Exploration Through Example

Example-driven development, Agile testing, context-driven testing, Agile programming, Ruby, and other things of interest to Brian Marick

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Wed, 11 May 2005

Approaches to legacy code

Some recent and pending encounters with legacy code have made me think there may be three different broad approaches.

Rewrite and throw away

In this approach, you declare that the application will be broken for some period of time. Like a cartoon character, you dive into it, produce a furious cloud of dust that obscures you (picture gears and such flying out of the cloud), and finish with a new thing (or a thing with a shiny new component installed).

Refactor into submission

This is the approach that Feathers teaches in his fine Working Effectively With Legacy Code. In it, you gradually wrap pieces of the application in tests, gingerly fixing dependencies to make that tractable. Then, when some part of the code is wrapped and decoupled, it can be changed safely. I think of this as islands of order growing amongst the seething chaos of the product.

Strangle

The term strangler application is due to Martin Fowler. The image is of a vine growing up around a tree, gradually killing it, until eventually the only thing left alive is the vine, roughly in the shape of the original tree. You don't (much) fix up existing code. Instead, when you need something new or changed, you begin by building fresh, greenfield code. The legacy code can call into the greenfield code, but access in the other direction is minimized and highly controlled. (I think it's fair to say a project Strangling its application is using Feathers's Chia Pet Pattern (pdf, slide 9) as the overwhelmingly dominant tool/metaphor.)

As the last parenthetical remark suggests, these approaches shade into one another. Nevertheless, they seem to me three distinguishable stances toward legacy code. My question is: how do you decide which stance to take?

In the 60's, renowned software engineer Rocket J. Squirrel drew on his extensive experience to make the definitive comment on Rewrite and Throw Away: "That trick never works!". Which isn't invariably true, of course, but it's a risky strategy. The other two methods can still deliver a steady stream of features to the business, albeit at a slower rate (in the short term). In this one, the team vanishes from view of the business, doing who knows what, promising great things for the future. That's an unstable situation, because the business is liable to get fed up and start insisting on features ASAP. It's also a dangerous situation for the programmers. Because they have to finish everything before anything works, they might not know they're way off course until far too late.

Still, Rewrite and Throw Away is easier than refactoring into submission. When faced by a tangled mass of code, it's hard to know where to start. And, in my (lack of) experience, you're much more likely to have a refactoring that flames out and has to be rolled back, just because of some complex interconnectedness you didn't grasp at first. Until you've learned-through-doing, it's frustrating, and frustration leads to short-cuts (which is how you got the legacy code in the first place, probably).

Strangling the application has the advantage that you've consciously decided you shan't fix the old code, so the dangers of touching it are (somewhat) lessened. It also has the advantage that you can quickly create a concrete architecture (a set of layered subsystems, say) to guide your forward movement. (This insight is due to Michael Thomas, who's writing me an article on strangling code.) I don't have the experience to speak to Strangling's disadvantages (except that I can imagine long debates about the concrete architecture preventing people from getting moving).

I have this idea that there must be a set of patterns or lore that would help people navigate among those choices. Who will write it down?

## Posted at 09:14 in category /coding [permalink] [top]