Lessons from the ASP Classic Compiler project

I have not done much with my ASP Classic Compiler project for over a year now. The lack of additional work is due to both the economic and the technical reasons. I will try to document the reasons here both for myself and for would–be open source developers.

How the project got started?

I joint my current employer at the end of 2008. The company had a large amount of ASP classic code in its core product. The company was in talk with a major client on customization so that there was a period that the development team had fairly light load. I had a chance to put in a lot of thinking on how to convert the ASP classic code to ASP.NET. One of the ideas was to compile ASP Classic code into .net IL so that they can be executed within the ASP.NET runtime.

I do not have a formal education in computer science; my Ph.D. is in physics. I had been very fond of writing parsers by following the book “Writing Compilers & Interpreters” since 1996. I wrote a VBA like interpreter and a mini web-browser, all in Visual Basic, using the knowledge acquired. Nevertheless, a full-featured compiler is still a major undertaking. I spent several months of my spare-time working on the parser and compiler, and studied the theory behind. I was able to implement all the VBScript 3.0 features and the compiler was able to execute several Microsoft best-practice applications. The code runs about twice as fast as ASP Classic. I had lots of fun experimenting with ideas such as using StringBuilder for string concatenation to drastically improve the speed of rendering. I was awarded Microsoft ASP/ASP.NET MVP in 2010. In 2011, I donated the project to the community by opening the source code after consulting with my employer.

Since then, I faced challenges on two fronts: adding VBScript 5.0 support and solving a large number of compatibility issues in the real-world. After working for a few more months, I found it difficult to sustain the project.

Lesson 1: the economy and business issues

1. Sustainability of an open source project requires business arrangements. I am an employee. Technical activities outside the work are welcome as long as it benefits the employer. However, excess amount of activities not relating to the work could be a loyalty issue.

2. ASP Classic is a shrinking platform. So ASP Classic Compiler would not be a sound investment for most businesses except for Microsoft who has interest in bridging customers from older to newer platform painlessly.

Lesson 2: the technical issues

1. VBScript is a terrible language without a formal specification and test suites. The best “spec” outside of the users’ guide is probably Eric Lippert’s blog. It is not getting much maintenance and it is losing favor. As a result, few other developers with knowledge in compiler would have interest in VBScript.

2. Like the IronPython project and the Rhino projects, I started by writing a compiler. I was very shrilled that my first implementation without much optimization is about twice as fast as VBScript in my benchmark test. However, I had many obstacles on compatibility and adding the debug feature. If I would do it again, I would probably implement it as an interpreter with an incremental compiler. The interpreter would have a smaller start-up overhead. I can then have a background thread compiling the high usage code. The delayed compiling would allow me to employee more sophisticated data-flow analysis and construct larger call-site blocks. The current naïve implementation on DLR results in too many very small call-sites so that type-checking at call-sites is a significant overhead.

In summary, I had lots of fun with the project. I significantly improved my theoretical knowledge of algorithms in general, and of parsers and compilers. I had the honor of being awarded Microsoft ASP.NET/IIS MVP for the past 3 years and enjoyed my private copy of MSDN subscription at home. After a working prototype, the remaining work to take it to a production quality software is more a business problem than else. I am actively contemplating some new forward-looking ideas and wish I have some results to share soon.


  • Thank you for all the comments. Classic ASP Compiler was open source under the Apache License v2.

    What do you think of sharing ASP classic and ASP.NET state?

  • @johnnycash11, if you use ASP Classic Compiler, it shares session state with ASP.NET. The downside is that it only supports VBScript 3.0 and still has a lot of compatibility issues. If you want to use ASP Classic and share state with ASP.NET, you might try my other project NSession (http://nsession.codeplex.com/). You would need to use the Session State Service that is part of ASP.NET.

  • VBScript (Visual Basic Scripting Edition) is an Active Scripting language developed by Microsoft that is modeled on Visual Basic. It is designed as a “lightweight” language with a fast interpreter for use in a wide variety of Microsoft environments.

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