Reusability is a Deal with the Devil

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OK, it’s not, but it has some similarities… 🙂

Reusability sounds like a great thing. Build something and reuse it often. What’s not to like? Well, a few of things.

If you build something with the intent of being able to reuse it, by definition you are building more than you need right now. It takes extra effort to build in the extensibility points. You need to test for the hypothetical future cases.

There are cases where this extra effort is worthwhile. If you know you are going to be building a lot of similar applications the work you are saving yourself should offset the extra work. When you are building an interface to third party systems it makes sense to build in extension points. If your business is to build libraries for other developers, of course they need to be reusable. But these are rare cases and usually only need limited reusability.

If you have existing code and want to make it reusable, that effort has a cost. If the current codebase is not good, you will pay a lot to make it reusable, and not just immediately. The code will always be buggy because it started that way. To further tax the debt metaphor, you will pay interest on that code forever.

There are times when it might be OK to take current code and reuse it. In these cases you will want to copy and paste it into a new project. It should not keep any ties to its past life. It is serving a different purpose. The chance that it will solve the same problems is low. Trying to keep it in sync will just lead to more bugs.

If you really need to reuse code, you have to have automated tests around it. You need that safety net. Without it, as the code evolves away from its original purpose, bugs will creep in. A change to business logic to solve the current problem may have hidden impacts on other parts of the codebase. Automated tests will save you from a 4 hour hunt for that method you didn’t know existed because of a dependency injection trick.

If you need to get a feature done fast, reuse is your solid fair weather friend. Just be careful when the weather changes.

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