In addition to ISAPI Extensions, IIS also allows for ISAPI Filters. An ISAPI Filter is a piece of unmanaged code that can run in response to events raised by IIS. During the lifecycle of a request, IIS passes through a number of steps that raise corresponding events. For example, an event is raised when the request first reaches IIS, when the request is about to be authenticated, when the rendered content is about to be sent back to the client, and so forth. ISAPI Filters are commonly used to provide capabilities such as URL rewriting, compression, specialized authentication and authorization, specialized logging, and so on.
When a request for an ASP.NET resource reaches IIS, it is routed to the ASP.NET engine, which renders the content for the requested resource. The ASP.NET engine behaves a lot like IIS in that it raises a number of events as the request passes through the ASP.NET HTTP pipeline. Furthermore, the ASP.NET engine delegates rendering of the requested resource to a particular class. Whereas IIS uses unmanaged ISAPI Extensions and Filters, ASP.NET uses managed classes called HTTP handlers and modules.
An HTTP handler is a class that is responsible for rendering a particular type of resource. For example, the code-behind class for an ASP.NET Web page is an HTTP handler, knowing how to render the markup for the particular Web page. It helps to think about handlers as specialized renderers that know how to create the markup for a particular type of resource.
Following Figure provides a graphical representation of the ASP.NET HTTP pipeline. Note that the process starts with a request arriving at IIS. Assuming the requested resource is configured to be handled by the ASP.NET ISAPI Extension, IIS dispatches the request to the unmanaged aspnet_isapi.dll ISAPI Extension. This ISAPI Extension passes off the request to the managed ASP.NET engine. During the request lifecycle, one or more HTTP modules may execute, depending on what modules have been registered and what events they have subscribed to. Finally, the ASP.NET engine determines the HTTP handler that is responsible for rendering the content, invoking the handler and returning the generated content back to IIS, which returns it back to the requesting client.
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