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
Accoun t
accoun tin g
da tabase
system

Auto -teller
system

Branch
Usa ge
counter
database
system

Maintenance
system

©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 7

©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 8
 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 9

 Equipment procurement process

©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 10

©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 11
 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 12

 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 13

 Order processing DFD

©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 14

 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 15

 Insulin pump DFD

©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 16

 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 17

 Statecharts
⚫ Allow the decomposition of a model into sub-
models (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 18

 Microwave oven model

©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 19

 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 20

 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 21

 Microwave oven operation

©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 22

©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 23
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 24
 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 25

 Library semantic model

©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 26

 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 27

 Data dictionary entries

Name Description Type Date

Details of the published article that may be ordered by
Article Entity 30.12.2002
people using LIBSYS.
The names of the authors of the article who may be due
authors Attribute 30.12.2002
a share of the fee.
The person or organisation that orders a co py of the
Buyer Entity 30.12.2002
article.
A 1:1 relationship between Article and the Copyright
fee- Relation 29.12.2002
Agency who should be paid the copyright fee.
payable-to
The address of the buyer. This is used to any paper
Address Attribute 31.12.2002
billing information that is required.
(Buyer)

©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 28

 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
• Inheritance models;(how object classes are related to each other)
• Aggregation models;( how objects are aggregated to form other
objects)
• Interaction models.(how objects interact with other objects)

©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 29

 Object models
⚫ OO requirement analysis involves modelling
real-world entities using object classes
⚫ More abstract entities are more difficult to
model using this approach
⚫ Process of identifying Objects and Object
class is recognised as one of difficult areas
in OOD as this requires 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 30

 Inheritance models
⚫ OOM involves identifying the classes of
object that are important in the domain being
studied
⚫ They are then organized into a taxonomy
⚫ Taxonomy is a classification scheme that
shows how an object class is related to other
classes through common attributes and
services
⚫ To display this taxonomy the classes are
organized into an inheritance hierarchy

©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 31

 Inheritance models

⚫ Object Classes at the top of the hierarchy reflect the

common or general 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 32

 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 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.
• Next example shows inheritance where every object class
inherits its attributes and operations from a single parent
class.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 33
 Library class hierarchy
Library item

Catalo gue n umber

Acquisition da te
Cost
Type
Status
Number o f copies
Acquir e ()
Catalo gue ()
Dispose ()
Issue ()
Return ()

Pub lished item Recor ded item

Title Title
Pub lisher Medium

Book Magazine Film Comp uter

pro gram
Auth or Year Director
Editio n Date o f release Version
Issue
Pub lication da te Distrib uto r Platfor m
ISBN

©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 34

 User class hierarchy
Library user
Name
Address
Phon e
Reg istration #

Reg ister ()
De-reg ister ()

Reader Borrower
Affiliation Items o n loan
Max. loans

Staff Student
Depar tment Major subject
Depar tment phon e Home ad dress

©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 35

 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 36

 Multiple inheritance

Book Voice recording

Auth or Speak er
Editio n Dur ation
Pub lication da te Recor ding da te
ISBN

Talking boo k

# Tapes

©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 37

 Object aggregation
⚫ An aggregation model shows how classes
that are collections are composed of other
classes(i.e) an object is an aggregate of a
set of other objects
⚫ Aggregation models are similar to the part-of
relationship in semantic data models.
⚫ UML notation for aggregation is to represent
the composition by including a diamond
shape on the source of the link

©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 38

 Object aggregation

©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 39

 Object behaviour modelling
⚫ A behavioural model shows the interactions
between objects to produce some particular
system behaviour that is specified as a use-case.
⚫ One way to model behaviour is to use UML
sequence diagrams that show the sequence of
actions involved in a use case.
⚫ Sequence diagrams in the UML are used to model
interaction between objects.
⚫ UML also includes collaboration diagrams that
show the sequence of messages exchanged by
objects
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 40
 Issue of electronic items

©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 41

 Object behaviour modelling
⚫ Objects and actors are aligned along the top
of the diagram
⚫ Labelled arrows indicate operations
⚫ Sequence of operations is from top to bottom

©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 42

 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 43

 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 44

 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 45

An analysis and design workbench

Structur ed Repor t
Data
diag ramming gener ation
dictionary
tools facilities

Cen tr al Query
Code
infor ma tion langua ge
gener ator
repository facilities

Forms Design, an al ysis

Impo rt/e xpo rt
cr ea tion and checkin g
facilities
tools tools

©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 46

 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 47

 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 48
 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 49