ch07

Report
Chapter 7
System Models
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 7
Slide 1
System models

Abstract descriptions of systems whose
requirements are being analysed
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 7
Slide 2
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 2000
Software Engineering, 6th edition. Chapter 7
Slide 3
Topics covered





Context models
Behavioural models
Data models
Object models
CASE workbenches
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 7
Slide 4
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 2000
Software Engineering, 6th edition. Chapter 7
Slide 5
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 2000
Software Engineering, 6th edition. Chapter 7
Slide 6
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 2000
Software Engineering, 6th edition. Chapter 7
Slide 7
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 2000
Software Engineering, 6th edition. Chapter 7
Slide 8
Context models



Context models are used to illustrate the
boundaries of a system
Social and organisational concerns may affect
the decision on where to position system
boundaries
Architectural models show the a system and its
relationship with other systems
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 7
Slide 9
The context of an ATM system
S ecuri ty
s ys tem
B ranch
accou nt ing
s ys tem
Acco un t
d at ab as e
Au to -t ell er
s ys tem
B ranch
cou nt er
s ys tem
Us ag e
d at abas e
M ain ten ance
s ys tem
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 7
Slide 10
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 2000
Software Engineering, 6th edition. Chapter 7
Slide 11
Equipment procurement process
Del iv ery
n ot e
S p eci fy
equ ip m ent
requ ir ed
Eq ui pm ent
s pec.
Vali dat e
s pecifi cat io n
Accep t
d el ivery of
equ ip m ent
Get co st
est im at es
S p ec. +
s up pl ier +
est im a t e
Eq ui pm ent
s pec.
S u pp li er
d at ab as e
C h eck ed
s pec.
S u pp li er li st
F i nd
s up pl iers
C h oo se
s up pl ier
Ord er
n ot ificat io n
Ord er
d et ail s +
B l an k ord er
fo rm
P l ace
equ ip m ent
o rd er
C h ecked and
s ig ned ord er fo rm
Del i very
n ot e
C h eck
d el iv ered
i tem s
Ins t al lat io n
i ns tru ct io ns
Ins t al l
equ ip m ent
Ins t al lat io n
accept ance
Accep t
d el iv ered
equ ip m ent
Eq ui pm ent
d et ail s
Eq ui pm en t
d at ab as e
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 7
Slide 12
Behavioural models


Behavioural models are used to describe the
overall behaviour of a system
Two types of behavioural model are shown here
•
•

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
Both of these models are required for a
description of the system’s behaviour
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 7
Slide 13
Data-processing models





