COM 6030 Software
Analysis and Design
Lecture 4- System modelling
Dr Richard Clayton & Dr Marian Gheorghe
Module (1st part) homepage http://www.dcs.shef.ac.uk/~marian
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Outline
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Modelling concepts.
Why use models in software development?
Types of model
Abstract models
Concrete (representational) models
Role of modelling
The UML
Reading: Somerville chapter 7, Bennett chapter 5.
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First part
of the course
Models
“An abstract model (or conceptual model) is a theoretical construct
that represents … processes, with a set of variables and a set of
logical and quantitative relationships between them.
Models in this sense are constructed to enable reasoning within an
idealized logical framework about these processes and are an
important component of scientific theories.
Idealized here means that the model may make explicit assumptions
that are known to be false in some detail, but by their simplification
of the model allow the production of acceptably accurate solutions”
Wikipedia. http://en.wikipedia.org/wiki
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Models of software systems
Models can provide a way to
specify clearly system design,
architecture, function
Models aid
communication between
client and designer
Model
Ambiguity
Resolve ambiguities
using a clearly specified
and standard set of
modelling tools
Models aid understanding of
• Functionality of the system
Business
process
• How well a software system matches
the desired process
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Software
system
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Models of software systems
 Ease project management tasks.
 Can provide complete views of a system, as well as
detailed views of subsystems.
 Clarify structures and relationships.
 Offer a communication framework for ideas within
and between teams.
 Can generate new ideas and possibilities.
 Allow quality assurance and testing scenarios to be
generated.
 Are platform independent.
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Models of software systems
Models can be used to describe different aspects of a system
 All models are developed from a particular perspective
 External perspective shows context of system and relationship to
environment (e.g. other systems in the business).
 Behavioural perspective shows dynamic and functional aspects of
the system (e.g. how data structures are processed).
 Structural perspective shows the structure of the system and data
(e.g. relationships between components of the system and data
structures).
 Models may be abstract (simplified) or concrete (detailed).
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Example – different perspectives
Model of ATM showing
external perspective.
Abstract model with no
detail
Account
database
User
ATM
Maintenance
system
Security
system
Model of cash withdrawal from ATM shows behavioural perspective.
Could show considerably more
Authenticate
user
Prompt for
operation
Select
withdrawal
Prompt for
amount
Enter
amount
Return card
to user
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Abstract models
 Abstract models provide an overview
(abstraction) of an entire system, and shows
the most important aspects.
 Details are not included.
 Abstract models are most useful in the
requirements analysis and design stages.
e.g., Context (external) model for ATM, is an
abstract model of the system environment
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Concrete models
During design, models become
Less abstract and more concrete.
More formal.
More detailed.
These models represent the system, and
maintain all (most) of the information
about the entity that is being modelled.
e.g. detailed class model for the bank
account of an ATM user.
This type of model is important in the
design, implementation, and testing
stages of software development.
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Account
-acNumber:int(frozen)
-acHolder:String(frozen)
-encodedPIN:int
#currentBallance:int
-overdraftLimit:int
-acFrozen:boo
-charges:ChargeTable
+getNumber():int
+getHolder():int
+matchPIN(p:PIN):bool
+getBallance(): int
+makeDeposit(a:int):bool
+makeWithdrawal(a:int):bool
+isFrozen():bool
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Role of models
 Modelling is used for design of software systems
before coding begins.
 Models are essential for large projects, and valuable
for smaller ones.
 Diagrams and models are used in other areas of
engineering.
 Models aid documentation.
 Models enable modular architecture to be developed.
 Modular approach enables code re-use.
 CASE tools enable code generation from model
diagrams.
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Types of models
Natural language models.
 Useful for gathering requirement details.
 Natural language is inherently ambiguous, aim for structured
approach.
 Can be over flexible and long.
Diagrammatic models.
 Can be ad hoc, or follow a specific notation with clearly defined
syntax.
 Can show static or dynamic relationships and behaviours.
Formal descriptions
 State based models using formal algebraic and logical approach.
Combinations
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Natural language & diagrammatic
model - ATM
1.
2.
3.
4.
5.
User inserts credit/debit card into slot.
System authenticates user PIN.
