How ISAPI Extensions Operate

ISAPI applications are compiled as dynamic link libraries (DLLs) that are loaded into the HTTP service when the service starts. ISAPI applications are actually extensions of the HTTP service. ISAPI applications remain in physical memory; run as threads within the IIS service process, Inetinfo.exe; and share the Inetinfo.exe memory space. When a client requests a page generated by an ISAPI extension, a waiting thread processes the request. When the request is complete, the thread returns to the thread pool until another request arrives. No further processing time or memory is required.