Chapter 10 -- Internet Server API (ISAPI)

Chapter 10

From the standpoint of developers working on server applications, the inclusion in Microsoft Windows NT 4 of the Internet Information Server (IIS) is one of the single biggest improvements over previous versions of the Windows NT operating system. The inclusion in Microsoft Windows 2000 of IIS 5 adds many improvements for server-side developers. In addition to Active Server Pages (ASP) (covered in Chapter 9), the Internet Server API (ISAPI) offers tremendous opportunities for developers who need to exploit the benefits of server-based systems.

Interestingly, ASP itself is implemented as an ISAPI DLL named ASP.DLL. Recall from Chapter 9 the configuration screen for IIS 5shown in Figure 10-1that allows the association between file extensions and the ISAPI DLLs that IIS calls when a program requests a file with that extension. This ability to change the behavior of IIS by selecting the DLL to be called is a powerful way to customize its operation. All of the DLLs listed in Figure 10-1 are ISAPI Extension DLLs.

Figure 10-1 IIS configuration screen.

If we need any proof that ASP.DLL is an ISAPI extension, we need only load the DLL into the Microsoft Visual C++ Dependency Walker to see the DLL's list of imported and exported functions. Figure 10-2 shows some of the functions exported from the Windows 2000 ASP.DLL. The most significant of these functions are GetExtensionVersion and HttpExtensionProc . As we will see, these are the functions that must be supported by any ISAPI Extension DLL.

Figure 10-2 Dependency Walker showing some of ASP.DLL's exported functions.