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Software Design

Software Design. Introduction. Material drawn from [Godfrey96,Parnas86,Parnas94]. Software Design. How to implement the what . Requirements Document (RD) is starting point. Software design is a highly-creative activity. Good designers are worth their weight in gold!

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Software Design

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  1. Software Design Introduction Material drawn from [Godfrey96,Parnas86,Parnas94] Software Design (Introduction)

  2. Software Design • How to implement the what. • Requirements Document (RD) is starting point. • Software design is a highly-creative activity. • Good designers are worth their weight in gold! • Highly sought after, head-hunted, well-paid. • Experience alone is not enough: • creativity, “vision”, all-around brilliance required. Software Design (Introduction)

  3. Software Design (Cont’d) • Some consider software design to be a “black art”: • difficult to prescribe how to do it • hard to measure a good design objectively • “I know a good design when I see it.” Software Design (Introduction)

  4. Requirements Engineering:An Overview • Basic goal: To understand the problem as perceived by the user. • Activities of RE are problem oriented. • Focus on what, not how • Don’t cloud the RD with unnecessary detail • Don’t pre-constrain design. • After RE is done, do software design: • solution oriented • how to implement the what Software Design (Introduction)

  5. Requirements Engineering:An Overview • Key to RE is good communication between customer and developers. • Work from Requirements Document as guide. Software Design (Introduction)

  6. Requirements Engineering • Basically, it’s the process of determining and establishing the precise expectations of the customer about the proposed software system. Software Design (Introduction)

  7. The Two Kinds of Requirements • Functional: The precise tasks or functions the system is to perform. • e.g., details of a flight reservation system • Non-functional: Usually, a constraint of some kind on the system or its construction • e.g., expected performance and memory requirements, process model used, implementation language and platform, compatibility with other tools, deadlines, ... Software Design (Introduction)

  8. The Purpose of RE • Raw user requirements are often: • vague • contradictory • impractical or impossible to implement • overly concrete • just plain wrong • The purpose of RE is to get a usable set of requirements from which the system may be designed and implemented, with minimal “surprises”. Software Design (Introduction)

  9. System Models Requirements Definition Requirements Specification Requirements Document Software Specification Requirements Analysis The RE Process leads to Requirements Definition produces Requirements Specification Software Specification included in Software Design (Introduction)

  10. The Requirements Document • The official statement of what is required of the system developers. • Includes system models, requirements definition, and requirements specification. • Not a design document. • States functional and non-functional requirements. • Serves as a reference document for maintenance. Software Design (Introduction)

  11. Requirements Document “Requirements” • Should be easy to change as requirements evolve. • Must be kept up-to-date as system changes. Software Design (Introduction)

  12. The Requirements Document Should State ... • Foreseen problems: • “won’t support Win-3.x apps” • Expected evolution: • “will port to MacOS in next version” • Response to unexpected events/usage: • “if input data in old format, will auto-convert” Software Design (Introduction)

  13. Requirements Document Structure • Introduction (describe need for system) • Functional Requirements • Non-Functional Requirements • System Evolution (describe anticipated changes) • Glossary (technical and/or new jargon) • Appendices • Index Software Design (Introduction)

  14. A Story ... Dear Mr. Architect, Please design and build me a house. I am not quite sure of what I need, so you should use your discretion. My house should have between two and forty-five bedrooms. Just make sure the plans are such that bedrooms can be easily added or deleted. When you bring the blueprints to me, I will make the final decision of what I want. Also bring me the cost breakdown for each configuration so that I can arbitrarily pick one. Software Design (Introduction)

  15. A Story … (Cont’d) Keep in mind that the house I ultimately chose must cost less than the one I am currently living in. Make sure, however, that you correct all the deficiencies that currently exist in my house (the floor of my kitchen vibrates when I walk across it, and the walls don’t have nearly enough insulation in them). Also keep in mind as you design this house that I wish to keep yearly maintenance cost as low as possible. This should mean the incorporation of extra-cost features like aluminum or vinyl siding. If you chose not to specify aluminum, be prepared to explain in detail. Software Design (Introduction)

  16. A Story … (Cont’d) Please take care that modern design practices and the latest materials are used in construction of the house. The house should be really nice. However, be alerted that the kitchen should be designed to accommodate among other things, my 1952 Gibson refrigerator. To assure that you are building the correct house for our family, make sure that you contact each of the children and also the in-laws. My mother-in-law will have very strong feelings about how the house ought to be designed since she visits with us at least once a year. Make sure that you weigh all these options carefully and make the right decision. I, however, retain the right to override any decision you come up with. Software Design (Introduction)

  17. A Story … (Cont’d) Please don’t bother me with small details right now. Your job is to develop the overall plans for this house. Get the big picture. It is not appropriate at this time to be choosing the color of the carpet. However, keep in mind that my wife likes green. Also do not worry at this time about acquiring resources to build this house. Your first priority is to develop detailed plans and specifications. However, once I accept these plans, I will expect to have the house under roof within 48 hours. Software Design (Introduction)

