System models
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System models
©Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 8
Slide 1
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 object modelling
To introduce some of the notations used in
the Unified Modeling Language (UML)
To show how CASE workbenches support
system modelling
©Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 8
Slide 2
Topics covered
Context models
Behavioural models
Data models
Object models
CASE workbenches
©Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 8
Slide 3
System modelling
System modelling helps the analyst to understand
the functionality of the system and models are used
to communicate with customers.
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.
©Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 8
Slide 4
Model types
Data processing model showing how the data is
processed at different stages.
Composition model showing how entities are
composed of other entities.
Architectural model showing principal sub-systems.
Classification model showing how entities have
common characteristics.
Stimulus/response model showing the system’s
reaction to events.
©Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 8
Slide 5
Context models
Context models are used to illustrate the
operational context of a system - they show
what lies outside the system boundaries.
Social and organisational concerns may
affect the decision on where to position
system boundaries.
Architectural models show the system and its
relationship with other systems.
©Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 8
Slide 6
The context of an ATM system
Security
system
Branch
accounting
system
Account
da tabase
Auto-teller
system
Branch
counter
system
Usage
database
Maintenance
system
©Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 8
Slide 7
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.
©Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 8
Slide 8
Equipment procurement process
Delivery
note
Specify
equipment
requir ed
Equipment
spec.
Checked
spec.
Valida te
specifica tion
Spec. +
supplier +
estima te
Equipment
spec.
Supplier
da tabase
Accept
deli very of
equipment
Get cost
estima tes
Supplier list
Find
suppliers
Choose
supplier
Delivery
note
Check
deliver ed
items
Installa tion
instructions
Order
notifica tion
Order
details plus
blank or der
for m
Place
equipment
order
Check ed and
signed or der f orm
Install
equipment
Installa tion
acceptance
Accept
deli ver ed
equipment
Equipment
details
Equipment
da tabase
©Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 8
Slide 9
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.
These models show different perspectives
so both of them are required to describe the
system’s behaviour.
©Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 8
Slide 10
Data-processing models
Data flow diagrams (DFDs) may be used to
model the system’s data processing.
These show the processing steps as data
flows through a system.
DFDs are an intrinsic part of many analysis
methods.
Simple and intuitive notation that customers
can understand.
Show end-to-end processing of data.
©Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 8
Slide 11
Order processing DFD
Completed
order f or m
Order
details +
blank
or der f orm
Complete
or der f orm
Signed
order f or m
Valida te
order
Signed
order f orm
Check ed and
signed or der
+ or der
notifica tion
Send to
supplier
Recor d
or der
Order
details
Signed
order f orm
Adjust
availab le
budget
Order
amount
+ account
details
Orders
file
©Ian Sommerville 2004
Budget
file
Software Engineering, 7th edition. Chapter 8
Slide 12
Data flow diagrams
DFDs model the system from a functional
perspective.
Tracking and documenting how the data
associated with a process is helpful to
develop an overall understanding of the
system.
Data flow diagrams may also be used in
showing the data exchange between a
system and other systems in its environment.
©Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 8
Slide 13
Insulin pump DFD
Blood
par ameters
Blood
Blood sugar
sensor
Blood sugar
anal ysis
Blood sugar
level
Insulin
requir ement
computa tion
Pump contr ol
commands
Insulin
Insulin
pump
©Ian Sommerville 2004
Insulin
deli very
contr oller
Insulin
requir ement
Software Engineering, 7th edition. Chapter 8
Slide 14
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.
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.
©Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 8
Slide 15
Statecharts
Allow the decomposition of a model into submodels (see following slide).
A brief description of the actions is included
following the ‘do’ in each state.
Can be complemented by tables describing
the states and the stimuli.
©Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 8
Slide 16
Microwave oven model
Full
pow er
Full po wer
do: set power
= 600
Timer
Waiting
Number
do: display
time
Full
po wer
Half
po wer
Half
power
Set time
Operation
do: get number
exit: set time
do: operate
oven
Door
closed
Timer
Cancel
Start
Door
open
Half po wer
do: set power
= 3 00
Enabled
Door
closed
Door
open
do: display
'Ready'
Waiting
do: display
time
Disab led
do: display
'Waiting'
©Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 8
Slide 17
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 ŌHalf powerÕ.
