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I'm going to define a software systems architecture as

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the set of principal design decisions about the system. Where

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principal here, implies a degree of importance, that grants

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a design decision architectural status. And the point here, as

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we discussed with Neno early on, is that when

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building a system, we make tons of design decisions, and

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most of them do not affect the architecture of

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the system. For example, the effect of choosing a for

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loop, instead of a while loop, in the code, or the

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fact of deciding that we are going to use data structure A

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instead of data structure B. Some decisions however, do affect the

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architecture of the system. And in some cases the distinction between these

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two kinds of design decisions is clear. In some other cases

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it is much fuzzier and it depends on the context. The

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bottom line here, is that if you believe that something is

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an important design decision, that becomes an architectural decision. That is a

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decision that impacts a system's architecture. In this spirit,

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we can see a software architecture as the blueprint

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for a software system, that we can use to

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construct and evolve the system. And the key point

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about software architecture is that this blueprint encompasses every

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facet of the system under development. It encompasses its

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structure, of course, but not only. It also involves

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the behavior of the system, the interactions within the system,

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and the non-functional properties of the system. And we will see

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how this happens in the rest of the lesson. Another important

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point about software architecture is that there is a temporal aspect

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to it. And the point here is that you don't build the

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software architecture in a single shot, but you do it iteratively,

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over time. So, basically, you go from having no architecture to

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your final architecture. So, at any point in time, there is

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a software architecture, but it will change over time. And this happens

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because design decisions are made, unmade and changed over a system's lifetime.