Modeling

ECE 417/617:Elements of Software Engineering

Stan BirchfieldClemson University

Overview• Modeling provides abstraction to bridge

the gap between– High-level (world)– Low-level (code)

• Types of modeling: – Analysis modeling

• Models problem domain (users, world)– System modeling

• Models solution domain (software)

Analysis modeling

• Let us first look at modeling the problem domain

Requirements elicitation• It is important to define what the software

is supposed to do before – defining how to do it, or– before actually doing it

• “The hardest single part of building a software system is deciding what to build.” – Fred Brooks

• Requirements elicitation – gathering requirements from users and other stakeholders

Difficulties in specifying requirements

• Customers often do not know what they want ... until they see it

• Customers often have a poor understanding of the ease or difficulty of implementing different capabilities

• The requirements change over time

Steps in gathering requirements

• Inception – establish basic understanding of problem

• Elicitation – Ask the users what is needed• Elaboration – Refine the model of the S/W

functions, features, and constraints• Negotiation – Reconcile conflicts by ranking

requirements and discussing priorities• Specification – Final work product describing the

function and performance of the S/W• Validation – Examine the specification to ensure

that all requirements have been stated unambiguously, inconsistencies have been corrected, etc.

Specifying requirements

• Requirements can be specified in a number of ways:– user scenarios– functions and feature lists– analysis models– specification

Traceability table• Captures the relationships between

– features and requirements– interfaces and requirements– requirements themselves (dependencies)– etc.

A01 A02 A03 ...R01 √R02 √ √R03 √...

aspectsre

quire

men

ts

User scenarios

• Usage scenarios– identify a thread of usage for the system– enable the S/W team to see how the

functions and features will be used by different classes of end users

– often called use cases

Use cases• Use case tells a stylized story about how an end-

user interacts with the system under a specific set of circumstances

• Can be either– narrative text– outline of tasks or interactions– template-based description, or– diagrammatic representation

• “A use-case captures a contract...” -- Alistair Cockburn, Writing Effective Use Cases. Addison-Wesley 2000. http://www.usecases.org/

Use case exampleUse case: Withdraw money

Level: User goal (Three levels: Summary, User goal, and Sub-function level)

Primary actor: Client

Goal in context: To withdraw money from the client’s account

Preconditions: User has an account, ATM has power and connectivity

Main scenario:1. Client inserts card2. Client types PIN3. Client specifies which account4. Client enters amount to withdraw5. Money is dispensed6. Card is ejected7. Client removes card

Extensions:1a. Card is invalid; card is ejected and client notified.2a. Pin is incorrect; client notified and given no more than two more attempts.4a. Amount exceeds limit; client notified, repeat step.7a. Client does not remove card within time limit; card is retracted.

System modeling

• Now let us look at modeling the solution domain

Data flow diagram (DFD)• Data flow diagram (DFD) developed in late 1970s

– part of Structured Design (one of the earliest methodologies for software development); aka Structured Systems Analysis and Design Method (SSADM), a waterfall method

– invented by Larry Constantine, who also developed concepts of coupling and cohesion

• DFD is a forerunner of UML and may complement it• Arcs are data; boxes are processes/actions

executeunittests

reviewtest

results

test results review decisiontest plan

source code

Gane and Sarson notation for DFDs

• squares – external entities• round rectangles – processes• arrows – data flow• open-ended rectangles – data

stores

http://www.agilemodeling.com/artifacts/dataFlowDiagram.htm

Data flow diagram (DFD)• DFDs are refined iteratively

– Level 0 is context-level DFD; represents s/w as a single bubble with input and output

– Level 1 is achieved by expanding the bubble into additional bubbles; perform grammatical parse on narrative describing bubble

– Continue refining until each bubble performs specific function; high cohesion

– Components: bubbles are processes, boxes are external entities, arrows are data or control objects, and double lines are data stores

• Process specification (PSPEC) describes all flow model processes that appear at the final level of refinement. It is a minispec for each transform at the lowest level of a DFD

• Program design language description (PDL) is basically pseudocode. One way to represent PSPEC

CRC modeling• Class Responsibility Collaborator (CRC) is a

lightweight model

• Write on 3”x5” index cards• Used in extreme programming• Can be used for

– detailed object-oriented design – conceptual modeling

http://www.agilemodeling.com/artifacts/crcModel.htm

CRC example

class is collection of objects

responsibility is anything a class knows or does

two types of collaboration:• request for information• request to do something

http://www.agilemodeling.com/artifacts/crcModel.htm

Creating CRC cardsIteratively• Find classes• Find responsibilities• Define collaborators• Move the cards

around

http://www.agilemodeling.com/artifacts/crcModel.htm

Unified modeling language (UML)

• Several competing object-oriented notations developed in 1980s and 1990s

• Rumbaugh and Booch began working together in 1994 at IBM Rational to standardize their notations (OMT and Booch)

• Result was Unified Modeling Language (UML)• Rights owned by Object Management Group

(OMG), www.omg.org• Good reference: M. Blaha and J. Rumbaugh,

Object-Oriented Modeling and Design with UML, 2nd ed.

UMLUnified modeling language (UML) includes

three models:• class model – structural aspects of

system (class diagrams)• state model – temporal, behavioral

aspects of system (state diagrams)• interaction model – collaboration of

individual objects (use cases, sequence diagrams, and activity diagrams)

A simple problem to provide brief overview of UML

1 5 V

light

switch

1. Use Case DiagramSimpleCircuit

FlipOn

FlipOff

ViewLight

User

Functionality from user’s point of view

2. Class Diagram

Battery5V

Switch

Resistor Light

Structure of system (objects, attributes, associations, operations)

3. Sequence Diagram

ResistorSwitch Battery Light

Messages between objects

UserFlipOn() HeatUp() Drain()

Shine()

3. Collaboration Diagram

Resistor

More compact, but harder to interpret

User1. FlipOn()

1.1 HeatUp()

1.3 Drain()1.2 Shine()

Battery

Switch

Light

4. Statechart Diagram

Transitions between states of one object(Extension of Finite State Machine (FSM) model)

LightOff

LightOn

flipSwitchOn

flipSwitchOff

4. Statechart Diagram (different objects)

Cold Hot

flipSwitchOn

flipSwitchOff

NotDraining

Draining

flipSwitchOn

flipSwitchOff

(Resistor) (Battery)

5. Activity Diagram

Actions are states

Flip Switch On Flip Switch Off

With swimlanes:Actor1 Actor2

Flip Switch On

Read Book

SummaryWe have looked at five UML diagrams:1. Use case diagrams [Interaction Model]

-- models functionality from user’s point of view

2. Class diagrams [Class Model]-- models structure of system using objects

3. Interaction diagrams [Interaction Model](sequence and collaboration)-- models messages passed between objects

4. Statechart diagrams [State Model]-- models transitions between states

5. Activity diagrams [Interaction Model]-- models flow control as transitions between activities

The actual UML spec has 12 diagrams, but these five will be sufficient for us.