Software Engineering, 254451 Slide 1 Chapter 6 Software Design and design methodology

Software Engineering, 254451 Slide 1

Chapter 6

Software Design and design methodology


Software Design

Deriving a solution which satisfies software requirements


Stages of Design

• Problem understanding เข้�าใจปั�ญหา– Look at the problem from different angles to discover the

design requirements.

• Identify one or more solutions หาวิ�ธี�การหนึ่��งหร�อมากกวิ�า– Evaluate possible solutions and choose the most appropriate depending on the

designer's experience and available resources.

• Describe solution abstractions อธี�บายวิ�ธี�– Use graphical, formal or other descriptive notations to

describe the components of the design.

• Repeat process for each identified abstraction ทำ�าซ้ำ��าprocessแต่�ละอ#นึ่until the design is expressed in primitive- fundamental terms.


The Design Process

• Any design may be modelled as a directed graph made up of entities with attributes which participate in relationships.

• The system should be described at several different levels of abstraction. อธี�บายระบบออกมาเปั$นึ่ระดั#บ

• Design takes place in overlapping stages. It is artificial to separate it into distinct phases but some separation is usually necessary. ออกแบบล�าดั#บแบบซ้ำ�อนึ่


Phases in the Design Process

Architecturaldesign

Abstractspecificatio

n

Interfacedesign

Componentdesign

Datastructuredesign

Algorithmdesign

Systemarchitecture

Softwarespecification

Interfacespecification

Componentspecification

Datastructure

specification

Algorithmspecification

Requirementsspecification

Design activities

Design products


Design Phases

• Architectural design: Identify sub-systems. ระบ&sub sys

• Abstract specification: Specify -ก�าหนึ่ดั sub-systems. ก�าหนึ่ดัsub sys

• Interface design: Describe sub-system interfaces. ออกแบบsub interface

• Component design: Decompose sub-systems into components. แต่กระบบออกมา(SA)

• Data structure design: Design data structures to hold problem data. ออกแบบdata (DB)

• Algorithm design: Design algorithms for problem functions.


Procedural Abstraction

• The most obvious design methods involve functional decomposition. แต่กออกมาเปั$นึ่ส่�วินึ่ย�อยๆ

• This leads to programs in which procedures represent distinct logical functions in a program.

• Examples of such functions:– “Display menu”– “Get user option”

• This is called procedural abstraction


Design

• Computer systems are not monolithic -massive : they are usually composed of multiple, interacting modules.

• Modularity has long been seen as a key to cheap, high quality software.

• The goal of system design is to decode:– What the modules are; ม�โมดั&ลอะไรบ�าง– What the modules should be; โมดั+ลทำ�าอะไร– How the modules interact with one-another ส่ามารถต่�ต่�อก#บดัม

ดั+ลอ��นึ่หร�อไม�


Modular programming

• In the early days, modular programming was taken to mean constructing programs out of small pieces: “subroutines”

• But modularity cannot bring benefits unless the modules are – Coherent ซ้ำ��งส่อดัคล�อง , ไม�ข้#ดัแย�งในึ่ต่#วิเอง – Autonomous ซ้ำ��งอย+�ไดั�ดั�วิยต่นึ่เอง , ซ้ำ��งม�อ�ส่ระในึ่การ

เล�อก – Robust


Programs as Functions

• Another view is programs as functions:

input output

x f f (x) the program is viewed as a function from a set I of legal inputs to a set O of outputs.

• There are programming languages (ML, Miranda, LISP) that directly support this view of programming

Well-suited to certain application domains - e.g., compilers

Less well-suited to distributed, non-terminating systems - e.g., process control systems, operating systems like WinNT, ATM machines


Object-Oriented Design

– The system is viewed as a collection of interacting objects.

– The system state is decentralized and each object manages its own state.

– Objects may be instances of an object class and communicate by exchanging

methods.


Five Criteria for Design Methods

• We can identify five criteria to help evaluate modular design methods (Pucc-D):– Modular decomposability;– Modular composability;– Modular understandability;– Modular continuity;– Modular protection.con de (tong-kao-jai) kan pong kun Com


Modular Decomposability

• This criterion is met by a design method if the method supports the decomposition of a problem into smaller sub-problems, which can be solved independently.

• In general method will be repetitive: sub-problems will be divided still further

• Top-down design methods fulfil this criterion; stepwise -ทำ�ละชั้#�นึ่ refinement is an example of such method


Top-down Design

• In principle, top-down design involves starting

at the uppermost components in the hierarchy and working down the hierarchy level by level.

