Code rewrites and joint cognitive systems

Way back in the year 2000, Joel Spolsky famously criticized the idea of doing a code rewrite.

The idea that new code is better than old is patently absurd. Old code has been used. It has been testedLots of bugs have been found, and they’ve been fixed. There’s nothing wrong with it.

Joel Spolsky, Things You Should Never Do, Part I

I think Spolsky is wrong here. His error comes from considering the software in isolation. The problem here isn’t the old code, it’s the interaction between the old code and the humans who are responsible for maintaining the software. If you draw the boundary around those people and the software together, you get what the cognitive systems engineering community calls a joint cognitive system.

One of the properties of joint cognitive systems is that the system has knowledge about itself. Being responsible for maintaining a legacy codebase is difficult because the joint cognitive system is missing important knowledge about itself.

Here’s Spolsky again:

When you throw away code and start from scratch, you are throwing away all that knowledge. 

But that knowledge is already gone! The people who wrote the code have left, and the current maintainers don’t know what their intent was. The joint cognitive system, the combination of code and the current maintainers, don’t know why it’s implemented the way it is.

Spolsky gestures at this, but doesn’t grasp its implications:

The reason that they think the old code is a mess is because of a cardinal, fundamental law of programming: It’s harder to read code than to write it.

Spolsky is missing the importance of a system’s ability to understand itself. Ironically, the computer scientist Peter Naur was writing about this phenomenon fifteen years earlier. In an essay titled Programming as Theory Building, he described the importance of having an accurate mental model or theory of the software, and the negative consequences of software being modified by maintainers with poor mental models.

It isn’t just about the software. It’s about the people and the software together.

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