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Now to drive home some of the points that I just
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made, I would like to show you a few real world examples
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of architectures that kind of went astray. The first example I
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want to use is an example from the Linux kernel. Actually, from
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an earlier version of the Linux kernel. A research group studied
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the documentation of Linux, and also
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interviewed several Linux developers. And by
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doing that, they were able to come up with a software
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architecture of Linux at different levels of obstruction. So the one that
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I'm showing you here on the left, is the
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software architecture at the level of Linux's main subsystems. So
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this is the prescriptive architecture of Linux at the level
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of Linux's main subsystems. So the researchers, after identifying this
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architecture, they showed it to the developers, and the
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developers agreed that, that was indeed the architecture of the
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system. The researchers then studied the source code of Linux
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and reverse engineered its actual architecture. So the architecture as
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implemented, it's descriptive architecture. And this one here, on the
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right, is the result. And as you can see, they found
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a number of differences or violations between the prescriptive architecture and
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the descriptive architecture. In particular, if we look at this architecture,
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we can see that pretty much everything talks to everything else,
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which is, in general, not a good thing. And in addition
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to that, there are also several things that don't really make
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much sense. For example the library calls the file system and
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also the network interface which doesn't make much sense. Another thing
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that is kind of weird is the fact that file system
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calls the kernel initialization code. Which is also a little bit
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weird. So basically, the bottom line here is that not even
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the developers realized how the actual architecture of the system was,
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and how it was different from the architecture they have conceived.
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And in fact another interesting thing here is the reaction of
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the developers when they were shown the actual architecture. So basically
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they justified the differences by saying things such as, well you
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know it had to be done fast, and therefore I changed it
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and then I didn't have time to go back and update the documentation
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and things of this sort. And by the way these are exactly some
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of the reasons that we mentioned early on in the lesson for the
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discrepancy between prescriptive and descriptive software
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architecture. So one last thing that I
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want to mention here as an aside and we can get back to
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that later is the fact that you can probably clearly show how representing
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software architectures graphically can be extremely useful, because it allows
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for easily seeing the structure of the system. Look at different
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views identify problematic points and so on. And we will see
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how that can be useful in many cases also later on.
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