• In practice, large systems design is never truly top-down. Some branches are designed before others. Designers reuse experience (and sometimes components) during the design process.


Hierarchical Design Structure

System level

Sub-systemlevel


Modular Composability

• A method satisfies this criterion if it leads to the production of modules that may be freely combined to produce new systems.

• Composability is directly related to the issue of reusability • Note that composability is often at odds โอกาส่ทำ��จะเปั$นึ่ไปัไดั�

with decomposability; top-down design, – for example, tends to produce modules that may not be composed

in the way desired

• This is because top-down design leads to modules which fulfil a specific function, rather than a general one


Examples

• The Numerical Algorithm Group (NAG) libraries contain a wide range of routines for solving problems in linear algebra, differential equations, etc.

• The Unix shell provides a facility called a pipe, written “”, whereby – the standard output of one program may be

redirected to the standard input of another; this convention favours composability.


Modular Understandability

• A design method satisfies this criterion if it encourages the development of modules which are easily understandable.

• COUNTER EXAMPLE 1. Take a thousand lines program, containing no procedures; it’s just a long list of sequential statements. Divide it into twenty blocks, each fifty statements long; make each block a method.

• COUNTER EXAMPLE 2. “Go to” statements.


Understandability

• Related to several component characteristics– Can the component be understood on its own?– Are meaningful names used?– Is the design well-documented?– Are complex algorithms used?

• Informally, high complexity means many relationships between different parts of the design.


Modular Continuity

• A method satisfies this criterion if it leads to the production of software such that a small change in the problem specification leads to a change in just one (or a small number of ) modules.

• EXAMPLE. Some projects enforce the rule that no numerical or textual literal should be used in programs: only symbolic constants should be used

• COUNTER EXAMPLE. Static arrays (as opposed to open arrays) make this criterion harder to satisfy.


Modular Protection

• A method satisfied this criterion if it yields architectures in which the effect of an abnormal condition at run-time only effects one (or very few) modules

• EXAMPLE. Validating input at source prevents errors from propagating -แพร�พ#นึ่ธี&0 throughout the program.

• COUNTER EXAMPLE. Using int types where subrange or short types are appropriate.


Five principles for Good Design

• From the discussion above, we can distil -refine five principles that should be adhered -ย�ดัม#�นึ่ to:– Linguistic modular units; – Few interfaces; ม�ให�นึ่�อย– Small interfaces ม�ให�เล1ก– Explicit interfaces; ชั้#ดัเจนึ่– Information hiding.

Information tee-nae-non(ชั้#ดัเจนึ่) kiew kub Ling – tua-lek (small), mee noi mak (few)

FES- ( interfaces)


Linguistic Modular Units

• A programming language (or design language) should support the principle of linguistic modular units:– Modules must correspond to linguistic units in the language used

• EXAMPLE. Java methods and classes• COUNTER EXAMPLE. Subroutines in BASIC are called

by giving a line number where execution is to proceed from; there is no way of telling, just by looking at a section of code, that it is a subroutine.


Few Interfaces

• This principle states that the overall number of communication channels between modules should be as small as possible:– Every module should communicate with as few others

as possible.

• So, in the system with n modules, there may be a minimum of n-1 and a maximum of links; your system should stay closer to the minimum

2

)1( nn


Few Interfaces


Small Interfaces (Loose Coupling)

• This principle states:– If any two modules communicate, they should

exchange as little information as possible. การแลกเปัล��ยนึ่ข้�อม&ลระหวิ�างดัมดั+ลให�นึ่�อย

• COUNTER EXAMPLE. Declaring all instance variables as public!


• A measure of the strength of the inter-connections between system components. ควิามส่#มพ#นึ่0ระหวิ�างโมดั+ล

• Loose coupling means component changes are unlikely to affect other components.– Shared variables or control information exchange lead to

tight coupling.– Loose coupling can be achieved by state decentralization

(as in objects) and component communication via parameters or message passing.

Coupling


Tight Coupling

Module A Module B

Module C Module D

Shared dataarea


Loose Coupling

Module A

A’s data

Module B

B’s data

Module D

D’s data

Module C

C’s data


• Object-oriented systems are loosely coupled because there is no shared state and objects communicate using message passing.

• However, an object class is coupled to its super-classes. Changes made to the attributes or operations in a super-class propagate to all sub-classes.

Coupling and Inheritance


Reusability

• A major obstacle to the production of cheap quality software is the intractability of the reusability issue.

• Why isn’t writing software more like producing hardware? Why do we start from scratch every time, coding similar problems time after time after time?

• Obstacles:– Economic;

– Organizational;

– Psychological.


Stepwise Refinement

• The simplest realistic design method, widely used in practice.