If PIN is invalid then system displays error message and returns card.
System displays list of options and user makes selection.
If user selection is incorrect then system displays error message and then displays list
of options again.
6. If user selects cash withdrawal then
6.1 System asks user to enter amount of cash to be withdrawn, and user enters selection.
6.2 System checks that amount entered does not exceed amount contained in cash dispenser,
and amount in user’s account.
6.3 System dispenses cash to user.
etc.
Diagrammatic model
Authenticate
user
Prompt for
operation
Select
withdrawal
Prompt for
amount
Enter
amount
Return card
to user
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Complementary models
Context models show the relationships of the system to other systems,
and are used to establish the system boundaries.
Behavioural models show the processes that are supported by the
system.
Data flow diagrams show the flow of information from one process to
another. Can be easily understood by client.
Statechart models show how the states of the system change in response
to external or internal stimuli.
Object models show attributes, methods and relationships of object
classes within the system.
During software system development, these types of model will evolve.
It is critical that they remain consistent with each other, so that they aid the
development process rather than hindering it.
There are several modelling approaches that aim to achieve this.
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Unified Modelling Language
UML is the standard diagrammatic notation for documenting
object-oriented software systems.
UML is a descriptive modelling language, and not a methodology;
there is no notion of a development process.
UML can be used as a notation for many different software
development methodologies.
UML has a syntax, which defines diagrams that are legal, and a
semantics, which defines what a legal diagram means.
UML’s notion of syntax and semantics is informal.
Varying interpretations: Analysis models of the perceived world
versus design models documenting implementations.
Why unified? UML is the result of a notation obtained from three
development methods popular in the early 90s: OMT (Rumbaugh),
Booch’s methods and OOSE (Jacobson)
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Diagrams used in UML
Static models: show the structure of a system
 Use case diagram - requirements capture
 Class diagram - analysis, design, implementation
 Object diagram - detailed design, implementation
 Component diagram - implementation, packaging
 Deployment diagram - implementation, distribution
Dynamic models: show the behaviour of a system
 Activity diagram - analysis, distributed design
 Statechart diagram - unit design, testing
 Sequence diagram – analysis, detailed design, testing
 Collaboration diagram - analysis, detailed design, testing
interaction
diagrams
Extension mechanisms: adornments to diagrams
 stereotypes, constraints, tagged values
 OCL - object constraint language
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Scope of UML
UML is a huge and detailed specification.
It is a standard notation with a wide user community.
UML is mainly used for
 Object oriented analysis
 Use cases and scenarios.
 Use case realisation and object identification.
 Activity/sequence diagrams for expressing control
mechanisms
 Class diagrams for conceptual modelling.
 Object oriented design
 Static models, class diagrams, object attributes and
associations.
 Dynamic models, interaction diagrams.
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System views
 Use case view: forces that shape the system's architecture, in
terms of use case diagrams and activity diagrams (interaction
diagrams)
 Design view: functional requirements of system, in terms of
classes, interfaces, collaborations, interaction and statechart
diagrams (also activity diagrams)
 Process view: concurrency and synchronisation of system, using
same diagrams as design view with emphasis on performance,
scalability, throughput
 Implementation view: physical realisation of system, in terms of
components, files, using component and package diagrams (also
interaction, activity diagrams)
 Deployment view: system's hardware topology, the distribution
of elements to nodes, using deployment diagrams (also
interaction, statechart, activity diagrams)
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System views offered by UML
UML provides a standard for visualising, constructing and
documenting software systems
 Abstract: Business process,
Design
Implementation
requirements, system
View
View
function
Use Case
 Concrete: Classes,
View
associations, components
Process
Deployment
View
View
 UML is a widely used modelling language, and you should be
familiar with the main ideas.
 UML provides ‘blueprints’ for software.
 UML does not solve all the problems of software development,
remember it is a notation not a methodology.
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Summary
 Models are abstract representations of software
systems.
 Models aid development of systems by clarifying
function, architecture, design, and process.
 Models show different but complementary
perspectives of a system.
 Models may be expressed in natural language, or as
systems of diagrams.
 The Unified Modelling Language (UML) is a standard
notation for modelling object-oriented systems.
 UML is a notation not a methodology.
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