ISAPI

Internet Server Application Programming Interface (ISAPI), 

2 Component (extensions and filters) must compiled into DLL 

• ISAPI extensions
• Called from qualified request to IIS. (URL, POST [Submit Method]) 
• run on IIS, it has access to all functionality in IIS.
• Client can access ISAPI extensions in static HTML or certain file extensions can mapped to be handled by ISAPI extension.

• ISAPI filters
• Called from all request to IIS.
• Used to enhance the functionality by IIS.
• Run on IIS Server and filter every request until they find one they need to process.
• Filter can programmed to examine and modify both incoming and outgoing streams of data. (logging)
• Change request data (URL or headers) send by client.
• Control physical file get mapped to the URL,
• Control Username and password (authentication)
• Change response 
• Encryption and decryption.