Slide 11.1 © The McGraw-Hill Companies, 2007 CHAPTER 11 CLASSICAL ANALYSIS.

Slide 11.1

© The McGraw-Hill Companies, 2007

CHAPTER 11

CLASSICAL ANALYSIS

Slide 11.2


Overview

The specification document Informal specifications Structured systems analysis Structured systems analysis: The MSG

Foundation case study Other semiformal techniques Entity-relationship modeling Finite state machines Petri nets Z

Slide 11.3


The Specification Document Must Be

At the end of the analysis workflow – the specification document should be produced

Specification - contract Informal enough for the client

The client is generally not a computer specialist Formal enough for the developers

It is the sole source of information for drawing up the design

These two requirements are mutually contradictory

Slide 11.4


Specification

Specification Clientprogrammer

SoftwareEngineer

Slide 11.5


11.1 The Specification Document

The specification document is a contract between the client and the developers

Typical constraints on the product must be included in the specificationDeadline – for deliveryParallel running with existing productPortability – can run under different operating systemsReliabilityRapid response time

For real-time softwareHard real-time constraints must be satisfied

Slide 11.6


Specification Document (contd)

Acceptance criteria – how the client accept the software It is vital to spell out a series of tests

If the product passes the tests, it is deemed have satisfied its specifications

Some acceptance criteria are restatements of constraints such as the response time should be 1 sec, and system can run under Linux and WindowsXP etc

Slide 11.7


Acceptance constraints

As a constraintThe system must run under the windows XP

As a test conditionThe system will be tested under the windows XP

operating system

Slide 11.8


Exercise

Derive one acceptance test based on specification from your group’s mini-project

Slide 11.9


11.2 Informal Specifications

Informal specifications are written in a natural languageExamples: English, Mandarin, Kiswahili, Hindi

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

Slide 11.10


Case study

The management of a retail chain sets a target sales figure for each shop for each month; and if a shop does not meet this target, a report is to be printed. Consider the following scenario: Suppose that the January sales target for one particular shop is $100,000 but actual sales are only $64,000, that is, 36% below target. In this case, a report must be printed. Now suppose further that the February target figure is $120,000 and the actual sales are only $100,000, 16.7% below target. Although sales are below the target figure, the % difference for February, is less than half of the previous month’s % difference; management believes that an improvement has been made, and no report is to be printed. If in March, the target is again $100,000 but the shop makes $98,000, only 2% below target. Because the % difference is small (5%), no report should be printed.

Slide 11.11


The Meaning of This Specification

The sales target for January was $100,000, but actual sales were only $64,000 (36% below target)Print the report

The sales target for February was $120,000, the actual sales were only $100,000 (16.7% below target)The percentage difference for February (16.7%) is less

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

Slide 11.12


The Meaning of This Specification (contd)

The sales target for March was $100,000, the actual sales were $98,000 (2% below target)The percentage difference is under 5%, so do not print

the report

Slide 11.13


But the Specifications Do Not Say This

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

specifications

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

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

Slide 11.14


But the Specifications Do Not Say This (contd)

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

The style is poor The specifications should state when the report should

be printed …… Rather than when it should not be printed

Slide 11.15


11.3 Structured Systems Analysis

Use of graphics to specify software was an important technique

Three popular graphical specification methods of 1970sDeMarcoGane and SarsenYourdon

All are equivalent All are equally good

Slide 11.16


11.3 Structured Systems Analysis (contd)

Many U.S. corporations use them for commercial products

Gane and Sarsen’s method is taught here because it is so widely used

Slide 11.17


11.3.1 Sally’s Software Shop Mini Case Study

Sally’s Software Shop 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. Sally’s Software Shop 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?

Slide 11.18


Sally’s Software Shop Mini Case Study (contd)

A better questionWhat business functions should she computerize

Accounts payable Accounts receivable Inventory

Still betterHow? Batch, or online? In-house or outsourcing?

Slide 11.19



The fundamental issue What is Sally’s 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

Slide 11.20



We assume: Sally wishes to computerize “in order to make more money” We need to perform cost–benefit analysis for each

section of her business

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

problem is! The software should run on an IBM mainframe

Gane and Sarsen’s methodNine-step method to analyze the client’s needStepwise refinement is used in many steps

Slide 11.22


Structured system analysis

Nine steps Step 1

Draw the data flow diagram to determine the logical data flow

What happens to the data!