• Not appropriate for large-scale, distributed systems: mainly applicable to the design of methods.

• Basic idea is:– Start with a high-level spec of what a method is to achieve;– Break this down into a small number of problems (usually no

more than 10)– For each of these problems do the same;– Repeat until the sub-problems may be solved immediately.


Explicit Interfaces

• If two modules must communicate, they must do it so that we can see it:– If modules A and B communicate, this must be

obvious from the text of A or B or both.

• Why? If we change a module, we need to see what other modules may be affected by these changes.


Information Hiding

• This principle states: ข้�อม&ลซ้ำ�อนึ่– All information about a module, (and particularly how the

module does what it does) should be private to the module unless it is specifically declared otherwise.

• Thus each module should have some interface, which is how the world sees it anything beyond that interface should be hidden.

• The default Java rule:– Make everything private


Cohesion

A measure of how well a component “fits together”.

• A component should implement a single logical entity or function.

• Cohesion is a desirable design component attribute as when a change has to be made, it is localized in a single cohesive component.

• Various levels of cohesion have been identified. จ�านึ่วินึ่ก�จกรรมภายในึ่โมดั+ล


Cohesion Levels

• Coincidental cohesion (weak)– Parts of a component are simply bundled together.

• Logical association (weak)– Components which perform similar functions are

grouped.

• Temporal cohesion (weak)– Components which are activated at the same time

are grouped.


Cohesion Levels• Communicational cohesion (medium)

– All the elements of a component operate on the same input or produce the same output.

• Sequential cohesion (medium)– The output for one part of a component is the input to another part.

• Functional cohesion (strong)– Each part of a component is necessary for the execution of a single

function.

• Object cohesion (strong)– Each operation provides functionality which allows object

attributes to be modified or inspected.


Cohesion as a Design Attribute

• Not well-defined. Often difficult to classify cohesion.

• Inheriting attributes from super-classes weakens cohesion.– To understand a component, the super-classes

as well as the component class must be examined.

– Object class browsers assist with this process.


Assignment

• 2 students in the Group Project work out for each Design

• 4 designs have to be done (explain next)• Report (Final Revision from 4 Designs)

- abstract (why design for each work)- conclusion and further work (mistake, development, suggestion)

• Presentation


Entity-Relationship

Diagram

Data FlowDiagram

State-TransitionDiagram

Data Dictionary

Process Specification (PSPEC)

Control Specification (CSPEC)

Data Object Description

THE ANALYSIS MODEL

proceduraldesign

interfacedesign

architecturaldesign

datadesign

THE DESIGN MODEL


Assignment in Design Methodology

• Procedural Design

• Interface design

• Architecture Design

• Data Design


1. Procedural Design

• Procedural Abstraction +Stepwise Refinement (pseudo code)

• Modularity (Modular Design)

• Software Procedure


Software Procedural Design

เนึ่�นึ่รายละเอ�ยดัในึ่การปัระมวิลผลแต่�ละโมดั+ล• ล�าดั#บเหต่&การณ์0• จ&ดัต่#ดัส่�นึ่ใจ• การทำ�างานึ่ซ้ำ��า• โครงส่ร�างข้�อม+ล


ModuleA


ModuleA


เปั$นึ่การค�ดัแยกรายละเอ�ยดัข้องปั�ญหาออกเปั$นึ่ระดั#บทำ��ชั้#ดัเจนึ่



“Enter” 1. เดั�นึ่ไปัทำ��ปัระต่+ 2 . ย��นึ่ม�อไปัทำ��ล+กบ�ดั 3 . หม&นึ่ล+กบ�ดั 4 . ดั�งปัระต่+ 5 . เดั�นึ่เข้�าปัระต่+



•งานึ่ร#บข้�อม+ลส่�วินึ่ส่+ง , ฐานึ่• งานึ่ต่รวิจเชั้1คข้�อม+ล

เทำ�าก#บศู+นึ่ย0 หร�อต่�ดัลบหร�อไม�• งานึ่ค�านึ่วิณ์ต่ามส่+ต่ร• งานึ่แส่ดังผล

Abstraction ระดั#บทำ��2

ซ้ำอฟต่0แวิร0ค�านึ่วิณ์หาพ��นึ่ทำ��ข้องส่ามเหล��ยม


Stepwise Refinement

• ออกแบบในึ่ล#กษณ์ะ Top-down• ข้ยายรายละเอ�ยดัเปั$นึ่ล�าดั#บข้#�นึ่

(Hierarchy)


open

walk to door;reach for knob;

open door;

walk through;close door.

