Programming Microsoft ASP.NET 4 - Dino Esposito [65]
A Web server generally provides an application programming interface (API) for enhancing and customizing the server’s capabilities. Historically speaking, the first of these extension APIs was the Common Gateway Interface (CGI). A CGI module is a new application that is spawned from the Web server to service a request. Nowadays, CGI applications are almost never used because they require a new process for each HTTP request, and this approach poses severe scalability issues and is rather inadequate for high-volume Web sites.
More recent versions of Web servers supply an alternate and more efficient model to extend the capabilities of the server. In IIS, this alternative model takes the form of the ISAPI interface. When the ISAPI model is used, instead of starting a new process for each request, the Web server loads a made-to-measure component—namely, a Win32 dynamic-link library (DLL)—into its own process. Next, it calls a well-known entry point on the DLL to serve the request. The ISAPI component stays loaded until IIS is shut down and can service requests without any further impact on Web server activity. The downside to such a model is that because components are loaded within the Web server process, a single faulty component can tear down the whole server and all installed applications. Some effective countermeasures have been taken over the years to smooth out this problem. Today, IIS installed applications are assigned to application pools and each application pool is served by a distinct instance of a worker process.
From an extensibility standpoint, however, the ISAPI model is less than optimal because it requires developers to create Win32 unmanaged DLLs to endow the Web server with the capability of serving specific requests, such as those for ASPX resources. Until IIS 7 (and still in IIS 7 when the classic mode is configured), requests are processed by IIS and then mapped to some ISAPI (unmanaged) component. This is exactly what happens with plain ASPX requests, and the ASP.NET ISAPI component is aspnet_isapi.dll. In IIS 7.x integrated mode, you can add managed components (HTTP handlers and HTTP modules) directly at the IIS level. More precisely, the IIS 7 integrated mode merges the ASP.NET internal runtime pipeline with the IIS pipeline and enables you to write Web server extensions using managed code. This is the way to go.
Today, if you learn how to write HTTP handlers and HTTP modules, you can use such skills to customize how any requests that hit IIS are served, and not just requests that would be mapped to ASP.NET. You’ll see a few examples in the rest of the chapter.
Writing HTTP Handlers
As the name suggests, an HTTP handler is a component that handles and processes a request. ASP.NET comes with a set of built-in handlers to accommodate a number of system tasks. The model, however, is highly extensible. You can write a custom HTTP handler whenever you need ASP.NET to process certain types of requests in a nonstandard way. The list of useful things you can do with HTTP handlers is limited only by your imagination.
Through a well-written handler, you can have your users invoke any sort of functionality via the Web. For example, you could implement click counters and any sort of image manipulation, including dynamic generation of images, server-side caching, or obstructing undesired linking to your images. More in general, an HTTP handler is a way for the user to send a command to the Web application instead of just requesting a particular page.
In software terms, an HTTP handler is a relatively simple class that implements the IHttpHandler interface. An HTTP handler can either work synchronously or operate in an asynchronous way. When working synchronously, a handler doesn’t return until it’s done with the HTTP request. An asynchronous handler, on the other hand, launches a potentially lengthy process and returns immediately after. A typical implementation of asynchronous handlers is asynchronous pages. An asynchronous HTTP handler is a class that implements a different