Slide 11.23



The data flow diagram (DFD) shows the logical data flow “What happens, not

how it happens” After you have

determined the requirements

Only 4 symbols:

Figure 11.1

Slide 11.24


Data flow diagram

DFD will be huge for complex system Developed iteratively Each flow of data starts and ends either at a

source or destination or at a data store Data are transformed by processes Diagram refined by adding new flow of data or an

existing flow is refined with more details

Slide 11.25


Step 1. Draw the DFD

First refinement Infinite number of possible interpretationsProcess_order – worker

Figure 11.2

Slide 11.26


Step 1 (contd)

Second refinement PENDING ORDERS is scanned daily

Figure 11.3

Slide 11.27


Step 1 (contd)

Portion of third refinement

Figure 11.4

Slide 11.28


Exercise

Apply DFD to define operations in a libraryFor example how books are being borrowed or returned

Slide 11.29


Step 1 (contd)

The final DFD is largerBut it is easily understood by the client

When dealing with larger DFDsSet up a hierarchy of DFDsUse proper symbol (such as a box) to represent DFD at

lower levelA box becomes a DFD at a lower level

Slide 11.30


Step 2. Decide What Parts to Computerize and How

It depends on how much client is prepared to spend

Large volumes of data, tight controlsBatch (collect sufficient order and then process together)

Small volumes of data, in-house microcomputerOnline

Cost/benefit analysis – to determine what to computerize

Slide 11.31


Step 3. Determine the Details of the Data Flows

Determine the data items for each data flow Refine each flow stepwise

Example;order:

order_identification

customer_details

package_details

We need a data dictionary for larger products – to keep track of all the data elements.

Slide 11.32


Sample Data Dictionary Entries

Figure 11.5

Slide 11.33


Step 4. Define the Logic of the Processes

We have process give educational discountSally must explain the discount she gives to educational

institutions 10% on up to 4 packages 15% on 5 or more

Slide 11.34


Step 4 . Define the Logic of the Processes (contd)

Translate this into a decision tree

Figure 11.6

Slide 11.35


Step 5. Define the Data Stores

Define the exact contents and representation (format) Specify where immediate access is required

Data immediate-access diagram (DIAD) How the data stored can be searched – by name, by

machine and by function

Slide 11.36


Step 6. Define the Physical Resources

For each file, specifyFile nameOrganization of the file (sequential, indexed, etc.)Storage medium – in DVD, tape, or RAID Blocking factor – how large is a block of dataRecords (to field level)Table information, if a DBMS (Data Base Management

System) is to be used

Slide 11.37


Step 7. Determine Input/Output Specifications

Specify Input forms – the paper form Input screens - GUIPrinted output

Slide 11.38


Step 8. Determine the Sizing

Obtain the numerical data that are needed in Step 9 to determine the hardware requirementsVolume of input (daily or hourly)Size, frequency, deadline of each printed reportSize, number of records passing between CPU and

mass storageSize of each file

Slide 11.39


Step 9. Determine the Hardware Requirements

Mass storage requirements – to store the data Mass storage for back-up Input needs – scanner, keyboard Output devices – printer, monitor Is the existing hardware adequate?

If not, recommend whether to buy or lease additional hardware

Slide 11.40


Steps to perform structured systems analysis

Draw the data flow diagram Decide what sections to computerize and how

(batch processing or online) Determine the details of the data flows Define the logic of the processes Define the data stores Define the physical resources Determine the input-output specifications Perform the sizing Determine the hardware requirements

Slide 11.41


However

Response times cannot be determined

The 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

Slide 11.42


Overall

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

Slide 11.43


11.4 Structured Systems Analysis: The MSG Foundation Case Study

Figure 11.9

Data flow diagram

Slide 11.44


Structured Systems Analysis: The MSG Foundation Case Study (contd)

As reflected in the DFD, the user can perform three different types of operations:

1. Update investment data, mortgage data, or operating expenses data:The USER enters an update_requestTo update investment data, process

perform_selected_update solicits the updated_investment_details from the USER, and sends then to the INVESTMENT_DATA store of data

Updating mortgage data or expenses data is similar

Slide 11.45



2. Print a listing of investments or mortgages:To print a list of investments, the USER enters an

investment_report_request. Process generate_listing_of_investments then obtains investment data from store INVESTMENT_DATA, formats the report, and then prints the report

Printing a listing of mortgages is similar

Slide 11.46



3. Print a report showing available funds for mortgages for the week:The USER enters a funds_availability_report_request. To determine how much money is available for

mortgages for the current week, process compute_availability_of_funds_and_generate_funds_report then obtains (see next slide):

