ISAPI

ISAPI stands for Internet Server Application Programming Interface. It is a software development kit that allows developers to extend the functionality of Microsoft's Internet Information Services (IIS) web server. ISAPI extensions are dynamic-link libraries (DLLs) that run within the IIS process, offering a way to handle web requests more efficiently than traditional CGI scripts.

ISAPI applications can be used for a variety of purposes, such as performing custom authentication, processing specific file types, or generating dynamic content. Compared to CGI, ISAPI generally offers better performance because the ISAPI DLLs are loaded into memory once and remain there, avoiding the overhead of starting a new process for each request.

There are two main types of ISAPI applications: ISAPI extensions and ISAPI filters. ISAPI extensions are responsible for handling specific requests, such as those for a particular file extension. ISAPI filters, on the other hand, can intercept and modify requests and responses as they pass through IIS, allowing for tasks like URL rewriting or custom logging.

While ISAPI was a significant advancement in web server extensibility, its use has declined somewhat with the rise of more modern technologies like ASP.NET and various open-source web frameworks. However, it remains a viable option for certain scenarios, particularly in environments that heavily rely on existing IIS infrastructure.