Lecture 6 & 7

CS540 Software Design 2Lecture 6 & 7

Specifications PhaseSpecifications Phase Specification document must be Specification document must be

Informal enough for clientInformal enough for client Formal enough for developersFormal enough for developers Free of omissions, contradictions, ambiguitiesFree of omissions, contradictions, ambiguities

Specification techniques includeSpecification techniques include Informal specificationsInformal specifications Structured systems analysisStructured systems analysis Object Oriented analysisObject Oriented analysis Entity-relationship modelingEntity-relationship modeling Finite state machinesFinite state machines Petri netsPetri nets Other formal techniques such as Z specificationOther formal techniques such as Z specification


Informal SpecificationsInformal Specifications

ExampleExample““If sales for current month are below If sales for current month are below target sales, then report is to be target sales, then report is to be printed, unless difference between printed, unless difference between target sales and actual sales is less than target sales and actual sales is less than half of difference between target sales half of difference between target sales and actual sales in previous month, or if and actual sales in previous month, or if difference between target sales and difference between target sales and actual sales for the current month is actual sales for the current month is under 5%”under 5%”


Meaning of SpecificationMeaning of Specification Sales target for January was $100,000, actual Sales target for January was $100,000, actual

sales were only $64,000 (36% below target)sales were only $64,000 (36% below target) Print reportPrint report

Sales target for February was $120,000, actual Sales target for February was $120,000, actual sales were only $100,000 (16.7% below target)sales were only $100,000 (16.7% below target) Percentage difference for February (16.7%) less than Percentage difference for February (16.7%) less than

half of previous month’s percentage difference (36%), half of previous month’s percentage difference (36%), do not print report do not print report

Sales target for March was $100,000, actual Sales target for March was $100,000, actual sales were $98,000 (2% below target)sales were $98,000 (2% below target) Percentage difference < 5%, do not printPercentage difference < 5%, do not print


But Specifications Do Not But Specifications Do Not Say ThisSay This

““[D]ifference between target sales and actual [D]ifference between target sales and actual sales”sales” There is no mention of percentage differenceThere is no mention of percentage difference

Difference in January was $36,000, difference Difference in January was $36,000, difference in February was $20,000in February was $20,000 Not less than half of $36,000, so report is printedNot less than half of $36,000, so report is printed

““[D]ifference … [of] 5%” [D]ifference … [of] 5%” Again, no mention of percentageAgain, no mention of percentage

Ambiguity—should the last clause read Ambiguity—should the last clause read “percentage difference … [of] 5%” or “percentage difference … [of] 5%” or “difference … [of] $5,000” or something else “difference … [of] $5,000” or something else entirely?entirely?

Style is poor Style is poor


Informal Specifications Informal Specifications (contd)(contd)

ClaimClaim This cannot arise with professional This cannot arise with professional

specifications writersspecifications writers RefutationRefutation

Text Processing case studyText Processing case study


Episode 1Episode 1 1969 — Naur Paper1969 — Naur Paper

Given a text consisting of words separated byGiven a text consisting of words separated by blankblank or or byby nlnl (new line) characters, convert it to line-by-line (new line) characters, convert it to line-by-line form in accordance with following rules:form in accordance with following rules:

(1) (1) line breaks must be made only where given text line breaks must be made only where given text has has blankblank or or nl nl ;;

(2)(2) each line is filled as far as possible, as long aseach line is filled as far as possible, as long as

(3)(3) no line will contain more than no line will contain more than maxposmaxpos characters characters Naur constructed a procedure (25 lines of Algol Naur constructed a procedure (25 lines of Algol

60), and informally proved its correctness)60), and informally proved its correctness)


Episode 2Episode 2

1970 — Reviewer in 1970 — Reviewer in Computing Computing ReviewsReviews First word of first line is preceded by a First word of first line is preceded by a

blankblank unless the first word is exactly unless the first word is exactly maxposmaxpos characters long characters long