Slide 11.47



investment_details from store INVESTMENT_DATA and computes the expected total annual return on investments

mortgage_details from store MORTGAGE_DATA and computes the expected income for the week, expected mortgage payments for the week, and expected grants for the week

annual_operating_expenses from store EXPENSES_DATA and computes the expected annual operating expense

Slide 11.48



Process compute_availability_of_funds_and_ generate_funds_report then uses these results to compute available_funds_for_week, and format and print the report

Slide 11.49


11.6 Entity-Relationship Modeling

Semi-formal techniqueWidely used for specifying databasesExample:

Figure 11.10

Slide 11.50


Entity-Relationship Diagrams (contd)

Many-to-many relationship

Figure 11.11

Slide 11.51


Entity-Relationship Diagrams (contd)

More complex entity-relationship diagrams

Figure 11.12

Slide 11.52


Exercise

Apply ERD to define something in a University

Slide 11.53


Finite state machine

Finite state machine is a formal specification technique

A system is defined by different states It is based on the

State InputTransition function Initial stateFinal state

A diagram or a table is used to describe how a state is transited to another state

Slide 11.54


11.7 Finite State Machines

Case studyA 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 six possible dial movements, namely 1L, 1R, 2L, 2R, 3L, and 3R. The combination to the safe is 1L, 3R, 2L; any other dial movement will cause the alarm to go off

Figure 11.13

Slide 11.55


Finite State Machines (contd)

The set of states J is {Safe Locked, A, B, Safe Unlocked, Sound Alarm}

The set of inputs K is {1L, 1R, 2L, 2R, 3L, 3R}

The transition function T is on the next slide

The initial state J is Safe Locked

The set of final states J is {Safe Unlocked, Sound Alarm}

Slide 11.56


Figure 11.14


Transition table

Slide 11.57


Extended Finite State Machines

FSM transition rules have the form (for a menu driven GUI)

current state [menu] and event [option selected] new state

Extend the standard FSM by adding global predicates ( a term designating a property or relation )

Predicate is something either “true” or “false” Transition rules then take the form

current state and event and predicate new state

Slide 11.58


11.7.1 Finite State Machines: The Elevator Problem Case Study

A product is to be installed to control n elevators in a building with m floors. The problem concerns the logic required to move elevators between floors according to the following constraints:1. Each elevator has a set of m buttons, one for each floor. These illuminate when pressed and cause the elevator to visit the corresponding floor. The illumination is canceled when the corresponding floor is visited by the elevator2. Each floor, except the first and the top floor, has two buttons, one to request an up-elevator, one to request a down-elevator. These buttons illuminate when pressed. The illumination is canceled when an elevator visits the floor, then moves in the desired direction 3. If an elevator has no requests, it remains at its current floor with its doors closed

Slide 11.59


Finite State Machines: The Elevator Problem Case Study (contd)

There are two sets of buttons

Elevator buttons In each elevator, one for each floor

Floor buttonsTwo on each floor, one for up-elevator, one for down-

elevator

Slide 11.60


Elevator Buttons

EB (e, f): Elevator Button in elevator e pressed to request floor f

Slide 11.61


Elevator Buttons (contd)

Two statesEBON (e, f): Elevator Button (e,f) ON

EBOFF (e,f): Elevator Button (e,f) OFF

If an elevator button is on and the elevator arrives at floor f, then the elevator button is turned off

If the elevator button is off and the elevator button is pressed, then the elevator button comes on

Figure11.15

Slide 11.62


Elevator Buttons (contd)

Two eventsEBP (e,f): Elevator Button (e,f) Pressed

EHAF (e,f): Elevator e Has Arrived at Floor f

Global predicateV (e,f): Elevator e is Visiting (stopped at) floor f

Transition RulesEBOFF (e,f) and EBP (e,f) and not V (e,f) EBON (e,f)

EBON (e,f) and EHAF (e,f) EBOFF (e,f)

Slide 11.63


Floor Buttons

FB (d, f): Floor Button on floor f that requests elevator traveling in

direction dDirection “Up” or “Down”

Slide 11.64


StatesFBON (d, f): Floor Button (d, f) ONFBOFF (d, f): Floor Button (d, f) OFF

If the floor button is on and an elevator arrives at floor f, traveling in the correct direction d, then the floor button is turned off

If the floor button is off and the floor button is pressed, then the floor button comes on

Floor Buttons (contd)

Figure 11.16

Slide 11.65


Floor Buttons (contd)

