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Supporting object persistence the ability to store and retrieve an object from a database can be quite complex. This was covered earlier in the chapter during the discussion about basic persistence and the concept of ORM. As you ll see in 18, data access logic is encapsulated within the formal data access layer assembly, which is invoked by the business objects. This data access assembly must be deployed to the physical tier that will execute the data access code. At the same time, however, you don t want to be in a position in which a change to your physical architecture requires every business object in the system to be altered. The ability to easily switch between having the data access code run on the client machine and having it run on an application server is the goal, with that change driven by a configuration file setting. On top of this, when using an application server, not every business object in the application should be directly exposed by the server. This would be a maintenance and configuration nightmare because it would require updating configuration information on all client machines any time a business object is added or changed.
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Note This is a lesson learned from years of experience with DCOM and MTS/COM+. Exposing large numbers of components, classes, and methods from a server almost always results in a tightly coupled and fragile relationship between clients and the server.
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Instead, it would be ideal if there were one consistent entry point to the application server so that every client could simply be configured to know about that single entry point and never have to worry about it again. This is exactly what the data portal concept provides, as shown in Figure 2-14.
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Figure 2-14. The data portal provides a consistent entry point to the application server. The data portal provides a single point of entry and configuration for the server. It manages communication with the business objects while they re on the server running their data access code. Additionally, the data portal concept provides the following other key benefits: Centralized security when calling the application server A consistent object-persistence mechanism (all objects persist the same way) Abstraction of the network transport between client and server (enabling support for WCF, remoting, web services, Enterprise Services, and custom protocols) One point of control to toggle between running the data access code locally or on a remote application server The data portal functionality is designed in several parts, as shown in Table 2-4.
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Table 2-4. Parts of the Data Portal Concept
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Client-side DataPortal Client-side proxy classes
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Functions as the primary entry point to the data portal infrastructure, for use by code in business objects Implements the channel adapter pattern to abstract the underlying network protocol from the application
CHAPTER 2 FRAMEWORK DE SIGN
Table 2-4. Parts of the Data Portal Concept (Continued)
Area
Message objects
Functionality
Transfers data to and from the server, including security information, application context, the business object s data, the results of the call, or any server-side exception data Exposes single points of entry for different server hosts, such as WCF, remoting, .asmx web services, and Enterprise Services Implements transactional and nontransactional data access behaviors, delegating all actual data access to appropriate business objects Implements data access behaviors for objects that are contained within other objects Provides an alternate model for the data portal, where the data portal creates and invokes a factory object instead of interacting directly with the business object
Server-side host classes Server-side data portal Server-side child data portal Object factory
Let s discuss each area of functionality in turn.
Client-Side DataPortal
The client-side DataPortal is implemented as a Shared class, which means that any Public methods it exposes become available to business object code without the need to create a DataPortal object. The methods it provides are Create(), Fetch(), Update(), Delete(), and Execute(). Business objects and collections use these methods to retrieve and update data, or in the case of a CommandBase-derived object, to execute server code on the server. The client-side DataPortal has a great deal of responsibility, however, since it contains the code to read and act on the client s configuration settings. These settings control whether the server-side data portal components will actually run on the server or locally on the client. It also looks at the business object itself, since a RunLocal attribute can be used to force persistence code to run on the client, even if the configuration says to run it on the server. Either way, the client-side DataPortal always delegates the call to the server-side data portal, which handles the actual object-persistence behaviors. However, if the client configuration indicates that the server-side data portal really will run on a server, the configuration will also specify which network transport should be used. It is the client-side DataPortal that reads that configuration and loads the appropriate client-side proxy object. That proxy object is then responsible for handling the network communication. As an object is implemented, its code will use the client-side DataPortal to retrieve and update the object s information. An automatic result is that the code in the business object won t need to know about network transports or whether the application is deployed into a 1-, 2-, or n-tier physical environment. The business object code always looks something like this: Public Shared Function GetCustomer(ByVal id As String) As Customer Return DataPortal.Fetch(Of Customer)(New SingleCriteria(Of Customer, _ String)(id)) End Function An even more important outcome is that any UI code using these business objects will look something like this: Dim cust As Customer = Customer.GetCustomer(myId)
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