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The next model that we will discuss is the spiral
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model, which was first described by Barry Boehm, which is
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the professor that we interviewed at the beginning of this
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lesson. In his paper from 1986 that was entitled A Spiral
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Model of Software Development and Enhancement. And one of the
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main characteristics of that paper is that it was describing the
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spiral model using a diagram, which is the one that
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I'm showing you here, and this diagram has become very very
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popular, and you probably saw it either in this
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form or one of the many variations of the
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diagram. So I'm not going to discuss all of
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the details of the spiral model, but I just want to
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give you an idea of its main characteristics. The
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spiral model is an incremental risk-oriented lifecycle model that has
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four main phases listed here: determine objectives, identify and
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resolve risks, development and tests, and plan the next iteration.
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A software project will go through these four phases in
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an iterative way. In the first phase, the requirements will
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be gathered. In the second phase, the risks and the
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alternate solutions will be identified, and a prototype will be produced.
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Software and tests for the software are produced in the
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development and test phase, which is the third step of the
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process. Finally, in the fourth phase, the output of the
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project, so far, is evaluated, and the next iteration is planned.
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So basically, what the spiral process prescribes is a
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way of developing software by going through these phases in
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an iterative way, in which we learn more and more
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of the software, we identify more and more, and account
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for, more and more risks and we go more
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and more towards our final solution, our final release. There
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are several advantages of using a spiral model. The first
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one is that the extensive risk analysis does reduce the
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chances of the project to fail. So there is a
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risk reduction advantage. The second advantage is that functionality can be
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added at a later phase because of the iterative nature of
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the process. And finally, software is produced early in the software
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lifecycle. So, at any iteration, we have something to show
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for our development. We don't wait until the end before producing
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something. And then of course there's also the advantage that we
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can get early feedback from the customer about what we produced.
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The main disadvantages on the other hand of
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the spiral model, are that the risk analysis requires
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a highly specific expertise. And unfortunately, the whole success
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of the process is highly dependent on risk analysis.
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So risk analysis has to be done right.
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And finally the spiral model is way more complex
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than other models, like for example, the water fall
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model. And therefore it can be costly to implement.
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