It can be very difficult for a Web-hosting company to administer multiple sites within a shared-server hosting environment utilizing Microsoft Internet Information Server (IIS). Luckily, IIS provides the Internet Server Application Programming Interface (ISAPI), a mechanism that opens up server-request processing and lets administrators customize this processing through the use of ISAPI filters. ISAPI filters are usually written in C and then compiled into dynamic link libraries (DLLs), which are loaded by the Web server on startup. The filters stay in memory until the server is shut down. In contrast, you may have heard of ISAPI extensions, which are loaded only when requested. When monitoring all incoming requests to a server—as we'll do with the LogFilter tool presented later in this article—a filter can often be more efficient than an extension because there's no loading/unloading overhead.
展开▼
