ISAPI (Internet Server Application Programming Interface) is a framework to expand IIS and a programming interface that can be used in the framework. When an Internet Server Application (hereafter abbreviated as ISA) is an application using ISAPI, it runs as a thread in IIS. Therefore, ISA's typically execute faster than traditional CGI applications. Also, less system resources such as memory are required. However, since ISA's run as a thread, they must be thread-safe. Do not forget to release resources, because, if a resource is not released, the resource may remain unusable until IIS itself is terminated. The following two types of applications can be created using ISAPI:
Unit to install each Web application. It is possible to change applications to be used for each request.
Web applications invoked in common throughout the Web site. ISAPI filter's are executed before or after an ISAPI extension and are used when processing common throughout applications is required.
