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To do that let's look at how architectural evolution

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occurs in practice. Ideally when a system evolves, its prescriptive

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architecture should be modified first. Just like when you

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modify a building. You change the blueprint and then you

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change the actual building. You don't go the other

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way around. In software, unfortunately this rarely ever happens in

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practice. In practice the system, and therefore it's descriptive

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architecture are often directly modified. Like in this case that

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I'm showing here. So what happens is that the architecture

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as conceived does not change. Whereas the architecture as implemented, does

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change. And therefore these two things start diverging. And this really

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happens for a number of reasons. So I'm just going to list

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a few of those reasons here. In some cases it

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just happens for plain sloppiness. I need to make this modification

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and I don't really want to go back and look at

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the prescriptive architecture modified. I'm just going to make the change, and

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maybe I'll fix the description later. And then you never really get

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to it. In other cases you do this because of the perception

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of short deadlines. If you have to do something by this afternoon,

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you're not going through a four

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month software architecture review, you normally just

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get to it, and do it. In some cases a prescriptive architecture

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is not even present, so there's a lack of documentation. So in these

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cases, clearly, you cannot go and modify something that does not even

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exist, and so you jump directly to the code and start modifying that.

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And as I said there's many, many more

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other reasons why that happen. But important point here is

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that it does happen and it does happen often

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and the result is that prescriptive and descriptive architectures diverge.