EventsFBP (d, f): Floor Button (d, f) PressedEHAF (1..n, f): Elevator 1 or … or n Has Arrived at Floor f

PredicateS (d, e, f): Elevator e is visiting floor f

Direction of motion is up (d = U), down (d = D), or no requests are pending (d = N)

Transition rulesFBOFF (d, f) and FBP (d, f) and not S (d, 1..n, f) FBON

(d, f)FBON (d, f) and EHAF (1..n, f) and S (d, 1..n, f) FBOFF

(d, f), d = U or D

Slide 11.66


Finite State Machines: The Elevator Problem Case Study (contd)

The state of the elevator consists of component substates, including:Elevator slowingElevator stoppingDoor openingDoor open with timer runningDoor closing after a timeout

Slide 11.67


Elevator Problem: FSM (contd)

We assume that the elevator controller moves the elevator through the substates

Three elevator statesM (d, e, f): Moving in direction d (floor f is next)

S (d, e, f): Stopped (d-bound) at floor f

W (e,f): Waiting at floor f (door closed)

For simplicity, the three stopped states S (U, e, f), S (N, e, f), and S (D, e, f) are grouped into one larger state

Slide 11.68


State Transition Diagram for Elevator

Figure 11.17

Slide 11.69


Elevator Problem: FSM (contd)

EventsDC (e,f): Door Closed for elevator e, floor f

ST (e,f): Sensor Triggered as elevator e nears floor f

RL: Request Logged (button pressed)

Transition RulesIf the elevator e is in state S (d, e, f) (stopped, d-bound, at floor f), and the doors close, then elevator e will move up, down, or go into the wait state

DC (e,f) and S (U, e, f) M (U, e, f+1)

DC (e,f) and S (D, e, f)M (D, e, f-1)

DC (e,f) and S (N, e, f) W (e,f)

Slide 11.70



The power of an FSM to specify complex systems

There is no need for complex preconditions and postconditionsReading the FSM diagram can determine the conditions

Specifications take the simple form current state and event and predicate next state

Slide 11.71



Using an FSM, a specification isEasy to write downEasy to validateEasy to convert into a designEasy to convert into code automaticallyMore precise than graphical methodsAlmost as easy to understandEasy to maintain

HoweverTiming considerations are not handled

Slide 11.72


Exercise

Apply FSM to design a dialing action of a mobile phone

Slide 11.73


Petri Net

Invented by Carl Adam Petri Currently has wide applications in computer

science in fields including performance evaluation, operating systems, and software engineering

Able to describe concurrent interrelated activities

Slide 11.74


Petri Nets Example

Slide 11.75


Petri Nets basic elements

Four partsA set of places P {p1, p2, p3, p4}A set of transitions T {t1, t2}An input function I

I(t1) = {p2, p4} i(t2) = {p2}

An output function O O(t1) = {p1} O(t2) = {p3, p3} as there are 2 arrows

Slide 11.76


Petri Nets

Marking – assignment of tokens to a Petri Net Token is represented by a The marking can be represented by a vector

(1,2,0,1) based on the sequence and number of tokens in the places

A transition is enabled if each of its input places has as many tokens in it as there are arcs from the place to that transition

Tokens will be moved from input places to an output when a transition is fired

Slide 11.77


Petri Net

Slide 11.78


Petri Net

Slide 11.79


Petri Net

Slide 11.80


Transition fire

T1 fires – one token would be removed from p2 an one from p4; one new token would be placed in p1

T2 fires – one token would be removed from p2, and 2 new tokens would be placed in p3

The number of tokens is not conserved Petri Nets are nondeterministic – if more than one

transition can fire, then any one of them can be fired

Slide 11.81


Petri Net

Inhibitor arc – marked by a small circle No token is needed on an inhibitor input arcs

Slide 11.82


Petri Nets example

Based on the elevator system There are n elevator and m floors Each floor is represented by a place Ff where

1<=f<=m An elevator is represented by a token A token in Ff denotes that an elevator is at floor f The elevator button for floor f is represented in the

Petri Net by place EBf

A token in EBf denotes that the elevator button for floor f is illuminated

Slide 11.83


Elevator problem

If the button EBf is not illuminated, due to the inhibitor arc, the transition EBf pressed is enabled and a token is in EBf

When the button is pressed again, EBf pressed will not fire because of the inhibitor arc (as there is now token in EBf)

If the elevator is to travel from floor g to floor f, transition Elevator in action is enabled because token in Fg and EBf

