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Maintenance in large applications: Every page that accesses the database needs its own set of SqlDataSource controls. This can turn into a maintenance nightmare, particularly if you have several pages using the same query (each of which requires a duplicate instance of the SqlDataSource). In a component-based application, you ll use a higher-level model. The web pages will communicate with a data access library, which will contain all the database details. Lack of flexibility: Every data access task requires a separate SqlDataSource. If you want to provide a user with multiple ways to view or query data, this can swamp your page with data sources objects, one for each command variant. This can get complicated fast. Inapplicability to other data tasks: The SqlDataSource doesn t properly represent some types of tasks. The SqlDataSource is intended for data display and data editing scenarios. However, this model breaks down if you need to connect to the database and perform another task, such as placing a shipment request into an order pipeline or logging an event. In these situations, you ll need custom database code. It will simplify your application if you have a single database component that encapsulates these tasks along with data retrieval and updating operations.

Remember, the EmployeeDB class uses error-handling blocks to make sure connections are properly closed, but it doesn t catch exceptions. (Best design practices are to let the exception notify the web page, which can then decide how best to inform the user.) You can handle errors with the ObjectDataSource in the same way you handle them with the SqlDataSource first, handle the Selected, Inserted, Updated, or Deleted event; second, check for an exception; and third, mark it as handled. For more information, see the Handling Errors section earlier in the chapter.

Based on this example, you can probably imagine a variety of different ways you can use HTTP handlers. For example, you could render a custom image, perform an ad hoc database query, or return some binary data. These examples extend the ASP.NET architecture but bypass the web-page model. The result is a leaner, more efficient component. You can also create HTTP handlers that work asynchronously. This means they create a new thread to do their work, instead of using one of the ASP.NET worker threads. This improves scalability in situations where you need to perform a task that takes a long amount of time but isn t CPU-intensive. A classic example is waiting to read an extremely slow network resource. ASP .NET allows a fixed set of worker threads to run only at one time (typically 25), so once this limit is reached additional requests will be queued, even if the computer has available CPU time. With asynchronous handlers, additional requests can be accepted, because the handler creates a new thread to process each request rather than using the worker process. Of course, there is a risk with this approach. Namely, if you create too many threads for the computer to manage efficiently, or if you try to do too much CPU-intensive work at once, the performance of the entire web server will be adversely affected. Asynchronous HTTP handlers aren t covered in this book, but in 7 you ll learn how to use asynchronous pages, which use asynchronous HTTP handlers behind the scenes.

In fact, in a well-abstracted multitier application, your web page may call a method such as Business.PlaceOrder() without worrying about whether this operation involves saving an order record in a database, sending a message to a message queue, communicating with a remote component, or using a combination of all these tasks.

Figure 2-2. Browsing to a website location Once you make your selection and click Open, Visual Studio returns you to the Create Web Site dialog box. Click OK, and Visual Studio will create the new web application. A new website starts with exactly one file a default.aspx start page.

By default, HTTP handlers do not have access to client-specific session state. That s because HTTP handlers are generally used for lower-level tasks, and skipping the steps needed to serialize and retrieve session state information achieves a minor increase in performance. However, if you do need access to session state information, you simply need to implement one of the following two interfaces: IRequiresSessionState IReadOnlySessionState If you require just read-only access to session state, you should implement the IReadOnlySessionState interface. If you need to modify or add to session information, you should implement the IRequiresSessionState interface. You should never implement both at the same time. These two interfaces are just marker interfaces and do not contain any methods. That means you don t need to write any extra code to enable session support. For example, if you want to use read-only session state with the SimpleHandler class, you would declare it in this way: public class SimpleHandler : IHttpHandler, IReadOnlySessionState {...} To actually access the Session object, you ll need to work through the HttpContext object that s submitted to the ProcessRequest() method. It provides a Session property.

To get around these limitations, you should consider the ObjectDataSource. The ObjectDataSource allows you to bind your page to a custom data access component. Best of all, you get almost all the same frills, such as design-time data binding and no need to write code in your web page.