Software Design

Introduction

Material drawn from [Godfrey96,Parnas86,Parnas94]

Software Design (Introduction) © SERG 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) © SERG 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) © SERG 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) © SERG Requirements Engineering: An Overview

• Key to RE is good communication between customer and developers. • Work from Requirements Document as guide.

Software Design (Introduction) © SERG 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) © SERG 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) © SERG 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) © SERG Requirements Analysis The RE Process leads to produces Requirements Definition

System Requirements Models Specification

Requirements Definition Software Specification included in Requirements Specification

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

Software Design (Introduction) © SERG 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) © SERG 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) © SERG 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) © SERG 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) © SERG 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) © SERG 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) © SERG 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) © SERG 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) © SERG 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) © SERG 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) © SERG 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) © SERG Back to Software Design ...

Software Design (Introduction) © SERG SoftwareSoftware architecturearchitecture

SubsystemSubsystem High-Level decompositiondecomposition (abstract) SubsystemSubsystem design dependenciesdependencies

SubsystemSubsystem interfacesinterfaces

modulemodule or or class class decompositiondecomposition

modulemodule or or class class dependenciesdependencies

modulemodule or or class class Low-Level interfacesinterfaces (detailed)

design DataData structures structures

AlgorithmsAlgorithms Software Design (Introduction) © SERG 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) © SERG 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) © SERG 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) © SERG Discussion

• What does “quality” mean to: – IBM? – Microsoft? – Netscape? – FAA? – IRS? – Intel? – ...

Software Design (Introduction) © SERG 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) © SERG 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) © SERG 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) © SERG 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) © SERG 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) © SERG RPD Payoff

The real payoff comes during maintenance and “the next time around”.

Software Design (Introduction) © SERG 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) © SERG 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) © SERG 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) © SERG 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) © SERG 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) © SERG 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) © SERG 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) © SERG 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) © SERG 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) © SERG 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) © SERG 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) © SERG 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) © SERG 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) © SERG The Cost of Software Failure

• Inability to keep up, • reduced performance, • decreasing reliability.

Software Design (Introduction) © SERG 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) © SERG 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) © SERG Decreasing Reliability

• As the software is maintained, errors are introduced. • Many studies have shown that each time an attempt is made to decrease the failure rate of a system, the failure rate got worse! • That means that, on average, more than one error is introduced for every repaired error.

Software Design (Introduction) © SERG Decreasing Reliability (Cont’d)

• Often the choice is to either: – abandon the project – stop fixing bugs • For a commercial product, Parnas was once told that the list of known unrepaired bugs exceeded 2,000.

Software Design (Introduction) © SERG Reducing the Cost of SW Aging

• We should be looking far beyond the first release to the time when the product is old. • Inexperienced programmers get a “rush” after the first successful compile or demonstration. • Experienced programmers realize that this is only the beginning ...

Software Design (Introduction) © SERG Reducing the Cost of SW Aging (Cont’d) • Responsible, professional, organizations realize that more work is invested between the time after the first successful run and the first release than is required to get the first successful run. • Extensive testing and rigorous reviews are necessary.

Software Design (Introduction) © SERG Preventive Medicine

• Design for success • Keep records (documentation) • Seek second opinions (reviews)

Software Design (Introduction) © SERG Design for Success

• Design for change. • This principle is known by various names: – information hiding – abstraction – separation of concerns – data hiding – object-orientation

Software Design (Introduction) © SERG Design for Change

• To apply this principle one begins by trying to characterize the changes that are likely to occur over the “lifetime” of a product. • Since actual changes cannot be predicted, predictions will be about classes of changes: – changes in the UI – change to a new windowing system – changes to data representation – porting to a new operating system ...

Software Design (Introduction) © SERG Design for Change (Cont’d)

• Since it is impossible to make everything equally easy to change, it is important to: – estimate the probabilities of each type of change – organize the software so that the items that are most likely to change are “confined” to a small amount of code

Software Design (Introduction) © SERG Why is Design for Change Ignored? • Textbooks fail to discuss the process of estimating the probability of change for various classes of changes. • Programmers are impatient because they are too eager to get the first version working. • Designs that result from this principle are different from the “natural” designs of the programmer’s intuition.

Software Design (Introduction) © SERG Why is Design for Change Ignored? (Cont’d)

• Few good examples of the application of the principle. Designers tend to mimic other designs they have seen. • Programmers tend to confuse design principles with languages. • Many practitioners lack training in software development.

Software Design (Introduction) © SERG Keeping Records (Documentation)

• Even when software is well designed, it is often not documented. • When documentation is present it is often: – poorly organized – inconsistent – incomplete – written by people who do not understand the system

Software Design (Introduction) © SERG Documentation

• Hence, documentation is ignored by maintainers. • Worse, documentation is ignored by managers because it does not speed up the initial release.

Software Design (Introduction) © SERG Second Opinions (Reviews)

• In engineering, as in medicine, the need for reviews by other professionals is never questioned. • In designing a building, ship, aircraft, there is always a series of design documents that are carefully reviewed by others.

Software Design (Introduction) © SERG Reviews • This is not true in the software industry: – Many programmers have no professional training in software at all. – Emphasis of CS degrees on mathematics and science; professional discipline is not a topic for a “liberal” education. – Difficult to find people who can serve as quality reviewers; no money to hire outsiders. – Time pressure misleads designers into thinking that they have no time for proper reviews. – Many programmers resent the idea of being reviewed. Software Design (Introduction) © SERG Reviews

• Every design should be reviewed and approved by someone whose responsibilities are for the long-term future of the product.

Software Design (Introduction) © SERG Why is Software Aging Inevitable? • Our ability to design for change depends on our ability to predict the future. • We can do so only approximately and imperfectly. • Over a period of years: – changes that violate original assumptions will be made – documentation will never be perfect – reviewers are bound to miss flaws ... Software Design (Introduction) © SERG Why is Software Aging Inevitable? (Cont’d)

• Preventive measures are worthwhile but anyone who thinks that this will eliminate aging is living in a dream world.

Software Design (Introduction) © SERG Software Geriatrics

• Retroactive Documentation: – A major step in slowing the age of older software, and often rejuvenating it, is to upgrade the quality of the documentation. • Retroactive Modularization: – Change structure so that each module hides a design decision that is likely to change.

Software Design (Introduction) © SERG Software Geriatrics (Cont’d)

• Amputation: – A section of code has been modified so often, and so thoughtlessly, that it is not worth saving. • Major Surgery (Restructuring): – Identify and eliminate redundant components and gratuitous dependencies.

Software Design (Introduction) © SERG Planning Ahead • It’s time to stop acting as if “getting it to run” was the only thing that matters. • Designs and changes have to be documented and carefully reviewed. • If it’s not documented, it’s not done. • In other areas of engineering, product obsolescence is recognized and included in design and marketing plans. • The same should be done for software engineering. Software Design (Introduction) © SERG References

• [Godfrey96] M. W. Godfrey class lecture notes in Software Engineering. • [Parnas86] D. L. Parnas, P. C. Clements, A Rational Design Process, How and Why to Fake It, IEEE TSE, Vol 12, No 2, 1986. • [Parnas94] D. L. Parnas, Software Aging (plenary talk), ICSE, May, 1994.

Software Design (Introduction) © SERG