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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.

Summary

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.

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.

For managed objects, object destruction and memory reclamation are decoupled. This supports reliable code in two ways. First, accessing managed objects that are already destroyed cannot corrupt the state of some other random object; instead, it often ends up in a welldefined reaction the ObjectDisposedException. The second benefit is that the runtime can

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

Table 2-4. Parts of the Data Portal Concept (Continued)

call the finalizer to perform last-chance cleanup before memory of an undestructed object is reclaimed. This is helpful to ensure reliable cleanup of non-memory resources, but it can also be the source of pitfalls like the finalization timing race condition or the graph promotion problem. Furthermore, normal finalizers are often not reliable enough for long-running servers like SQL Server 2005. To further increase reliability, the CLR version 2.0 introduces CERs, which ensure that regions of constrained code cannot be interrupted by an asynchronous exception. One way to avoid these pitfalls and to benefit from CERs is the use of the SafeHandle base class.

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

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.

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