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As with web pages, you can write HttpModules to operate asynchronously. Since they run for every request, it’s particularly important for scalability to use async operations if your HttpModule does any I/O, including database requests. The following are possible applications of HttpModules: • • • • • • Enforcing site-wide cookie policies Centralized monitoring and logging Programmatically setting or removing HTTP headers Wiring up post-processing of response output, such as removing extra whitespace Session management Authorization and authentication
■ Note Cassini, the development web server that’s used by Visual Studio, sends all requests through all HttpModules, even when the site is configured in Classic mode.
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Here’s a list of the requirements for the example: • • • • • Allow the default theme to be modified at runtime. Set a long-lasting cookie on each client that contains a unique ID, and record the value of that ID in the database. If the Page or the HttpModule itself sets a cookie, make sure that the P3P privacy header is also set (see Chapter 3). Differentiate between clients that retain cookies and those that don’t (as a highlevel indicator of whether they might be spider and not a real user). For all .aspx requests, log the client’s unique ID and details about the page request in the database, using a background worker thread.
Init() Method
Create MainModule.cs in your App_Code folder: using using using using using using using using using System; System.Collections.Generic; System.Data; System.Data.SqlClient; System.Diagnostics; System.Text; System.Threading; System.Web; System.Web.UI;
Requirements for the Example HttpModule
I covered a simple example of an HttpModule in Chapter 4. However, since HttpModules can play a very important role in establishing site-wide consistency and because of their potential impact on performance, let’s walk through a much more detailed example that includes some of the techniques that I’ve discussed.
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Managing Application Policies
During the design and coding of your web site, you will often come up with actions that you would like to take for every page on your site, or perhaps for every control of a certain type. I call those actions application policies. You might apply them before or after a page generates its content or as part of page or control processing. Application policies can have a big impact on performance in areas such as session management, caching, URL rewriting, output filtering, and control rendering. They also form an important part of the ultra-fast approach, since they allow you to easily centralize, manage, and monitor certain aspects of your application. That helps improve agility while minimizing code duplication and simplifying many system-wide debugging and analysis tasks. In this chapter, I will cover the following: • • • • • • • • Using HttpModules, which allow you to apply policies at any point in the IIS request life cycle Using HttpHandlers, which allow you to bypass the policies and associated overhead imposed by the Page handler Implementing a page base class, which will allow you to override Page policies and to add others of your own Implementing URL rewriting programmatically to help shorten your URLs Using page adapters, tag mapping, and control adapters to implement application policies Using HTTP redirects and their alternatives Improving your user’s experience by flushing the response buffer early Reducing the size of your page with HTML whitespace filtering
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Custom HttpModules
requests have a life cycle that starts with an incoming HTTP request. IIS and the runtime then step through a number of states, ultimately producing the HTTP response. At each state transition, IIS and the runtime invoke all registered event handlers. Depending on the event, those event handlers might be located in your Global.asax file or in a standard or custom HttpModule. When your site is running in IIS Integrated mode, as I suggest in Chapter 4, the pipeline events are as shown in Figure 4-2. The events happen for both static and dynamic content, and you can write event handlers in either managed or unmanaged code. For example, you could write an HttpModule in C# that handles events for your static images or even for files processed by a nonmanaged handler, such as PHP. One of the differences between Global.asax and HttpModules is that events in Global.asax are executed only for requests that are processed by the handler, even when IIS is running in Integrated mode. From an architectural perspective, I tend to discourage use of Global.asax as much as possible, since integrating a new HttpModule from another project into your site is much easier than merging multiple Global.asax files together. However, a couple of events are available only in Global.asax, and not in HttpModules, such as Application_Start and Application_Stop.