Full power
The oven power is set to 600 watts. The display shows ŌFull powerÕ.
Set time
The cooking time is s et to the userÕs input value. The display shows the cooking time
selected and is updated as the time is set.
Disabled
Oven operation is disabled for safety. Interior oven light is on. Display shows ŌNot
readyÕ.
Enabled
Oven operation is enabled. Interior oven light is off. Display shows ŌReady to cookÕ.
Operation
Oven in operation. Interior oven light is on. Display shows the timer countdown. On
completion of cooking, the buzzer is sounded for 5 s econds. Oven light is on. Display
shows ŌCooking completeÕ while buzzer is sounding.
©Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 8
Slide 18
Microwave oven stimuli
Stimulus
Description
Half power
The user has pressed the half power button
Full power
The user has pressed the full power button
Timer
The user has pressed one of the timer buttons
Number
The user has pressed a numeric key
Door open
The oven door switch is not closed
Door closed
The oven door switch is closed
Start
The user has pressed the start button
Cancel
The user has pressed the cancel button
©Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 8
Slide 19
Microwave oven operation
Operation
Checking
OK
do: check
status
Time
Cook
do: run
generator
Emitter
fault
Turntable
fault
Alarm
do: display
event
Timeout
Done
do: buzzer on
for 5 secs.
Door open
Disabled
©Ian Sommerville 2004
Cancel
Waiting
Software Engineering, 7th edition. Chapter 8
Slide 20
Semantic data models
Used to describe the logical structure of data
processed by the system.
An entity-relation-attribute model sets out the
entities in the system, the relationships between
these entities and the entity attributes
Widely used in database design. Can readily be
implemented using relational databases.
No specific notation provided in the UML but objects
and associations can be used.
©Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 8
Slide 21
Library semantic model
Article
title
authors
pdf file
fee
Source
published-in
m
n
fee-payable-to
1
1
delivers
1
in
1
1
n
Order
order number
total payment
date
tax status
n
places
1
Copyright
Agency
name
has-links
address
title
publisher
issue
date
pages
Country
1
in
1
copyright form
tax rate
Buyer
name
address
billing info
©Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 8
Slide 22
Data dictionaries
Data dictionaries are lists of all of the names used in
the system models. Descriptions of the entities,
relationships and attributes are also included.
Advantages
•
•
Support name management and avoid duplication;
Store of organisational knowledge linking analysis, design
and implementation;
Many CASE workbenches support data dictionaries.
©Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 8
Slide 23
Data dictionary entries
Name
Description
Type
Date
Article
Details of the published article that may be ordered by
Entity
people using LIBSYS.
30.12.2002
authors
The names of the authors of the article who may be due
Attribute
a share of the fee.
30.12.2002
Buyer
The person or organisation that orders a co py of the
Entity
article.
30.12.2002
A 1:1 relationship between Article and the Copyright
Relation
Agency who should be paid the copyright fee.
29.12.2002
The address of the buyer. This is used to any paper
Attribute
billing information that is required.
31.12.2002
feepayable-to
Address
(Buyer)
©Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 8
Slide 24
Object models
Object models describe the system in terms of
object classes and their associations.
An object class is an abstraction over a set of
objects with common attributes and the services
(operations) provided by each object.
Various object models may be produced
ã
ã
ã
Inheritance models;
Aggregation models;
Interaction models.
âIan Sommerville 2004
Software Engineering, 7th edition. Chapter 8
Slide 25
Object models
Natural ways of reflecting the real-world
entities manipulated by the system
More abstract entities are more difficult to
model using this approach
Object class identification is recognised as a
difficult process requiring a deep
understanding of the application domain
Object classes reflecting domain entities are
reusable across systems
©Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 8
Slide 26
Inheritance models
Organise the domain object classes into a hierarchy.
Classes at the top of the hierarchy reflect the
common features of all classes.
Object classes inherit their attributes and services
from one or more super-classes. these may then be
specialised as necessary.
Class hierarchy design can be a difficult process if
duplication in different branches is to be avoided.
©Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 8
Slide 27
Object models and the UML
The UML is a standard representation devised by
the developers of widely used object-oriented
analysis and design methods.
It has become an effective standard for objectoriented modelling.
