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Software Design Software Design (Introduction)(Introduction) © SERG

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!– Highly sought after, head-hunted, well-paid.

• Experience alone is not enough:– creativity, “vision”, all-around brilliance required.


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.”


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


Requirements Engineering:An Overview

• Key to RE is good communication between customer and developers.

• Work from Requirements Document as guide.


Requirements Engineering

• Basically, it’s the process of determining and establishing the precise expectations of the customer about the proposed software system.


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, ...


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”.


The RE ProcessRequirements

Analysis

RequirementsDefinition

RequirementsSpecification

SoftwareSpecification

SystemModels

RequirementsDefinition

RequirementsSpecification

RequirementsDocument

SoftwareSpecification

included in

produces

leads to


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.


Requirements Document “Requirements”

• Should be easy to change as requirements evolve.

• Must be kept up-to-date as system changes.


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”


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


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 makesure 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 ofwhat I want. Also bring me the cost breakdown for each configuration sothat I can arbitrarily pick one.


A Story … (Cont’d)

Keep in mind that the house I ultimately chose must cost less than theone I am currently living in. Make sure, however, that you correct allthe deficiencies that currently exist in my house (the floor of mykitchen vibrates when I walk across it, and the walls don’t have nearlyenough insulation in them).

Also keep in mind as you design this house that I wish to keep yearlymaintenance cost as low as possible. This should mean the incorporationof extra-cost features like aluminum or vinyl siding. If you chose notto specify aluminum, be prepared to explain in detail.



Please take care that modern design practices and the latest materialsare used in construction of the house. The house should be really nice.However, be alerted that the kitchen should be designed to accommodateamong other things, my 1952 Gibson refrigerator.

To assure that you are building the correct house for our family, makesure that you contact each of the children and also the in-laws. Mymother-in-law will have very strong feelings about how the house oughtto be designed since she visits with us at least once a year. Make surethat you weigh all these options carefully and make the right decision.I, however, retain the right to override any decision you come up with.



Please don’t bother me with small details right now. Your job is todevelop the overall plans for this house. Get the big picture. It isnot 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 thishouse. Your first priority is to develop detailed plans andspecifications. However, once I accept these plans, I will expect tohave the house under roof within 48 hours.



While you are designing this house specifically for me, keep in mindthat sooner or later I will have to sell this house. It should haveappeal to potential buyers. Please make sure that before you finalizethe plans, there is a consensus of the population in my area that theylike the features this house has.

You are advised to run up and look at my neighbor’s house he hadconstructed last year. We like it a great deal. It has many featuresthat we would like to have in our new home, particularly the 75-footswimming pool. With careful engineering I believe that you can designthis into our new house without impacting the construction cost.



Please prepare a complete set of blueprints. It is not necessary atthis time to do the real design since these blueprints will be used onlyfor construction bids. Please be advised however, that any increase ofcost in the future as a result of design changes will result in yougetting your hands slapped.

You must be thrilled to be working on such an interesting project suchas this. To be able to use new kinds of construction and to be givensuch freedom in your designs is something that doesn’t happen veryoften. Contact me as rapidly as possible with your design ideas.I am enthusiastic about seeing what you can come up with.



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.


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.


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.


Back to Software Design ...


SoftwarearchitectureSoftware

architecture

SubsystemdecompositionSubsystem

decomposition

SubsystemdependenciesSubsystem

dependencies

Subsysteminterfaces

Subsysteminterfaces

module or classdecomposition

module or classdecomposition

module or classdependencies

module or classdependencies

module or classinterfaces

module or classinterfaces

AlgorithmsAlgorithms

Data structuresData structures

Low-Level(detailed)

design

High-Level(abstract)

design


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.


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.


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.


Discussion

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


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.


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.


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.


A Rational Design Process:How and Why to Fake it

“Many have sought a software design process that allows a program to bederived systematically from a precise statement of requirements. …although we will not succeed in designing a real product that way, we canproduce documentation that makes it appear that the software was designed by such a process …”

D. L. Parnas


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.


RPD Payoff

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


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!


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.


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.


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.


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 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 “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 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 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!


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.


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.


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.


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.


The Cost of Software Failure

• Inability to keep up,

• reduced performance,

• decreasing reliability.


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.


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.


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.


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.


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 ...


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.


Preventive Medicine

• Design for success

• Keep records (documentation)

• Seek second opinions (reviews)


Design for Success

• Design for change.

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


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 ...


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


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.


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.


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


Documentation

• Hence, documentation is ignored by maintainers.

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


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.


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.


Reviews

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


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 ...


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 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 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.


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.


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.

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