Episode 3Episode 3

1971 — London found 3 more faults1971 — London found 3 more faults Including: procedure does not terminate Including: procedure does not terminate

unless a word longer than maxpos characters unless a word longer than maxpos characters is encountered is encountered


Episode 4Episode 4

1975 — Goodenough and Gerhart 1975 — Goodenough and Gerhart found 3 further faultsfound 3 further faults Including—last word will not be output unless Including—last word will not be output unless

it is followed by blank or nl it is followed by blank or nl Goodenough and Gerhart then produced new set Goodenough and Gerhart then produced new set

of specifications, about four times longer than of specifications, about four times longer than Naur’sNaur’s


Case Study (contd)Case Study (contd)

1985 — Meyer detected 12 faults in 1985 — Meyer detected 12 faults in Goodenough and Gerhart’s Goodenough and Gerhart’s specificationsspecifications

Goodenough and Gerhart’s specifications Goodenough and Gerhart’s specifications Were constructed with the greatest of careWere constructed with the greatest of care Were constructed to correct Naur’s specificationsWere constructed to correct Naur’s specifications Went through two versions, carefully refereedWent through two versions, carefully refereed Were written by experts in specificationsWere written by experts in specifications With as much time as they neededWith as much time as they needed For a product about 30 lines longFor a product about 30 lines long

What chance do we have of writing fault-free What chance do we have of writing fault-free specifications for a real product?specifications for a real product?


Episode 5Episode 5

1989 — Schach found fault in 1989 — Schach found fault in Meyer’s specificationsMeyer’s specifications Item (2) of Naur’s original requirement (“each Item (2) of Naur’s original requirement (“each

line is filled as far as possible”) is not satisfiedline is filled as far as possible”) is not satisfied


Informal SpecificationsInformal Specifications

ConclusionConclusion Natural language is Natural language is notnot a good way to specify a good way to specify

productproduct FactFact

Many organizations still use natural language, Many organizations still use natural language, especially for commercial productsespecially for commercial products

ReasonsReasons Uninformed managementUninformed management Undertrained computer professionalsUndertrained computer professionals Management gives in to client pressureManagement gives in to client pressure Management is unwilling to invest in trainingManagement is unwilling to invest in training


Structured Systems AnalysisStructured Systems Analysis

Three popular graphical specification Three popular graphical specification methods of ’70smethods of ’70s DeMarcoDeMarco Gane and SarsenGane and Sarsen YourdonYourdon

All equivalentAll equivalent All equally goodAll equally good Many corporations use them for Many corporations use them for

commercial products commercial products Gane and Sarsen used for object-oriented Gane and Sarsen used for object-oriented

designdesign


Structured Systems Analysis Structured Systems Analysis Case StudyCase Study

Sally’s Software Store buys software from various Sally’s Software Store buys software from various suppliers and sells it to the public. Popular software suppliers and sells it to the public. Popular software packages are kept in stock, but the rest must be packages are kept in stock, but the rest must be ordered as required. Institutions and corporations ordered as required. Institutions and corporations are given credit facilities, as are some members of are given credit facilities, as are some members of the public. Sally’s Software Store is doing well, with the public. Sally’s Software Store is doing well, with a monthly turnover of 300 packages at an average a monthly turnover of 300 packages at an average retail cost of $250 each. Despite her business retail cost of $250 each. Despite her business success, Sally has been advised to computerize. success, Sally has been advised to computerize. Should she?Should she?

Better questionBetter question What sections?What sections?

Still betterStill better How? Batch, or online? In-house or out-service?How? Batch, or online? In-house or out-service?



Fundamental issue Fundamental issue What is Sally’s objective in computerizing her What is Sally’s objective in computerizing her

business?business? Because she sells software?Because she sells software?