Token in EBf and Fg will be removed and a new token is placed in Ff

Slide 11.84


Petri Net example

Slide 11.85


Petri Net

Slide 11.86


11.9 Z

Z is a formal specification languageRequires knowledge in set theory, functions,

mathematics etc A Z specification consists of four sections:

1. Given sets, data types, and constants2. State definition3. Initial state4. Operations

Slide 11.87


1. Given sets

Given sets are sets that need not be defined in detail

Names appear in brackets

Here we need the set of all buttons

The specification therefore begins [Button]

Slide 11.88


2. State Definition

Z specification consists of a number of schemataA schema is an outline of a theoryA schema is used to introduce the state variablesA schema consists of a group of variable declarations,

plusDeclarations – the names and types of the entities

introduced in the schemaA list of predicates that constrain the values of variables

and relationships between the entities in declarations

Figure 11.25

Slide 11.89


Z: The Elevator Problem Case Study (contd)

In this problem there are four subsets of Button

The floor buttonsThe elevator buttons buttons (the set of all buttons in the elevator problem) pushed (the set of buttons that have been pushed)

Slide 11.90


Symbols used in Z schema

?

!

if and only if

union

change status

and

or

status not change

input

output

mapping

combining

belong

Slide 11.91


Schema Button_State

Figure 11.26

Slide 11.92


3. Initial State

The state when the system is first turned on

Button_Init ^= [Button_State' | pushed' = ]

(The caret ^ should be on top of the first equals sign =. Unfortunately, this is hard to type in PowerPoint!)

Pushed’ = the updated set

Slide 11.93


4. Operations

ΔButton_state – the operation changes the state A button pushed for the first time is turned on, and

added to set pushed Without the third precondition, the results would be

unspecified

Slide 11.94


Z: The Elevator Problem Case Study (contd)

If an elevator arrives at a floor, the corresponding button(s) must be turned off or removed from the set pushed Pushed \ {button?}

The solution does not distinguish between up and down floor buttons

Figure 11.28

Slide 11.95


11.9.2 Analysis of Z

Z is the most widely used formal specification language

It has been used to specifyCICS (part) – a software system of IBMAn oscilloscopeA CASE toolMany large-scale projects (especially in Europe)

Slide 11.96


Analysis of Z (contd)

Difficulties in using ZThe large and complex set of symbolsTraining in mathematics is needed

Slide 11.97


Analysis of Z (contd)

Reasons for the great success of Z It is easy to find faults in Z specificationsThe specifier must be extremely preciseWe can prove correctness (we do not have to)Only high-school math needed to read ZZ decreases development timeA “translation” of a Z specification into English (or

another natural language) is clearer than an informal specification

Slide 11.98


11.10 Other Formal Techniques

Anna For Ada

Gist, RefineKnowledge-based

VDM Uses denotational semantics

CSP CSP specifications are executableLike Z, CSP has a high squiggle factor

Slide 11.99


11.11 Comparison of Classical Analysis Techniques

Formal methods arePowerful, butDifficult to learn and use

Informal methods haveLittle power, but areEasy to learn and use

There is therefore a trade-off Ease of use versus power

Slide 11.100


Comparison of Classical Analysis Techniques (contd)

Figure 11.29

Slide 11.101


Which Analysis Technique Should Be Used?

It depends on theProjectDevelopment teamManagement teamMyriad other factors

It is unwise to ignore the latest developments

Slide 11.102


11.12 Testing during Classical Analysis

Specification inspectionAided by fault checklist

Results of Doolan [1992]2 million lines of FORTRAN1 hour of inspecting saved 30 hours of execution-based

testing

Slide 11.103


11.16 Challenges of Classical Analysis

A specification document must be Informal enough for the client; butFormal enough for the development team

Analysis (“what”) should not cross the boundary into design (“how”)

Do not try to assign modules to process boxes of DFDs until the classical design phase

Slide 11.104


FSM example

Use FSM to describe the operation of a lamp with a push switch.

Consider the control of a small part of a chemical plant. Temperature and pressure levels must be monitored for safety reasons. Sensors are installed to generate appropriate signals when either of these levels exceeds some predefined values. A trivial policy for managing the plant is the following: when either one of the signals is raised by the corresponding sensor, the control system shuts the plant off and raises an alarm signal; the system is restarted manually when the cause of the failure has been rectified.

Slide 11.105


Z example

Apply Z specification to describe part of the library system Identify the setState definition Initial stateOperation of book return

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