Data flow diagrams are used to model the
system’s data processing
These show the processing steps as data flows
through a system
Intrinsic part of many analysis methods
Simple and intuitive notation that customers can
understand
Show end-to-end processing of data
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 7
Slide 14
Order processing DFD
S i gned
o rd er fo rm
C om p let ed
o rd er fo rm
Or d er
d et ail s +
bl an k
o rd er fo rm
C o m pl et e
o rd er fo rm
Vali da t e
o rd er
S i gn ed
o rd er fo rm
S en d t o
s up pl ier
R ecord
o rd er
Ord er
d et ail s
S i gn ed
o rd er fo rm
C h eck ed and
s ig ned ord er
+ o rd er
n ot ificat io n
Ad j us t
avail abl e
bu dg et
Ord er
am ou nt
+ acco un t
d et ail s
Ord ers
fil e
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 7
B u dg et
fil e
Slide 15
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 2000
Software Engineering, 6th edition. Chapter 7
Slide 16
CASE toolset DFD
Inp ut
d es ig n
Vali d
d es ig n
Des ig n
edi to r
R eferenced
d es ig ns
Des ig n
d at ab as e
©Ian Sommerville 2000
Ch eck ed
des ig n
Des ig n
cro ss checker
a nd
Des ig n
analys is
Des ig n
anal y ser
C h eck ed
d es ig n
Co de s kelet on
g en erat or
Us er
repo rt
Repo rt
gen erato r
Ou t pu t
cod e
Software Engineering, 6th edition. Chapter 7
Des ig n
d at ab as e
Slide 17
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
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 7
Slide 18
Microwave oven model
F u ll
p ower
F u ll pow er
d o: set po wer
= 6 00
Ti m er
Wait in g
d o: di sp lay
ti m e
Hal f
p ow er
Nu m b er
F u ll
p ow er
d o: get nu m ber
exi t: s et t im e
Hal f
p ow er
Do or
clo sed
Ti m er
Do or
o pen
Hal f p ower
d o: set po wer
= 3 00
Op erati on
d o: op erate
o ven
S et ti m e
C ancel
S t art
En abl ed
Do or
clo sed
d o: di sp lay
'R eady '
Do or
o pen
Wait in g
d o: di sp lay
ti m e
Di s ab l ed
d o: di sp lay
'Wait in g'
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 7
Slide 19
Microwave oven state
description
S ta te
W a itin g
H a lf p o w e r
F u ll p o w e r
S e t tim e
D is a b le d
E n a b le d
O p e ra tio n
©Ian Sommerville 2000
D e s c rip tio n
T h e o v e n is w a itin g f o r in p u t. T h e d is p la y s h o w s th e
c u rre n t tim e .
T h e o v e n p o w e r is s e t to 3 0 0 w a tts . T h e d is p la y
s h o w s ‘H a lf p o w e r’.
T h e o v e n p o w e r is s e t to 6 0 0 w a tts . T h e d is p la y
s h o w s ‘F u ll p o w e r’.
T h e c o o k in g tim e is s e t to th e u s e r’s in p u t v a lu e . T h e
d is p la y s h o w s th e c o o k in g tim e s e le c te d a n d is
u p d a te d a s th e tim e is s e t.
O v e n o p e ra tio n is d is a b le d f o r s a f e ty . In te rio r o v e n
lig h t is o n . D is p la y s h o w s ‘N o t re a d y ’.
O v e n o p e ra tio n is e n a b le d . In te rio r o v e n lig h t is o f f .
D is p la y s h o w s ‘R e a d y to c o o k ’.
O v e n in o p e ra tio n . In te rio r o v e n lig h t is o n . D is p la y
of
c o m p le tio n
tim e r c o u n td o w n . O n
s h o w s th e
c o o k in g , th e b u z z e r is s o u n d e d f o r 5 s e c o n d s . O v e n
lig h t is o n . D is p la y s h o w s ‘C o o k in g c o m p le te ’ w h ile
b u z z e r is s o u n d in g .
Software Engineering, 6th edition. Chapter 7
Slide 20
Microwave oven stimuli
S tim u lu s
H a lf p o w e r
F u ll p o w e r
T im e r
Number
Door open
D o o r c lo s e d
S ta rt
C ancel
©Ian Sommerville 2000
D e s c r ip tio n
T h e u s e r h a s p re s s e d
T h e u s e r h a s p re s s e d
T h e u s e r h a s p re s s e d
T h e u s e r h a s p re s s e d
T h e o v e n d o o r s w itc h
T h e o v e n d o o r s w itc h
T h e u s e r h a s p re s s e d
T h e u s e r h a s p re s s e d
th e h a lf p o w e r b u tto n
th e f u ll p o w e r b u tto n
o n e o f th e tim e r b u tto n s
a n u m e ric k e y
is n o t c lo s e d
is c lo s e d
th e s ta rt b u tto n
th e c a n c e l b u tto n
Software Engineering, 6th edition. Chapter 7
Slide 21
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 2000
Software Engineering, 6th edition. Chapter 7
Slide 22
Microwave oven operation
O p e r ati on
C h ec k in g
d o: c he c k
s ta tu s
Tu rn ta b le
fa ul t
Ti m e
C o ok
d o: run
ge ne ra t or
OK
Em i tt e r
fa ul t
Ti m eo ut
D o ne
A l a rm
d o: bu z z e r on
for 5 s e c s.
d o: di sp la y
ev e nt
D o or
o pe n
D i s ab le d
©Ian Sommerville 2000
C a nc e l
Wa it in g
Software Engineering, 6th edition. Chapter 7
Slide 23
Semantic data models