She needs an in-house system with sound and light She needs an in-house system with sound and light effectseffects

Because she uses her business to launder “hot” Because she uses her business to launder “hot” money?money? She needs a product that keeps five different sets of She needs a product that keeps five different sets of

books, and has no audit trailbooks, and has no audit trail Assume: Computerization “in order to make Assume: Computerization “in order to make

more money” more money” Cost/benefit analysis for each section of business Cost/benefit analysis for each section of business



The danger of many standard approaches The danger of many standard approaches First produce the solution, then find out what First produce the solution, then find out what

the problem is!the problem is! Gane and Sarsen’s methodGane and Sarsen’s method

Nine-step method Nine-step method Stepwise refinement is used in many stepsStepwise refinement is used in many steps


Case Study (contd)Case Study (contd) Data flow diagram (DFD) shows logical data Data flow diagram (DFD) shows logical data

flow flow ““what happens, not how it happens”what happens, not how it happens”


Step 1. Draw the DFDStep 1. Draw the DFD

First refinementFirst refinement Infinite number of possible interpretationsInfinite number of possible interpretations


Step 1 (contd)Step 1 (contd) Second refinementSecond refinement pending orders scanned pending orders scanned

daily daily


Step 1 (contd)Step 1 (contd)

Portion of third Portion of third refinementrefinement


Step 1 (contd)Step 1 (contd) Final DFD Final DFD

Larger, But easily understood by client Larger, But easily understood by client Larger DFDsLarger DFDs

HierarchyHierarchy Box becomes DFD at lower levelBox becomes DFD at lower level

Frequent problemFrequent problem Process P at level L, expanded at level L+1Process P at level L, expanded at level L+1 Correct place for sources and destinations of data for Correct place for sources and destinations of data for

process P is level L+1process P is level L+1 Clients cannot understand DFD—sources and destinations Clients cannot understand DFD—sources and destinations

of data for P are “missing”of data for P are “missing” SolutionSolution

Draw “correct” DFD, modify by moving sources and Draw “correct” DFD, modify by moving sources and destinations of data one or more levels updestinations of data one or more levels up


Step 2. Decide What Parts to Step 2. Decide What Parts to ComputerizeComputerize

Depends on how much client is prepared to Depends on how much client is prepared to spendspend

Large volumes, tight controlsLarge volumes, tight controls BatchBatch

Small volumes, in-house microcomputerSmall volumes, in-house microcomputer OnlineOnline

Cost/benefit analysisCost/benefit analysis


Step 3. Refine Data FlowsStep 3. Refine Data Flows

Data items for each data flowData items for each data flow Refine each flow stepwiseRefine each flow stepwise Refine furtherRefine further Need a data dictionaryNeed a data dictionary


Step 3. Refine Data Flows Step 3. Refine Data Flows (contd)(contd)

Sample data dictionary entriesSample data dictionary entries


Step 4. Refine Logic of Step 4. Refine Logic of ProcessesProcesses

Have processHave process give educational discountgive educational discount Sally must explain discount for educational Sally must explain discount for educational

institutionsinstitutions 10% on up to 4 packages, 15% on 5 or more10% on up to 4 packages, 15% on 5 or more

Translate into decision treeTranslate into decision tree


Step 4 (contd)Step 4 (contd) Advantage of decision treeAdvantage of decision tree

Missing items are quickly apparentMissing items are quickly apparent

Can also use decision tables Can also use decision tables CASE tools for automatic translationCASE tools for automatic translation


Step 5. Refine Data StoresStep 5. Refine Data Stores Define exact contents and representation (format) Define exact contents and representation (format)

COBOL: specify to pic levelCOBOL: specify to pic level Ada: specify digits or deltaAda: specify digits or delta

Specify where immediate access is requiredSpecify where immediate access is required Data immediate access diagram (DIAD)Data immediate access diagram (DIAD)


Step 6. Define Physical Step 6. Define Physical ResourcesResources

For each file, specifyFor each file, specify File nameFile name Organization (sequential, indexed, etc.)Organization (sequential, indexed, etc.) Storage mediumStorage medium Blocking factorBlocking factor Records (to field level)Records (to field level)


