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SWE 205 - Introduction to Software Engineering

Lecture 12 – System Modeling

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Lecture Objectives To explain why the context of a system

should be modelled as part of the RE process

To describe behavioural modelling, data modelling and state machine models

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System modelling System modelling helps the analyst to

understand the functionality of the system and models are used to communicate with customers.

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System modelling Different models present the system

from different perspectives External perspective showing the system’s

context or environment; Behavioural perspective showing the

behaviour of the system; Structural perspective showing the system

or data architecture.

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Context models Context models are used to illustrate

the operational context of a system - they show what lies outside the system boundaries.

Developing a simple architectural model is the first step. Architectural models show the system and

its relationship with other systems.

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The context of an ATM system

Auto-tellersystem

Securitysystem

Maintenancesystem

Accountdatabase

Usagedatabase

Branchaccounting

system

Branchcountersystem

Architecture models do not show the relationshipBetween the other systems in the envirnoment.

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Process models Process models show the overall

process and the processes that are supported by the system.

Data flow models may be used to show the processes and the flow of information from one process to another.

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Equipment procurement process

Get costestimates

Acceptdelivery ofequipment

Checkdelivered

items

Validatespecification

Specifyequipmentrequired

Choosesupplier

Placeequipment

order

Installequipment

Findsuppliers

Supplierdatabase

Acceptdelivered

equipment

Equipmentdatabase

Equipmentspec.

Checkedspec.

Deliverynote

Deliverynote

Ordernotification

Installationinstructions

Installationacceptance

Equipmentdetails

Checked andsigned order form

Orderdetails plusblank order

form

Spec. +supplier +estimate

Supplier listEquipment

spec.

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Behavioural models Behavioural models are used to

describe the overall behaviour of a system.

Two types of behavioural model are: Data processing models that show how

data is processed as it moves through the system;

State machine models that show the systems response to events.

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Behavioural models These models show different

perspectives so both of them are required to describe the system’s behaviour.

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Data-processing models Data flow diagrams (DFDs) may be

used to model the system’s data processing.

DFDs show the processing steps as data flows through a system. Simple and intuitive notation that

customers can understand. Show end-to-end processing of data.

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Order processing DFD

Completeorder form

Orderdetails +

blankorder form

Validateorder

Recordorder

Send tosupplier

Adjustavailablebudget

Budgetfile

Ordersfile

Completedorder form

Signedorder form

Signedorder form

Checked andsigned order

+ ordernotification

Orderamount

+ accountdetails

Signedorder form

Orderdetails

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Data flow diagrams DFDs model the system from a

functional perspective. Each transformation represents a single

function or process. Tracking and documenting how the data

associated with a process is helpful to develop an overall understanding of the system.

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Data flow diagrams Data flow diagrams may also be used in

showing the data exchange between a system and other systems in its environment.

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Insulin pump DFD

Insulinrequirementcomputation

Blood sugaranalysis

Blood sugarsensor

Insulindelivery

controller

Insulinpump

Blood

Bloodparameters

Blood sugarlevel

Insulin

Pump controlcommands Insulin

requirement

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State machine models These model the behaviour of the system in

response to external and internal events. They show the system’s responses to stimuli

so are often used for modelling real-time systems.

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State machine models State machine models show system

states as nodes and events as arcs between

these nodes. When an event occurs, the system moves

from one state to another. Statecharts are an integral part of the

UML and are used to represent state machine models.

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Microwave oven modelFull power

Enabled

do: operateoven

Fullpower

Halfpower

Halfpower

Fullpower

Number

Dooropen

Doorclosed

Doorclosed

Dooropen

Start

do: set power= 600

Half powerdo: set power

= 300

Set time

do: get numberexit: set time

Disabled

Operation

Cancel

Waiting

do: displaytime

Waiting

do: displaytime

do: display 'Ready'

do: display'Waiting'

Timer

Timer

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Microwave oven state description

State Description

Waiting The oven is waiting for input. The display shows the current time.

Half power The oven power is set to 300 watts. The display shows Hشalf powerص.

Full power The oven power is set to 600 watts. The display shows Fشull powerص.

Set time The cooking time is s et to the userصs input value. The display shows the cooking timeselected and is updated as the time is set.

Disabled Oven operation is disabled for safety. Interior oven light is on. Display shows Nشotreadyص.

Enabled Oven operation is enabled. Interior oven light is off. Display shows شReady to cookص.

Operation Oven in operation. Interior oven light is on. Display shows the timer countdown. Oncompletion of cooking, the buzzer is sounded for 5 s econds. Oven light is on. Displayshows شCooking complete wصhile buzzer is sounding.

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Microwave oven stimuli

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Microwave oven operation

Cookdo: run

generator

Done

do: buzzer onfor 5 secs.

Waiting

Alarm

do: displayevent

do: checkstatus

Checking

Turntablefault

Emitterfault

Disabled

OK

Timeout

Time

Door open Cancel

Operation

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Key Points A model is an abstract system view.

Complementary types of model provide different system information.

Context models show the position of a system in its environment with other systems and processes.

Data flow models may be used to model the data processing in a system.

State machine models model the system’s behaviour in response to internal or external events

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Announcements Major 1: Today (i.e. Wed November 5,

2008) 8:00 PM – 9:30 PM (Bld 22, room 130)