having the data access code running on the client machine and placing it on a separate application server just by changing a configuration
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