An interface between the operating system and the application programs, it extends the capabilities of Web servers. Used by programmers who write applications that can interact with other applications, a server API is a published interface. Software developers write programs that become part of the Web server itself. Usually, these programs are DLLs that are loaded into memory and "stay resident" at all times. Some common server APIs and the servers they support include the following:
ISAPI, Microsoft Internet Information Server; NSAPI, Netscape Commerce and Enterprise Server; and WSAPI, O'Reilly Web Site and Web Site Pro.