Specification Phase

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Specification Phase

• Chapter Ten

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SPECIFICATION DOCUMENT

• The specification document must be
• Informal enough for client
• Formal enough for developers
• Free of omissions, contradictions, ambiguities

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SPECIFICATION DOCUMENT

• Constraints
• Cost
• Time
• Portability
• Reliability
• Response time
• Acceptance Criteria
• Vital to spell out series of tests
• Product passes tests, deemed to satisfy specs.
• Some are restatements of constraints

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SOLUTION STRATEGY

• General approach to building the product
• Find strategies without worrying about
  constraints
• Modify/select strategies in light of constraints
• Keep written record of all discarded strategies
  and why discarded
• To protect spec team
• To prevent unwise new solutions during
  maintenance phase

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SPECIFICATION METHODS

• Informal
• Semi-formal
• formal

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INFORMAL SPECIFICATIONS

• Example
• if sales of 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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INFORMAL SPECIFICATIONS

• Based on Natural Languages
• Can be ambiguous
• Claim
• ambiguity can not arise when specs done by
  professionals
• Refutation
• Text Processing case (see textbook)

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INFORMAL SPECIFICATIONS

• Conclusion Natural Language NOT good way to
  specify product
• Fact
• Many organizations still use natural language,
  especially for commercial products
• Reasons
• Uninformed management
• Under-trained computer professionals
• Management gives in to client pressure
• Management unwilling to invest in training

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STRUCTURED SYSTEMS ANALYSIS

• Three popular graphical specification methods
• DeMarco
• Gane and Sarsen
• Yourdon
• Equivalent
• Equally good
• Many US corporations use them for commercial
  products
• Gane and Sarsen used for Object-Oriented design

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SSA 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. Institution and
  corporations are given credit facilities, as are
  some individuals. Sallys store is doing well,
  but despite her business success Sally has been
  advised to computerize. Should she?
• Better question what part?

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CASE STUDY

• Fundamental issue
• What is Sallys objective in computerizing her
  business?
• Assume
• Objective is to make more money
• Cost/benefit analysis for each section of
  business.

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CASE STUDY

• Gane and Sarsen Technique
• A nine step method
• Stepwise refinement in many steps

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CASE STUDY

• 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

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STEP 1- Second Refinement
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STEP 1- Portion of third refinement
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STEP 1 contd

• Final DFD
• Larger, BUT easily understood by client
• Larger DFDs
• Hierarchy of DFDs
• Each box becomes DFD at a lower level

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STEP 2 Decide what parts to computerize

• cost/benefit analysis
• STEP 3 Refine data flow
• Data items for each data flow
• Refine each flow stepwise
• order
• order identifications
• customer details
• package details
• Refine further
• Need data dictionary

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STEP4 Refine Logic of Processes

• Example
• 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 5 Refine Data Stores

• Define contents and representation (format)
• Specify where immediate access is required
• define type of queries
• define Data Immediate Access Diagram (DIAD)

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STEP 6 Define Physical Resources

• For each file, specify
• File name
• Organization
• Storage medium
• Records (to field level)
• DBMS
• define each table
• STEP 7 Determine I/O Specs
• Specify input forms, input screens, printed output

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STEP 8 Perform Sizing

• Determine data for step 9
• Volume of input (daily or hourly)
• Size, frequency. Deadline for each printer report
• Size, number of records processed
• Size of each file

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STEP 9 Hardware Requirements

• Mass Storage requirements
• Input needs
• Output devices
• Is existing hardware adequate?
• However
• Response time can NOT be determined

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ENTITY-RELATIONSHIP DIAGRAMS

• Semi-Formal technique
• Data-Oriented
• Widely used in specifying databases
• used for Object-Oriented Analysis
• Example Author, Biography, Reader

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ENTITY-RELATIONSHIP DIAGRAMS
1-to-many relationships
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ENTITY-RELATIONSHIP DIAGRAMS
Many-to-many relationships
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FORMAL TECHNIQUES

• Finite State Machines
• Petri Nets
• Z

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FINITE STATE MACHINES

• A safe has a combination lock that can be in one
  of three positions labeled 1, 2 and 3. The dial
  can be turned left or right (L or R). Thus there
  are 6 possible dial movements 1L, 1R, 2L, 2R,
  3L, and 3R. The combination to the safe is 1L,
  3R, 2L. Any other dial movement cause the alarm
  to go off.

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FINITE STATE MACHINES

• Set of states (SafeLocked, A, B, SafeUnlocked,
  SoundAlarm)
• Set of inputs( 1L, 1R, 2L, 2R, 3L, 3R)
• Transition function
• Initial State SafeLocked
• Set if final states (SafeUnlocked, SoundAlarm)

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FSM--State Transition Diagram(STD)
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FSM--State Transition Table
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EXTENDED FSM

• Extended FSM with global predicates
• predicate condition in terms of global
  state/event or a combination of them
• Transition rules have form
• State and Event and Predicate gt New State

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ELEVATOR PROBLEM
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ELEVATOR PROBLEM
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ELEVATOR PROBLEM
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ELEVATOR PROBLEM
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ELEVATOR PROBLEM
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ELEVATOR PROBLEM
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ELEVATOR PROBLEM
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POWER OF FSM TO SPEC COMPLEX SYSTEMS

• Specs take simple form
• Specification is
• Easy to write down
• Easy to validate
• Easy to convert into design
• Easy to generate code automatically
• More precise than graphical methods
• Almost as easy to understand
• However
• Timing considerations not handled

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WHO IS USING FSMS?

• Commercial Products
• Menu driven
• Various states/screens
• Automatic code generation a major plus
• System Software
• Operating systems
• Word processors
• Spreadsheets
• CASE tools
• Statemate

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PETRI NETS

• Powerful technique for specifying systems with
  potential timing problems
• Concurrency and synchronization
• Race conditions
• Deadlock
• originally used by automata theorists
• now used in performance evaluation, O/S, and SE
  fields

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Formal Specifications - Cont.

• Z
• a formal specification language
• uses set theory, functions, and discrete math.
• Initially difficult to learn
• the most widely language

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Testing during Spec. Phase

• Walkthrough
• Inspection (more powerful)
• team of inspectors checks specs against a
  checklist
• If a formal tech. is used other testing tech.
  such as correctness-proving methods can be
  applied, simulation of FSM/Petri Nets etc.

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CASE tools

• Two classes
• Graphical tools
• DFD, FSMs, Petri nets, ER diagrams
• Data dictionary
• the two should be integrated
• E.g.,
• Analyst/Designer, Excelerator, software through
  pictures, system architect, and Teamwork

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Metrics

• Five fundamental metrics
• size
• number of items in the data dictionary
• cost
• duration
• effort
• quality
• number faults found during inspection
• This information is tentative at best