5.1-Software Life-Cycle Models - (Chap. 2 of Schach) - Part 1.pdf

Object-Oriented and Classical Software Engineering Eighth Edition, WCB/McGraw-Hill, 2011 Stephen R. Schach CHAPTER 2 SOFTWARE LIFE-CYCLE MODELS COMP1010 | www.newcastle.edu.au Overview S...

Object-Oriented and Classical Software Engineering Eighth Edition, WCB/McGraw-Hill, 2011 Stephen R. Schach CHAPTER 2 SOFTWARE LIFE-CYCLE MODELS COMP1010 | www.newcastle.edu.au Overview Software development in theory Winburg mini case study Lessons of the Winburg mini case study Teal tractors mini case study Iteration and incrementation Winburg mini case study revisited Risks and other aspects of iteration and incrementation Managing iteration and incrementation COMP1010 | www.newcastle.edu.au 2.1 Software Development in Theory Ideally, software is developed as described in Chapter 1 – Linear – Starting from scratch Figure 2.1 COMP1010 | www.newcastle.edu.au Software Development in Practice In the real world, software development is totally different – We make mistakes – The client’s requirements change while the software product is being developed COMP1010 | www.newcastle.edu.au 2.2 Winburg Mini Case Study Episode 1: The first version is implemented Episode 2: A fault is found – The product is too slow because of an implementation fault – Changes to the implementation are begun Episode 3: A new design is adopted – A faster algorithm is used Episode 4: The requirements change – Accuracy has to be increased Epilogue: A few years later, these problems recur COMP1010 | www.newcastle.edu.au Evolution-Tree Model Winburg Mini Case Study Figure 2.2 COMP1010 | www.newcastle.edu.au Waterfall Model The linear life cycle model with feedback loops – The waterfall model cannot show the order of events Figure 2.3 COMP1010 | www.newcastle.edu.au Return to the Evolution-Tree Model The explicit order of events is shown At the end of each episode – We have a baseline, a complete set of artifacts (constituent components) Example: – Baseline at the end of Episode 3: o Requirements1, Analysis1, Design3, Implementation3 COMP1010 | www.newcastle.edu.au 2.3 Lessons of the Winburg Mini Case Study In the real world, software development is more chaotic than the Winburg mini case study Changes are always needed – A software product is a model of the real world, which is continually changing – Software professionals are human, and therefore make mistakes COMP1010 | www.newcastle.edu.au 2.4 Teal Tractors Mini Case Study While the Teal Tractors software product is being constructed, the requirements change The company is expanding into Canada Changes needed include: – Additional sales regions must be added – The product must be able to handle Canadian taxes and other business aspects that are handled differently – Third, the product must be extended to handle two different currencies, USD and CAD COMP1010 | www.newcastle.edu.au Teal Tractors Mini Case Study (contd) These changes may be – Great for the company; but – Disastrous for the software product COMP1010 | www.newcastle.edu.au Moving Target Problem A change in the requirements while the software product is being developed Even if the reasons for the change are good, the software product can be adversely impacted – Dependencies will be induced COMP1010 | www.newcastle.edu.au Moving Target Problem (contd) Any change made to a software product can potentially cause a regression fault – A fault in an apparently unrelated part of the software If there are too many changes – The entire product may have to be redesigned and reimplemented COMP1010 | www.newcastle.edu.au Moving Target Problem (contd) Change is inevitable – Growing companies are always going to change – If the individual calling for changes has sufficient clout, nothing can be done about it There is no solution to the moving target problem COMP1010 | www.newcastle.edu.au 2.5 Iteration and Incrementation In real life, we cannot speak about “the analysis phase” – Instead, the operations of the analysis phase are spread out over the life cycle The basic software development process is iterative – Each successive version is intended to be closer to its target than its predecessor COMP1010 | www.newcastle.edu.au Miller’s Law At any one time, we can concentrate on only approximately seven chunks (units of information) To handle larger amounts of information, use stepwise refinement – Concentrate on the aspects that are currently the most important – Postpone aspects that are currently less critical – Every aspect is eventually handled, but in order of current importance This is an incremental process COMP1010 | www.newcastle.edu.au Iteration and Incrementation (contd) Figure 2.4 COMP1010 | www.newcastle.edu.au Iteration and Incrementation (contd) Iteration and incrementation are used in conjunction with one another – There is no single “requirements phase” or “design phase” – Instead, there are multiple instances of each phase COMP1010 | www.newcastle.edu.au Figure 2.2 (again) Iteration and Incrementation (contd) The number of increments will vary — it does not have to be four COMP1010 | www.newcastle.edu.au Classical Phases versus Workflows Sequential phases do not exist in the real world Instead, the five core workflows (activities) are performed over the entire life cycle – Requirements workflow – Analysis workflow – Design workflow – Implementation workflow – Test workflow COMP1010 | www.newcastle.edu.au Workflows All five core workflows are performed over the entire life cycle However, at most times one workflow predominates Examples: – At the beginning of the life cycle o The requirements workflow predominates – At the end of the life cycle o The implementation and test workflows predominate Planning and documentation activities are performed throughout the life cycle COMP1010 | www.newcastle.edu.au Iteration and Incrementation (contd) Iteration is performed during each incrementation Figure 2.5 COMP1010 | www.newcastle.edu.au Iteration and Incrementation (contd) Again, the number of iterations will vary—it is not always three COMP1010 | www.newcastle.edu.au 2.6 The Winburg Mini Case Study Revisited Consider the next slide The evolution-tree model has been superimposed on the iterative-and- incremental life-cycle model The test workflow has been omitted — the evolution-tree model assumes continuous testing COMP1010 | www.newcastle.edu.au The Winburg Mini Case Study Revisited Figure 2.6 COMP1010 | www.newcastle.edu.au More on Incrementation (contd) Each episode corresponds to an increment Not every increment includes every workflow Increment B was not completed Dashed lines denote maintenance – Episodes 2, 3: Corrective maintenance – Episode 4: Perfective maintenance COMP1010 | www.newcastle.edu.au 2.7 Risks and Other Aspects of Iter. and Increm. We can consider the project as a whole as a set of mini projects (increments) Each mini project extends the – Requirements artifacts – Analysis artifacts – Design artifacts – Implementation artifacts – Testing artifacts The final set of artifacts is the complete product COMP1010 | www.newcastle.edu.au Risks and Other Aspects of Iter. and Increm. (contd) During each mini project we – Extend the artifacts (incrementation); – Check the artifacts (test workflow); and – If necessary, change the relevant artifacts (iteration) COMP1010 | www.newcastle.edu.au Risks and Other Aspects of Iter. and Increm. (contd) Each iteration can be viewed as a small but complete waterfall life-cycle model During each iteration we select a portion of the software product On that portion we perform the – Classical requirements phase – Classical analysis phase – Classical design phase – Classical implementation phase COMP1010 | www.newcastle.edu.au Strengths of the Iterative-and-Incremental Model There are multiple opportunities for checking that the software product is correct – Every iteration incorporates the test workflow – Faults can be detected and corrected early The robustness of the architecture can be determined early in the life cycle – Architecture — the various component modules and how they fit together – Robustness — the property of being able to handle extensions and changes without falling apart COMP1010 | www.newcastle.edu.au Strengths of the Iterative-and-Incremental Model (contd) We can mitigate (resolve) risks early – Risks are invariably involved in software development and maintenance We have a working version of the software product from the start – The client and users can experiment with this version to determine what changes are needed Variation: Deliver partial versions to smooth the introduction of the new product in the client organization COMP1010 | www.newcastle.edu.au Strengths of the Iterative-and-Incremental Model (contd) There is empirical evidence that the life-cycle model works The CHAOS reports of the Standish Group (see overleaf) show that the percentage of successful products increases COMP1010 | www.newcastle.edu.au Strengths of the Iterative-and-Incremental Model (contd) CHAOS reports from 1994 to 2006 COMP1010 | www.newcastle.edu.au Figure 2.7 Strengths of the Iterative-and-Incremental Model (contd) Reasons given for the decrease in successful projects in 2004 include: – More large projects in 2004 than in 2002 – Use of the waterfall model – Lack of user involvement – Lack of support from senior executives COMP1010 | www.newcastle.edu.au 2.8 Managing Iteration and Incrementation The iterative-and-incremental life-cycle model is as regimented as the waterfall model … … because the iterative-and-incremental life-cycle model is the waterfall model, applied successively Each increment is a waterfall mini project COMP1010 | www.newcastle.edu.au QUESTIONS ? COMP1010 | www.newcastle.edu.au