Lecture 6 & 7: Structured Analysis

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Lecture 6 7 Structured Analysis

• Anita S. Malik
• anitamalik_at_umt.edu.pk
• Adapted from Schach (2004) Chapter 11

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Specifications Phase

• Specification document must be
• Informal enough for client
• Formal enough for developers
• Free of omissions, contradictions, ambiguities
• Specification techniques include
• Informal specifications
• Structured systems analysis
• Object Oriented analysis
• Entity-relationship modeling
• Finite state machines
• Petri nets
• Other formal techniques such as Z specification

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Informal Specifications

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

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Meaning of Specification

• Sales target for January was 100,000, actual
  sales were only 64,000 (36 below target)
• Print report
• Sales target for February was 120,000, actual
  sales were only 100,000 (16.7 below target)
• Percentage difference for February (16.7) less
  than half of previous months percentage
  difference (36), do not print report
• Sales target for March was 100,000, actual sales
  were 98,000 (2 below target)
• Percentage difference lt 5, do not print

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But Specifications Do Not Say This

• Difference between target sales and actual
  sales
• There is no mention of percentage difference
• Difference in January was 36,000, difference in
  February was 20,000
• Not less than half of 36,000, so report is
  printed
• Difference of 5
• Again, no mention of percentage
• Ambiguityshould the last clause read percentage
  difference of 5 or difference of
  5,000 or something else entirely?
• Style is poor

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Informal Specifications (contd)

• Claim
• This cannot arise with professional
  specifications writers
• Refutation
• Text Processing case study

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Episode 1

• 1969 Naur Paper
• Given a text consisting of words separated by
  blank or by nl (new line) characters, convert it
  to line-by-line form in accordance with following
  rules
• (1) line breaks must be made only where given
  text has blank or nl
• (2) each line is filled as far as possible, as
  long as
• (3) no line will contain more than maxpos
  characters
• Naur constructed a procedure (25 lines of Algol
  60), and informally proved its correctness)

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Episode 2

• 1970 Reviewer in Computing Reviews
• First word of first line is preceded by a blank
  unless the first word is exactly maxpos
  characters long

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Episode 3

• 1971 London found 3 more faults
• Including procedure does not terminate unless a
  word longer than maxpos characters is encountered

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Episode 4

• 1975 Goodenough and Gerhart found 3 further
  faults
• Includinglast word will not be output unless it
  is followed by blank or nl
• Goodenough and Gerhart then produced new set of
  specifications, about four times longer than
  Naurs

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Case Study (contd)

• 1985 Meyer detected 12 faults in Goodenough and
  Gerharts specifications
• Goodenough and Gerharts specifications
• Were constructed with the greatest of care
• Were constructed to correct Naurs specifications
• Went through two versions, carefully refereed
• Were written by experts in specifications
• With as much time as they needed
• For a product about 30 lines long
• What chance do we have of writing fault-free
  specifications for a real product?

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Episode 5

• 1989 Schach found fault in Meyers
  specifications
• Item (2) of Naurs original requirement (each
  line is filled as far as possible) is not
  satisfied

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Informal Specifications

• Conclusion
• Natural language is not a good way to specify
  product
• Fact
• Many organizations still use natural language,
  especially for commercial products
• Reasons
• Uninformed management
• Undertrained computer professionals
• Management gives in to client pressure
• Management is unwilling to invest in training

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Structured Systems Analysis

• Three popular graphical specification methods of
  70s
• DeMarco
• Gane and Sarsen
• Yourdon
• All equivalent
• All equally good
• Many corporations use them for commercial
  products
• Gane and Sarsen used for object-oriented design

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Structured Systems Analysis Case Study

• Sallys Software Store buys software from various
  suppliers and sells it to the public. Popular
  software packages are kept in stock, but the rest
  must be ordered as required. Institutions and
  corporations are given credit facilities, as are
  some members of the public. Sallys Software
  Store is doing well, with a monthly turnover of
  300 packages at an average retail cost of 250
  each. Despite her business success, Sally has
  been advised to computerize. Should she?
• Better question
• What sections?
• Still better
• How? Batch, or online? In-house or out-service?

