Classic vs. Integrated mode in IIS 7

IIS 6.0 and previous versions allowed the development of .NET application components via the ASP.NET platform. ASP.NET integrated with IIS via an ISAPI extension, and exposed its own application and request processing model. This effectively exposed two separate server pipelines, one for native ISAPI filters and extension components, and another for managed application components. ASP.NET components would execute entirely inside the ASP.NET ISAPI extension bubble and only for requests mapped to ASP.NET in the IIS script map configuration.

IIS 7.0 integrates the ASP.NET runtime with the core web server, providing a unified request processing pipeline that is exposed to both native and managed components known as modules.

The many benefits of integration include:

• Allowing services provided by both native and managed modules to apply to all requests, regardless of handler. For example, managed Forms Authentication can be used for all content, including ASP pages, CGIs, and static files.
• Empowering ASP.NET components to provide functionality that was previously unavailable to them due to their placement in the server pipeline. For example, a managed module providing request rewriting functionality can rewrite the request prior to any server processing, including authentication.
• A single place to implement, configure, monitor and support server features such as single module and handler mapping configuration, single custom errors configuration, single url authorization configuration.

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Classic mode is where IIS only works with ISAPI extensions and ISAPI filters directly. This is how IIS 6 and below behaved. Using classic mode, ASP.NET is simply an ISAPI extension (aspnet_isapi.dll) and an ISAPI filter (aspnet_filter.dll). When using classic mode the server uses two piplines to handle requests, one for native code and the other for managed code. In this mode the application doesn't take full advantage of everything IIS 7.X has to offer.

Integrated mode handles all requests through a unified pipeline for IIS and is tightly integrated with ASP.NET through that same pipeline. ASP.NET sees every relevant request and manipulates things along the way rather than acting as an external plugin. With integrated mode ASP.NET runs much more efficiently in IIS and will yield greater performance for your site.