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So at this point, we have talked quite a bit about

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requirements engineering, but we haven't

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really discussed what are requirements exactly.

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So what is a requirement? To define that I am going

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to use this diagram which is a classical one. So you might

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have seen it before. So, discussing this diagram allows me to

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point out a few interesting things about requirements and define them

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in a better way. At a high level this diagram contains

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two main parts, the domain of the machine, which is the hardware,

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operating system, libraries and so on, on which the

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software will run. And the domain of the application, which

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is a world in which the software will operate.

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And the machine domain is characterized by computers, which are

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the hardware devices, and programs, which is the software

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that runs on these devices. The application domain, conversely, is

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characterized by domain properties, which are things that are true

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of the world anyways, whether I'm building my system or

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not, and requirements, which are things in the world we

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would like to achieve by delivering the system that we are

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building. Basically, to put it in a different way, the former,

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the domain properties, represents the assumptions that we make on the

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domain. And the latter, the requirements, are the actual requirements

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that we aim to collect. So we have something here, right,

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at the intersection of this application domain and this machine domain.

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And what is that? And this is what we normally call

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the specification, which is a description, often a formal description,

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of what the system that we are building should do to

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meet the requirements. So this is a bridge between these two

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domains. And as the graphical depiction shows, the specification is written

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in terms of shared phenomena. Things that are observable in both

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the machine domain and the application domain. And just to make

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things a little more concrete, I want to give you a

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couple of examples of what these phenomena, these shared phenomena, are.

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And we can think about two main kinds of phenomena.

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The first one are events in the real world that the

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machine can directly sense. For example, a button being pushed or

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a sensor being activated. These are events that happen here, but

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that the machine can detect. So they're events that can

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be used to define the specification. And the second type of

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phenomena are actions in the real world that the machine can

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directly cause. For example, an image appearing on a screen or

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a device being turned on and off. Again, this is something

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that the machine can make happen and then can have manifestation in

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the real world. And again this is therefore something on which

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the specification can predicate, something that

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we can describe in our specification.

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And this is sort of a philosophical discussion, but even if

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you don't care about the philosophical discussion, the one take away point

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that I would like for you to get from this discussion is

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the fact that when writing a specification you have to be aware

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of the fact that you're talking about shared phenomena. Events in the real

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world that the machine can sense and actions in the real world that the

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machine can cause. So this is what the specification is about, a bridge between

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these two worlds that define what the

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system should do to satisfy the requirements.