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Case Study (contd)

• Fundamental issue
• What is Sallys objective in computerizing her
  business?
• Because she sells software?
• She needs an in-house system with sound and light
  effects
• Because she uses her business to launder hot
  money?
• She needs a product that keeps five different
  sets of books, and has no audit trail
• Assume Computerization in order to make more
  money
• Cost/benefit analysis for each section of
  business

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Case Study (contd)

• The danger of many standard approaches
• First produce the solution, then find out what
  the problem is!
• Gane and Sarsens method
• Nine-step method
• Stepwise refinement is used in many steps

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Case Study (contd)

• Data flow diagram (DFD) shows logical data flow
• what happens, not how it happens

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Step 1. Draw the DFD

• First refinement
• Infinite number of possible interpretations

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Step 1 (contd)

• Second refinement
• pending orders scanned daily

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Step 1 (contd)

• Portion of third refinement

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Step 1 (contd)

• Final DFD
• Larger, But easily understood by client
• Larger DFDs
• Hierarchy
• Box becomes DFD at lower level
• Frequent problem
• Process P at level L, expanded at level L1
• Correct place for sources and destinations of
  data for process P is level L1
• Clients cannot understand DFDsources and
  destinations of data for P are missing
• Solution
• Draw correct DFD, modify by moving sources and
  destinations of data one or more levels up

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Step 2. Decide What Parts to Computerize

• Depends on how much client is prepared to spend
• Large volumes, tight controls
• Batch
• Small volumes, in-house microcomputer
• Online
• Cost/benefit analysis

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Step 3. Refine Data Flows

• Data items for each data flow
• Refine each flow stepwise
• Refine further
• Need a data dictionary

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Step 3. Refine Data Flows (contd)

• Sample data dictionary entries

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Step 4. Refine Logic of Processes

• Have process give educational discount
• Sally must explain discount for educational
  institutions
• 10 on up to 4 packages, 15 on 5 or more
• Translate into decision tree

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Step 4 (contd)

• Advantage of decision tree
• Missing items are quickly apparent
• Can also use decision tables
• CASE tools for automatic translation

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Step 5. Refine Data Stores

• Define exact contents and representation (format)
  
• COBOL specify to pic level
• Ada specify digits or delta
• Specify where immediate access is required
• Data immediate access diagram (DIAD)

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Step 6. Define Physical Resources

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

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Step 7. Determine Input/Output Specs

• Specify input forms, input screens, printed output

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Step 8. Perform Sizing

• Numerical data for Step 9 to determine hardware
  requirements
• Volume of input (daily or hourly)
• Size, frequency, deadline of each printed report
• Size, number of records passing between CPU and
  mass storage
• Size of each file

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Step 9. Hardware Requirements

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

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However

• Response times cannot be determined
• Number of I/O channels can only be guessed
• CPU size and timing can only be guessed
• Nevertheless, no other method provides these data
  for arbitrary products
• The method of Gane and Sarsen/De Marco/Yourdon
  has resulted in major improvements in the
  software industry

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Structured Analysis (Summary)

• Nine Steps
• Draw the Data Flow Diagram (DFD)
• Decide what section to computerize
• Determine the details of the data flows
• Define the logic of the processes
• Define the data stores
• Define the physical resources
• Determine input-output specifications
• Perform sizing
• Determine the hardware requirements

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Air Gourmet Case Study Structured Sys. Anal.

• Data flow diagram reflects centrality of SPECIAL
  MEAL DATA
• See Appendix E for remainder of structured
  systems analysis

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Testing during the Specification Phase

• Walkthrough of the document
• Inspect the document against a checklist. A study
  done on inspections showed each hour invested in
  inspections at analysis stage saved 30 hrs of
  execution-based fault detection and correction.

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Metrics for Analysis Phase

• Size, cost, duration, effort and quality
• Size number of pages in the specification
  document
• Quality note the number of faults of each type
  found during inspection
• Duration and effort number of items in the data
  dictionary, number of files, data items,
  processes etc.

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Challenges of Analysis Phase

• The documents generated during the analysis phase
  should be informal enough for the client to
  understand and formal enough for the development
  team to use as the description of the product to
  be built
• To keep within the what boundary and not cross
  to the how boundary