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A Web server generally knows how to serve a few types of resources (static HTML pages, text files, images) and forwards other requests to ad hoc modules that basically exist to extend the Web server s core capabilities. For this to happen, the Web server provides a documented 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. As you can easily understand, the CGI approach is rather inadequate for modern, high-volume Web sites because it creates severe scalability issues. IIS supports CGI applications, but you will seldom use this feature unless you have serious backward-compatibility issues. In the past decade, Web servers started supplying an alternative and more efficient model to extend the capabilities of the server. In IIS, this alternative model takes the form of the Internet Server Application Programming Interface (ISAPI). An ISAPI extension is a Win32 DLL that gets loaded into the IIS worker process that s in charge for any given Web application. In IIS 6.0 and later, this worker process is w3wp.exe. The DLL communicates with the host process by exposing a well-known set of entry-point functions the Win32 ancestor of modern service contracts. To start servicing a request, the worker process just ensures the ISAPI extension DLL is loaded in memory and then calls a well-known entry point in its public interface. In Figure 2-1, you recognize the ISAPI extension for ASP.NET requests in the aspnet_isapi.dll component. Figure 2-2 offers a view of the IIS 7.0 metabase configuration tool where the mapping between .aspx resources and aspnet_isapi.dll is established.

478 479

Achieving High Availability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .477

Depending on how IIS is configured to work, the w3wp.exe process might load the aspnet_ isapi.dll extension. In turn, the ISAPI extension will load the CLR in the worker process and launch the ASP.NET runtime pipeline to actually process the request. (I ll return to the discussion of IIS working mode in just a moment.) In the IIS jargon, ISAPI extensions are plain Win32 DLLs designated to do some server-side processing and return a response. The extensibility model of IIS, though, includes another flavor of component as well ISAPI filters.

Availability Planning for Mailbox Servers . . . . . . . . . . . . . . . . . . . . . . . . . . .480 Continuous Replication Designing and Configuring DAGs 487 495

Availability Planning for Client Access Servers. . . . . . . . . . . . . . . . . . . . . . .500 Client Access Load Balancing and Failover Solutions 500

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Availability Planning for Transport Servers. . . . . . . . . . . . . . . . . . . . . . . . . .509 Shadow Redundancy 509

ISAPI filters are components that intercept specific server events before the server itself handles them. Upon loading, the filter indicates what event notifications it will handle. If any of these events occur, the filter can process them or pass them on to other filters. ISAPI filters can accomplish tasks such as implementing custom authentication schemes, compression, encryption, logging, and request analysis. The ability to examine, and if necessary modify, both incoming and outgoing streams of data makes ISAPI filters powerful and flexible. Filters are also a delicate gear in the IIS machinery. They can facilitate applications and let them take control of customizable aspects of the engine. For this same reason, though, ISAPI filters can also degrade performance if they re not written carefully or if they re used when not strictly necessary. Filters, in fact, can run only in-process.

Planning Cross-site Failovers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 513 Cross-site DAG Considerations Cross-site Considerations for Client Access and Transport 513 514