repeat until door opensturn knob clockwise;if knob doesn't turn, then take key out; find correct key; insert in lock;endifpull/push doormove out of way;end repeat

Stepwise Refinement


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Program


Modularity

การแบ�งซ้ำอฟต่0แวิร0ออกเปั$นึ่ส่�วินึ่ๆ เร�ยกวิ�า Mudule

• ชั้��อ• องค0ปัระกอบ


ควิามเปั$นึ่อ�ส่ระ

โมดั+ล

Modular Design


Modular Types

• Sequence Module• Increment Module• Parallel Module


Sequence Module

• Subroutine• Function• Procedure


2. Architecture Design

• Software Architecture

- system as a whole, e.g. figure of Client/Server

• Hierarchy Design

- Control Hierarchy


Software Architecture Design

• the hierarchical structure of module• the structure of data


P1P2

P3P4

P5

S1 S4 S

5

S2

S3


S5

S4

S3

S2

S1

S3

S2

S1

P


Control Hierarchy

จ#ดัล�าดั#บข้องโมดั+ลต่�างๆในึ่โปัรแกรมให�อย+�ในึ่ล#กษณ์ะข้องการควิบค&มทำ��เปั$นึ่ล�าดั#บ(Hierarchy of control)

“Program Structure”


M

n

o

p

q

f g

h

a

d

e

l m

b

c

i j

k

r

Fan

- Fan out

Width

Depth

- in


- Fan out

Width

Depth

- Fan in

จ�านึ่วินึ่ระดั#บ (level ) ข้องการควิบค&ม(control)

ควิามกวิ�าง,ส่�วินึ่ทำ��ม�ระยะกวิ�างมากทำ��ส่&ดัจ�านึ่วินึ่โมดั+ลทำ��ถ+กควิบค&มโดัยต่รงโดัยโมดั+ลหนึ่��งๆจ�านึ่วินึ่โมดั+ลทำ��ควิบค&มโมดั+ลหนึ่��งๆ


M is Superordinate to a , b , c

h is Subordinate to e


3. Interface Design (GUI)

– WIMP (Windows, Icons, Mouse and Pointing device)

has conceptual structure representing screen of functions, which is implemented using control screen and interface structure called WIMP

– Human machine interface (HMI) are intelligent operator interface terminals, which allow the user to interact with an HMI unit and use it to control other devices.

Effectiveness? Efficiency? Satisfaction?


Graphic User Interface (GUI)


HMI



Interface Design


4. Data Design

• Data Abstraction

• Data relational system (Figure of each Table links one to another)

• Data Entity/Data Structure


“Paycheck” 1. ชั้��อผ+�ส่# �งจ�าย 2. จ�านึ่วินึ่เง�นึ่ 3. วิ#นึ่ , เดั�อนึ่ , ปั; 4. ธีนึ่าคารทำ��จ�าย

ฯลฯ

Data Abstraction


Data Structure

อธี�บายถ�งควิามส่#มพ#นึ่ธี0ทำางต่รรกะข้องข้�อม+ลในึ่ระบบ


โครงส่ร�างข้�อม+ลม�ผลกระทำบโดัยต่รงต่�อโครงส่ร�างข้องระบบ • การเข้�าถ�งข้�อม+ล

• การปัระมวิลผล• วิ�ธี�การ ฯลฯ


Data Design/Selection

Baptism1.parish_code 2 date 3 forename 4 sex 5 father_forename 6 mother_forename 7 surname 8 residence 9 illegitimacy 10 ill_fathers_sname 11 ill_fathers_occupation 12 ill_fathers_residence 13 date_of_birth 14 age 15 alternate_sname

Marriage1 parish_code 2 date 3 groom_fname 4 groom_sname 5 groom_status 6 groom_occupation 7 groom_resid 8 groom_age 10 bride_fname 11 bride_sname 12 bride_status 13 bride_resid 14 bride_age

Burial1 parish_code 2 date 3 forename 4 surname 5 sex 6 status 7 residence 8 relationship 9 illegitimacy 10 kin_fname_1 11 kin_fname_2 12 kin_sname 13 kin_occupation 14 kin_res 15 date_of_death 16 age


Example of Baptism link Marriage


Example of linkage e.g. baptism to marriage linkage as a query prototype:


• Cohesion• Coupling

มาต่รวิ#ดัค&ณ์ภาพข้องควิามเปั$นึ่อ�ส่ระ


ผ+�ออกแบบควิรหล�กเล��ยง

• Low Cohesion Spectrum• High Coupling Spectrum

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Software Engineering, 254451 Slide 1 Chapter 6 Software Design and design methodology