  18. A Story … (Cont’d) While you are designing this house specifically for me, keep in mind that sooner or later I will have to sell this house. It should have appeal to potential buyers. Please make sure that before you finalize the plans, there is a consensus of the population in my area that they like the features this house has. You are advised to run up and look at my neighbor’s house he had constructed last year. We like it a great deal. It has many features that we would like to have in our new home, particularly the 75-foot swimming pool. With careful engineering I believe that you can design this into our new house without impacting the construction cost. Software Design (Introduction)

  19. A Story … (Cont’d) Please prepare a complete set of blueprints. It is not necessary at this time to do the real design since these blueprints will be used only for construction bids. Please be advised however, that any increase of cost in the future as a result of design changes will result in you getting your hands slapped. You must be thrilled to be working on such an interesting project such as this. To be able to use new kinds of construction and to be given such freedom in your designs is something that doesn’t happen very often. Contact me as rapidly as possible with your design ideas. I am enthusiastic about seeing what you can come up with. Software Design (Introduction)

  20. A Story … (Cont’d) P.S. My wife has just told me that she disagrees with many on the instructions I’ve given you in this letter. As architect it is your responsibility to resolve these issues. I have tried in the past and have been unable to accomplish this. If you can’t handle this, I’ll have to look for a new architect. P.P.S. Perhaps what I need is not a house at all, but a travel trailer. Please advise me as early as possible if that is the case. Software Design (Introduction)

  21. RE Summary • RE focuses on determining what the customer wants, and not how it will be implemented. • RE is hard to get correct; it requires good communication skills. • Requirements may change over time. • RE requires iteration. Software Design (Introduction)

  22. RE Summary (Cont’d) • The customer often doesn’t have good grasp of what he wants. • Errors made at the requirements stage are very expensive to fix later. • You might well implement the stated requirements correctly, but it won’t be the system the customer really wants. Software Design (Introduction)

  23. Back to Software Design ... Software Design (Introduction)

  24. Software architecture Subsystem decomposition Subsystem dependencies Subsystem interfaces module or class decomposition module or class dependencies module or class interfaces Data structures Algorithms High-Level (abstract) design Low-Level (detailed) design Software Design (Introduction)

  25. Top-Down vs Bottom-Up Design • Top-down Design: • Start with a coarsely-grained view of system, and repeatedly refine components until you have concrete sub-components. • Bottom-up Design: • Start with existing components and “glue” them together to get what you want. Software Design (Introduction)

  26. Top-Down vs Bottom-Up Design (Cont’d) • Top-down is the “ideal” of most design methods, but it’s rarely followed absolutely: • some branches of development are expanded before others are even started • doesn’t adequately account for reuse of existing components: • COTS products, libraries, previous versions of the same system. Software Design (Introduction)

  27. Design Quality • Software design “quality”, as with other ideas on quality, is an elusive concept: • It depends on priorities of your company and the customers: • fastest to implement • easiest to implement • easiest to maintain, “evolve”, port • most efficient/reliable/robust end-product. Software Design (Introduction)

  28. Discussion • What does “quality” mean to: • IBM? • Microsoft? • Netscape? • FAA? • IRS? • Intel? • ... Software Design (Introduction)

  29. Some Desirable Design Attributes • Hierarchical: A good design should be organized into a well-designed hierarchy of components. • Modular: Separate distinct concerns (data and processing) into distinct containers (i.e., subsystems, modules, and/or classes). Hide implementation details and provide clean, simple interfaces for each container. Software Design (Introduction)

  30. Some Desirable Design Attributes (Cont’d) • Independent: Group similar things together; limit the amount of “special knowledge” that unrelated components may share. If you change your mind about something, the impact will be localized. Software Design (Introduction)

  31. Some Desirable Design Attributes (Cont’d) • Simple Interfaces: Endless flexibility adds complexity. Complex interfaces mean: • hard to understand by users and developers (e.g., Unix man page syndrome) • many possible variations of use • inconvenient to change interface in order to eliminate “bad options”. • You can get away with “flexible interfaces” in a low-level localized setting, but the larger the scale, the simpler the interface should be. Software Design (Introduction)

  32. A Rational Design Process:How and Why to Fake it “Many have sought a software design process that allows a program to be derived systematically from a precise statement of requirements. … although we will not succeed in designing a real product that way, we can produce documentation that makes it appear that the software was designed by such a process …” D. L. Parnas Software Design (Introduction)

  33. RDP - “Faking It” • The “rational design process” is an irrational ideal. • Question: If we rarely act in a purely top-down way when we develop software, why do most design methods assume we do? • Possible answers: • It’s simpler than trying to model real-life. • Wide variation in problems, possible solutions. • There is utility in the structure of the process, and its ongoing documentation. Software Design (Introduction)

