A tale about side effects
I once was part of a project where my team inherited a ten-year-old code base built by another software shop. The client had decided to replace the development team to reduce the ongoing maintenance costs and improve the development speed for new features. So, we got the contract.
As was to be expected, it was not easy to gain an understanding of what the code actually did, and the changes we made in one area of the code base often had side effects in other areas. But we managed by testing exhaustively, adding automated tests, and refactoring a lot.
After some time of successfully maintaining and extending the code base, the client requested a new feature. And they wanted us to build it in a way that was very awkward for the users of the software. So, I proposed to do it in a more user-friendly way that was even less expensive to implement since it needed fewer overall changes. It needed a small change in a certain very central component, however.
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