Used to describe the logical structure of data
processed by the system
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 2000
Software Engineering, 6th edition. Chapter 7
Slide 24
Software design semantic
model
Des ig n
1
n am e
d es crip ti on
C -d at e
M -dat e
i s-a
h as -no des
1
h as -li nk s
1
n
n
1
No de
h as -li nk s
1
Li nk
n
n am e
t yp e
n am e
t yp e
2
1
l in ks
1
1
h as -lab els
h as -lab els
Lab el
n
©Ian Sommerville 2000
n am e
t ex t
i co n
n
Software Engineering, 6th edition. Chapter 7
Slide 25
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 2000
Software Engineering, 6th edition. Chapter 7
Slide 26
Data dictionary entries
Nam e
D e sc rip tio n
1 :N re la tio n b e tw e e n e n titie s o f typ e N o d e o r
h a s-la b e ls
L in k a n d e n titie s o f typ e L a b e l.
H o ld s stru ctu re d o r u n stru ctu re d in fo rm a tio n
Label
a b o u t n o d e s o r lin ks. L a b e ls a re re p re se n te d b y
a n ico n (w h ich ca n b e a tra n sp a re n t b o x) a n d
a sso cia te d te xt.
A
1 :1
re la tio n
b e tw e e n
d e sig n
e n titie s
L in k
re p re se n te d a s n o d e s. L in ks a re typ e d a n d m a y
be nam ed.
E a ch la b e l h a s a n a m e w h ich id e n tifie s th e typ e
n a m e (la b e l) o f la b e l. T h e n a m e m u st b e u n iq u e w ith in th e
se t o f la b e l typ e s u se d in a d e sig n .
E a ch n o d e h a s a n a m e w h ich m u st b e u n iq u e
n a m e (n o d e ) w ith in a d e sig n . T h e n a m e m a y b e u p to 6 4
ch a ra cte rs lo n g .
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 7
T yp e
D a te
R e la tio n
5 .1 0 .1 9 9 8
E n tity
8 .1 2 .1 9 9 8
R e la tio n
8 .1 2 .1 9 9 8
A ttrib u te
8 .1 2 .1 9 9 8
A ttrib u te
1 5 .1 1 .1 9 9 8
Slide 27
Object models



Object models describe the system in terms of
object classes
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
•
•
•
Inheritance models
Aggregation models
Interaction models
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 7
Slide 28
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 2000
Software Engineering, 6th edition. Chapter 7
Slide 29
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 is a difficult process if
duplication in different branches is to be avoided
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 7
Slide 30
The Unified Modeling Language



Devised by the developers of widely used object-oriented
analysis and design methods
Has become an effective standard for object-oriented
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 2000
Software Engineering, 6th edition. Chapter 7
Slide 31
Li brary i tem
Library class hierarchy
C at al og ue n u m ber
Acq ui si ti on d at e
C o st
Ty pe
S t at us
Nu m b er of co pi es
Acq ui re ()
C atal ogu e ()
Di s po se ()
Iss u e ()
R etu rn ()
P u bl is hed it em
R ecord ed i tem
T i tl e
M edi um
Ti tl e
P ub li sh er
B o ok
Au th or
Ed it io n
P u bl icat io n d at e
IS B N
M ag azi ne
Year
Iss u e
F i lm
Di recto r
Dat e of releas e
Di s tri bu to r
C o m pu ter
p ro g ram
Versi on
P l at fo rm
User class hierarchy
Li br ary us er
Nam e
Ad d ress
P h on e
R egi st rat io n #
R egi st er ()
De-r egi st er ()
Reader
B o rro wer
Item s o n l oan
M ax. lo ans
Affil iat io n
S t aff
Dep artm en t
Dep artm en t ph on e
S t ud ent
M ajo r s ub ject
Ho m e ad d ress
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
Can lead to semantic conflicts where
attributes/services with the same name in
different super-classes have different semantics
Makes class hierarchy reorganisation more
complex
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 7
Slide 34
Multiple inheritance
B o ok
Vo ice record in g
Au th or
Ed it io n
P u bl icat io n d at e
IS B N
S p eaker
Du rat io n
Record in g d at e
Talk in g bo ok
# Tapes
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 7
Slide 35
Object aggregation


Aggregation model shows how classes which
are collections are composed of other classes
Similar to the part-of relationship in semantic
data models
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 7
Slide 36
Object aggregation
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 7
Slide 37
Object behaviour modelling


A behavioural model shows the interactions
between objects to produce some particular
system behaviour that is specified as a use-case
Sequence diagrams (or collaboration diagrams)
in the UML are used to model interaction
between objects
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 7
Slide 38
Issue of electronic items
Ecat :
C atal og
: Lib rary Item
Li b1 :
Net S erver
: Lib rary Us er
Lo ok up
Di s pl ay
Iss u e
Iss u e li cence
Accep t li cence
C o m press
Del iv er
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 7
Slide 39
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 2000
Software Engineering, 6th edition. Chapter 7
Slide 40
An analysis and design
workbench
Dat a
d icti on ary
S t ructu red
d iag ramm in g
t ools
R epo rt
g en erat io n
facil it ies
C o de
gen erato r
C entral
i nfo rmat io n
repo sito ry
Qu ery
l an gu ag e
facil it ies
Fo rm s
creati on
t oo ls
Des ig n, anal y si s
and ch ecki ng
t ools
Im po rt/ ex po rt
facil it ies
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 7
Slide 41
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 2000
Software Engineering, 6th edition. Chapter 7
Slide 42
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 2000
Software Engineering, 6th edition. Chapter 7
Slide 43
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
Object models describe the logical system
entities and their classification and aggregation
CASE workbenches support the development of
system models
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 7
Slide 44

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