Notation
•
•
•
Object classes are rectangles with the name at the top,
attributes in the middle section and operations in the
bottom section;
Relationships between object classes (known as
associations) are shown as lines linking objects;
Inheritance is referred to as generalisation and is shown
‘upwards’ rather than ‘downwards’ in a hierarchy.
©Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 8
Slide 28
Library class hierarchy
Library item
Catalo gue n umber
Acquisition da te
Cost
Type
Status
Number of copies
Acquire ()
Catalo gue ()
Dispose ()
Issue ()
Return ()
Pub lished item
Recor ded item
Title
Medium
Title
Pub lisher
Book
Author
Edition
Pub lication da te
ISBN
©Ian Sommerville 2004
Ma gazine
Computer
pro gram
Film
Director
Da te of release
Distrib utor
Year
Issue
Version
Pla tfor m
Software Engineering, 7th edition. Chapter 8
Slide 29
User class hierarchy
Library user
Name
Address
Phone
Reg istration #
Reg ister ()
De-reg ister ()
Reader
Borrower
Items on loan
Max. loans
Affiliation
Staff
Depar tment
Depar tment phone
©Ian Sommerville 2004
Student
Major subject
Home ad dress
Software Engineering, 7th edition. Chapter 8
Slide 30
Multiple inheritance
Rather than inheriting the attributes and services
from a single parent class, a system which supports
multiple inheritance allows object classes to inherit
from several super-classes.
This can lead to semantic conflicts where
attributes/services with the same name in different
super-classes have different semantics.
Multiple inheritance makes class hierarchy
reorganisation more complex.
©Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 8
Slide 31
Multiple inheritance
Book
Voice recording
Author
Edition
Pub lication da te
ISBN
Speak er
Duration
Recor ding da te
Talking book
# Tapes
©Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 8
Slide 32
Object aggregation
An aggregation model shows how classes
that are collections are composed of other
classes.
Aggregation models are similar to the part-of
relationship in semantic data models.
©Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 8
Slide 33
Object aggregation
Study pack
Course title
Number
Year
Instructor
OHP slides
Assignment
Credits
Slides
Exercises
Lectur e
notes
Videota pe
Tape ids .
Text
Solutions
Text
Diag rams
#Pr ob lems
Description
©Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 8
Slide 34
Object behaviour modelling
A behavioural model shows the interactions
between objects to produce some particular
system behaviour that is specified as a usecase.
Sequence diagrams (or collaboration
diagrams) in the UML are used to model
interaction between objects.
©Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 8
Slide 35
Issue of electronic items
Ecat:
Catalog
:Library Item
Lib1:
NetServer
:Library User
Lookup
Display
Issue
Issue licence
Accept licence
Compress
Deliver
©Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 8
Slide 36
Structured methods
Structured methods incorporate system
modelling as an inherent part of the method.
Methods define a set of models, a process
for deriving these models and rules and
guidelines that should apply to the models.
CASE tools support system modelling as
part of a structured method.
©Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 8
Slide 37
Method weaknesses
They do not model non-functional system
requirements.
They do not usually include information
about whether a method is appropriate for a
given problem.
The may produce too much documentation.
The system models are sometimes too
detailed and difficult for users to understand.
©Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 8
Slide 38
CASE workbenches
A coherent set of tools that is designed to
support related software process activities
such as analysis, design or testing.
Analysis and design workbenches support
system modelling during both requirements
engineering and system design.
These workbenches may support a specific
design method or may provide support for a
creating several different types of system
model.
©Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 8
Slide 39
An analysis and design workbench
Da ta
dictionary
Structur ed
diag ramming
tools
Repor t
gener ation
facilities
Code
gener ator
Centr al
infor ma tion
repository
Query
langua ge
facilities
Forms
cr ea tion
tools
Design, anal ysis
and checking
tools
Impor t/e xpor t
facilities
©Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 8
Slide 40
Analysis workbench components
Diagram editors
Model analysis and checking tools
Repository and associated query language
Data dictionary
Report definition and generation tools
Forms definition tools
Import/export translators
Code generation tools
©Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 8
Slide 41
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
©Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 8
Slide 42
Key points
Semantic data models describe the logical
structure of data which is imported to or
exported by the systems.
Object models describe logical system
entities, their classification and aggregation.
Sequence models show the interactions
between actors and the system objects that
they use.
Structured methods provide a framework for
developing system models.
©Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 8
Slide 43
