What are the best practices for ISAPI extension development and debugging?

1 Choose the right tools

To develop ISAPI extensions, you need a C or C++ compiler, such as Visual Studio, and the IIS Software Development Kit (SDK), which contains the ISAPI header files, libraries, and samples. You also need a debugger, such as WinDbg or Visual Studio Debugger, that can attach to the IIS worker process (w3wp.exe) and set breakpoints, watch variables, and analyze call stacks. Additionally, you may want to use tools such as Fiddler or Wireshark to monitor the HTTP traffic between the client and the server.

2 Follow the coding guidelines

ISAPI extensions are loaded into the IIS worker process, so they share the same memory space and resources with other extensions and applications. To avoid memory leaks, crashes, or performance issues, coding guidelines should be followed. This includes using thread-safe and exception-safe code, as ISAPI extensions can run in a multi-threaded environment. Global variables should be avoided as they can cause concurrency problems or data corruption. It is also important to use the ISAPI functions and macros to interact with the IIS core and the client, release any resources that have been allocated in your extension, handle errors and return values properly, and log any relevant information for troubleshooting.

3 Test and debug locally

Before deploying an ISAPI extension to a production server, you should test and debug it locally on your development machine. This requires configuring IIS to load the extension DLL and map it to a URL path or file extension. You must also grant the appropriate permissions to the IIS worker process and the extension DLL. Additionally, debugging mode in IIS should be enabled, so you can attach a debugger. To verify expected behavior and output, use a web browser or HTTP client to send requests to the extension URL. A debugger can be used to set breakpoints, step through the code, and inspect variable values. Lastly, tools like DebugDiag or ADPlus can be utilized to capture crash dumps or memory dumps if unexpected errors occur.

4 Optimize for performance

ISAPI extensions can have a significant impact on IIS and web application performance, so it’s important to optimize your extension code and configuration for efficiency and scalability. To improve the performance of your ISAPI extensions, you should use the Keep-Alive feature to reuse connections, as well as asynchronous I/O operations to free up worker threads. Additionally, you can use caching techniques to store frequently used data or output in memory or on disk. Performance counters such as ISAPI Extension Requests/sec or ISAPI Extension Request Bytes Sent/sec can also be useful for monitoring activity and resource usage, helping you to identify any bottlenecks or issues.

5 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?