Developer .NET UPDATE, March 25, 2003

Understanding the Application Components

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UNDERSTANDING THE APPLICATION COMPONENTS

In my March 11 column "Putting Architecture Patterns to Work" (http://www.winnetmag.com/articles/index.cfm?articleid=38337), I discussed the Exception Management Application Block. (If you tried to download this application block previously, you might have encountered a glitch. Microsoft has now fixed the problem.) By customizing this application block, you can control the behavior of your application's exception-handling components. This week, let's look at how you can set up database access in your applications. In particular, I want to discuss two types of components associated with application data: business entities and data access logic components.

A business entity represents the data and logic associated with an element in your application. For example, developers commonly create a person class in their applications. This class generally maintains state for a person and encapsulates key behavior associated with that person.

Most developers know that business entities are associated with their applications' business logic and that business entities shouldn't contain logic that's specific to a UI. Having UI information inside a class that contains business logic limits the class's reuse. What most developers don't realize, however, is that the same can be true for embedding database-specific logic in a business entity. Just as you might want to have a class compatible with more than one UI, you might want to have a class that can use data from more than one data source. For this reason, the data store doesn't encapsulate the data tier.

Whether you're using a rich database such as a Microsoft SQL Server database or an easily transferable Document Type Definition (DTD)/XML file, the data-access logic of an application can be represented by a unique set of classes. A data access logic component is a stateless class that implements the logic associated with retrieving data from a data store. The idea is that an application can encapsulate knowledge of the data store in a set of data access logic components so that you can isolate changes to the data store from the remainder of your application. These components are stateless because they don't maintain your application data; the business classes manage the state and data. You use data access logic components to manage the transfer of data from your application to the data store. The data access logic components handle the create, read, update, and delete actions for the data store.

You might be tempted to cache data as part of the data access logic components. Although this action is possible, I don't recommend it. By definition, the data access logic component is stateless and a cache is a maintained state.

To learn more about business entities and data access logic components, I highly recommend that you read "Designing Data Tier Components and Passing Data Through Tiers" (http://msdn.microsoft.com/architecture/default.aspx?pull=/library/en-us/dnbda/html/boagag.asp). This paper first defines common elements (e.g., data access logic component), then presents some standard advantages and disadvantages associated with various implementations. Understanding the implementations' advantages and disadvantages is important because no single correct implementation exists when you pass data from the data layer to your UI. Which implementation is best depends on factors such as the type of data and how the data will be used.

When you're reviewing the Microsoft implementations, keep your options open. For example, if your application runs in two tiers and the business objects and database tables are tightly mapped, you might find that isolating data access within private methods of your classes works just as effectively as using data access logic components. I realize that what I just said might sound like I'm recommending that you don't use data access logic components. However, as I mentioned earlier, more than one correct solution typically exists.

Next week, I'll discuss database access in the Windows .NET Framework. I'll show you some of the specific tools and options that you can use to plan and implement an application's data store.

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