Step 7. Determine Step 7. Determine Input/Output SpecsInput/Output Specs

Specify input forms, input screens, printed Specify input forms, input screens, printed outputoutput


Step 8. Perform SizingStep 8. Perform Sizing

Numerical data for Step 9 to determine Numerical data for Step 9 to determine hardware requirementshardware requirements Volume of input (daily or hourly)Volume of input (daily or hourly) Size, frequency, deadline of each printed Size, frequency, deadline of each printed

reportreport Size, number of records passing between Size, number of records passing between

CPU and mass storageCPU and mass storage Size of each fileSize of each file


Step 9. Hardware Step 9. Hardware RequirementsRequirements

DASD requirementsDASD requirements Mass storage for back-upMass storage for back-up Input needsInput needs Output devices Output devices Is existing hardware adequate?Is existing hardware adequate? If not, recommend buy/leaseIf not, recommend buy/lease


HoweverHowever Response times cannot be determinedResponse times cannot be determined Number of I/O channels can only be guessedNumber of I/O channels can only be guessed CPU size and timing can only be guessedCPU size and timing can only be guessed Nevertheless, no other method provides these Nevertheless, no other method provides these

data for arbitrary productsdata for arbitrary products

The method of Gane and Sarsen/De The method of Gane and Sarsen/De Marco/Yourdon has resulted in major Marco/Yourdon has resulted in major improvements in the software industryimprovements in the software industry


Structured Analysis Structured Analysis (Summary)(Summary)

Nine Steps:Nine Steps:1.1. Draw the Data Flow Diagram (DFD)Draw the Data Flow Diagram (DFD)2.2. Decide what section to computerizeDecide what section to computerize3.3. Determine the details of the data flowsDetermine the details of the data flows4.4. Define the logic of the processesDefine the logic of the processes5.5. Define the data storesDefine the data stores6.6. Define the physical resourcesDefine the physical resources7.7. Determine input-output specificationsDetermine input-output specifications8.8. Perform sizingPerform sizing9.9. Determine the hardware requirementsDetermine the hardware requirements


Air Gourmet Case Study: Air Gourmet Case Study: Structured Sys. Anal.Structured Sys. Anal.

Data flow diagram Data flow diagram reflects centrality reflects centrality of SPECIAL MEAL of SPECIAL MEAL DATADATA

See Appendix E See Appendix E for remainder of for remainder of structured structured systems analysissystems analysis


Testing during the Testing during the Specification PhaseSpecification Phase

Walkthrough of the documentWalkthrough of the document Inspect the document against a checklist. A study Inspect the document against a checklist. A study

done on inspections showed each hour invested done on inspections showed each hour invested in inspections at analysis stage saved 30 hrs of in inspections at analysis stage saved 30 hrs of execution-based fault detection and correction.execution-based fault detection and correction.


Metrics for Analysis PhaseMetrics for Analysis Phase

Size, cost, duration, effort and qualitySize, cost, duration, effort and quality Size: number of pages in the specification Size: number of pages in the specification

documentdocument Quality: note the number of faults of each type Quality: note the number of faults of each type

found during inspectionfound during inspection Duration and effort: number of items in the data Duration and effort: number of items in the data

dictionary, number of files, data items, processes dictionary, number of files, data items, processes etc.etc.


Challenges of Analysis PhaseChallenges of Analysis Phase

The documents generated during the analysis The documents generated during the analysis phase should be informal enough for the client to phase should be informal enough for the client to understand and formal enough for the understand and formal enough for the development team to use as the description of development team to use as the description of the product to be builtthe product to be built

To keep within the ‘what’ boundary and not cross To keep within the ‘what’ boundary and not cross to the ‘how’ boundaryto the ‘how’ boundary

Lecture 6 & 7

Business

Transcript of Lecture 6 & 7