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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 ANALYSISThree 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

• Sally’s 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. Sally’s 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 Sally’s 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 DFDFirst 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 ==> 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