  34. RPD Payoff The real payoff comes during maintenance and “the next time around”. Software Design (Introduction)

  35. The Role of Documentation • Documentation plays a major role in the development of any large, long-lived software system BUT poor documentation is a monumental, ubiquitous problem. • “Most programmers regard documentation as a necessary evil, written as an afterthought only because some bureaucrat requires it. They do not expect it to be useful.” • This attitude is a self-fulfilling prophecy! Software Design (Introduction)

  36. The Role of Documentation (Cont’d) • While most documents are incomplete and inaccurate, these problems can be fixed easily. More serious problems are: • Poor organization. • Boring, redundant, verbose prose: • Boredom leads to inattentive reading and undiscovered errors. • Confusing and inconsistent terminology. • “Myopia”, can’t see the forest through the trees. • Focus is on documenting small details rather than on important design decisions. Software Design (Introduction)

  37. So What’s to be Done? • Look at the various stages of the “Rational Design Process”, and consider the documents that are to be produced at each step. • Even if you don’t follow the steps in that order, go back and fill in the blanks! Pretend that you did follow the process precisely. • It isn’t just a paper trail you’re creating. Software Design (Introduction)

  38. Programs vs Proofs • Designing a software system is a lot like proving a mathematical theorem. • The construction of an original proof is a painful process; you make lots of mistakes, pursue bad paths, ... • However, once you’ve figured out how to get there, you clean up the proof and present it as if no mistakes had ever been made. Software Design (Introduction)

  39. Programs vs Proofs • If you want to prove a similar but different theorem, you can re-use ideas from the first proof! • The main difference is that during software development, you also record why you chose each path, what alternatives were considered, and why other paths were not chosen. Software Design (Introduction)

  40. Software Aging “Programs, like people, get old. We can’t prevent aging, but we can understand its causes, take steps to limit its effects, temporarily reverse some of the damage it has caused, and prepare for the day when the software is no longer viable. ... (We must) lose our preoccupation with the first release and focus on the long term health of our products.” D.L. Parnas Software Design (Introduction)

  41. Software “Aging”? • “It does not make sense to talk about software aging!” • Software is a mathematical product, mathematics does not decay with time. • If a theorem was correct 200 years ago, it will be correct tomorrow. • If a program is correct today, it will be correct 100 years from now. • If a program is wrong 100 years from now, it must have been wrong when it was written. • All of the above statements are true, but not really relevant. Software Design (Introduction)

  42. Software Does Age • Software aging is gaining in significance because: • of the growing economic importance of software, • software is the “capital” of many high-tech firms. Software Design (Introduction)

  43. Software Does Age • The authors and owners of new software products often look at aging software with disdain. • “If only the software had been designed using today’s languages and techniques …” • Like a young jogger scoffing at an 86 year old man (ex-champion swimmer) and saying that he should have exercised more in his youth! Software Design (Introduction)

  44. The Causes of Software Aging • There are two types of software aging: • Lack of Movement: Aging caused by the failure of the product’s owners to modify it to meet changing needs. • Ignorant Surgery: Aging caused as a result of changes that are made. • This “one-two punch” can lead to rapid decline in the value of a software product. Software Design (Introduction)

  45. Lack of Movement • Unless software is frequently updated, its user’s will become dissatisfied and change to a new product. • Excellent software developed in the 60’s would work perfectly well today, but nobody would use it. • That software has aged even though nobody has touched it. • Actually, it has aged because nobody bothered to touch it. Software Design (Introduction)

  46. Ignorant Surgery • One must upgrade software to prevent aging. • Changing software can cause aging too. • Changes are made by people who do not understand the software. • Hence, software structure degrades. Software Design (Introduction)

  47. Ignorant Surgery (Cont’d) • After many such changes nobody understands the software: • the original designers no longer understand the modified software, • those who made the modification still do not understand the software. • Changes take longer and introduce new bugs. • Inconsistent and inaccurate documentation makes changing the software harder to do. Software Design (Introduction)

  48. The Cost of Software Failure • Inability to keep up, • reduced performance, • decreasing reliability. Software Design (Introduction)

  49. Inability To Keep Up • As software ages, it grows bigger. • “Weight gain” is a result of the fact that the easiest way to add a feature is to add new code. • Changes become more difficult as the size of the software increases because: • There is more code to change, • it is more difficult to find the routines that must be changed. • Result: Customers switch to a “younger” product to get the new features. Software Design (Introduction)

  50. Reduced Performance • As the size of the program grows, it places more demands on the computer memory. • Customers must upgrade their computers to get acceptable response. • Performance decreases because of poor design that has resulted from long-term ad hoc maintenance. • A “younger” product will run faster and use less memory because it was designed to support the new features. Software Design (Introduction)

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