0
System models

©Ian Sommerville 2006

Software Engineering, 8th 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 mod...
Topics covered
Context models
Behavioural models
Data models
Object models
CASE workbenches

©Ian Sommerville 2006

Softwa...
System modelling
System modelling helps the analyst to understand
the functionality of the system and models are used
to c...
Model types
Data processing model showing how the data is
processed at different stages.
Composition model showing how ent...
Context models
Context models are used to illustrate the
operational context of a system - they show
what lies outside the...
The context of an ATM system
Security
sy stem
Branch
accounting
sy stem

Account
da tabase
Auto-teller
sy stem

Branch
cou...
Process models
Process models show the overall process
and the processes that are supported by the
system.
Data flow model...
Equipment procurement process
Delivery
note

Specify
equipm ent
requir ed

Equipment
spec.

Checked
spec.
V
alida te
speci...
Behavioural models
Behavioural models are used to describe the
overall behaviour of a system.
Two types of behavioural mod...
Data-processing models
Data flow diagrams (DFDs) may be used to
model the system’s data processing.
These show the process...
Order processing DFD

Completed
order for m
Order
details +
blank
or der form

Complete
or der form

Signed
order for m
V
...
Data flow diagrams
DFDs model the system from a functional
perspective.
Tracking and documenting how the data
associated w...
Insulin pump DFD

Blood

Blood
parameters
Blood sugar
sensor

Blood sugar
analy sis

Blood sugar
level
Insulin
requir emen...
State machine models
These model the behaviour of the system in
response to external and internal events.
They show the sy...
Statecharts
Allow the decomposition of a model into submodels (see following slide).
A brief description of the actions is...
Microwave oven model
Full
po w er

Full po wer
do: set power
= 600
Timer

Waiting
Number

do: display
time

Half
po wer

F...
Microwave oven state description
State

Description

Waiting

The oven is waiting for input. The display shows the current...
Microwave oven stimuli

©Ian Sommerville 2006

Software Engineering, 8th edition. Chapter 8

Slide 19
Microwave oven operation
Operation
Checking

OK

do: check
status

Tim e
Cook
do: run
generator

Emitter
fault

Turntable
...
Semantic data models
Used to describe the logical structure of data
processed by the system.
An entity-relation-attribute ...
Library semantic model
Article
title
authors
pdf file
fee

m

published-in

Source
n

fee-payable-to

1

1
delivers

1
in
...
Data dictionaries
Data dictionaries are lists of all of the names used in
the system models. Descriptions of the entities,...
Data dictionary entries

©Ian Sommerville 2006

Software Engineering, 8th edition. Chapter 8

Slide 24
Object models
Object models describe the system in terms of
object classes and their associations.
Combine some of the use...
Object models
Natural ways of reflecting the real-world
entities manipulated by the system
More abstract entities are more...
Inheritance models
Organise the domain object classes into a hierarchy.
Classes at the top of the hierarchy reflect the
co...
Object models and the UML
The UML is a standard representation devised by
the developers of widely used object-oriented
an...
Library class hierarchy
Library item
Catalo gue n umber
Acquisition da te
Cost
T pe
y
Status
Num ber of copies
Acquire ()
...
User class hierarchy
Library user
Name
Address
Phone
Reg istration #
Reg ister ()
De-reg ister ()

Reader

Borrower
Items ...
Multiple inheritance
Rather than inheriting the attributes and services
from a single parent class, a system which support...
Multiple inheritance
Book

Voice recording

Author
Edition
Pub lication da te
ISBN

Speak er
Duration
Recording da te

T
a...
Object aggregation
An aggregation model shows how classes
that are collections are composed of other
classes.
Aggregation ...
Object aggregation
Study pack
Course title
Number
Year
Instructor

OHP slides

Assignment
Credits

Exercises
#Pr ob lems
D...
Object behaviour modelling
A behavioural model shows the interactions
between objects to produce some particular
system be...
Issue of electronic items
Ecat:
Catalog

:Library Item

Lib1:
NetServer

:Library User

Lookup
Display
Issue
Issue licence...
Structured methods
Structured methods incorporate system
modelling as an inherent part of the method.
It is a systematic w...
Method weaknesses
They do not model non-functional system
requirements.
They do not usually include information
about whet...
CASE workbenches
A coherent set of tools that is designed to
support related software process activities
such as analysis,...
An analysis and design workbench
Data
dictionary

Structur ed
diag ramming
tools

Repor t
gener ation
facilities

Code
gen...
Analysis workbench components
Diagram editors
Model analysis and checking tools
Repository and associated query language
D...
Key points
A model is an abstract system view.
Complementary types of model provide
different system information.
Context ...
Key points
Semantic data models describe the logical
structure of data which is imported to or
exported by the systems.
Ob...
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  1. 1. System models ©Ian Sommerville 2006 Software Engineering, 8th edition. Chapter 8 Slide 1
  2. 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 2006 Software Engineering, 8th edition. Chapter 8 Slide 2
  3. 3. Topics covered Context models Behavioural models Data models Object models CASE workbenches ©Ian Sommerville 2006 Software Engineering, 8th edition. Chapter 8 Slide 3
  4. 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 2006 Software Engineering, 8th edition. Chapter 8 Slide 4
  5. 5. 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 2006 Software Engineering, 8th edition. Chapter 8 Slide 5
  6. 6. 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 2006 Software Engineering, 8th edition. Chapter 8 Slide 6
  7. 7. The context of an ATM system Security sy stem Branch accounting sy stem Account da tabase Auto-teller sy stem Branch counter sy stem Usage database Maintenance sy stem ©Ian Sommerville 2006 Software Engineering, 8th edition. Chapter 8 Slide 7
  8. 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 2006 Software Engineering, 8th edition. Chapter 8 Slide 8
  9. 9. Equipment procurement process Delivery note Specify equipm ent requir ed Equipment spec. Checked spec. V alida te specification Spec. + supplier + estima te Equipm ent spec. Supplier da tabase Accept deli very of equipment Get cost estim a tes Supplier list Find suppliers Choose supplier Delivery note Check delivered items Installa tion instructions Order notifica tion Order details plus blank or der for m Place equipment order Check ed and signed or der form Install equipment Installa tion acceptance Accept delivered equipment Equipment details Equipment da tabase ©Ian Sommerville 2006 Software Engineering, 8th edition. Chapter 8 Slide 9
  10. 10. 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 2006 Software Engineering, 8th edition. Chapter 8 Slide 10
  11. 11. 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 2006 Software Engineering, 8th edition. Chapter 8 Slide 11
  12. 12. Order processing DFD Completed order for m Order details + blank or der form Complete or der form Signed order for m V alida te order Signed order form Checked and signed or der + or der notifica tion Send to supplier Record or der Order details Signed order form Adjust availab le budget Order amount + account details Orders file ©Ian Sommerville 2006 Budget file Software Engineering, 8th edition. Chapter 8 Slide 12
  13. 13. 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 2006 Software Engineering, 8th edition. Chapter 8 Slide 13
  14. 14. Insulin pump DFD Blood Blood parameters Blood sugar sensor Blood sugar analy sis Blood sugar level Insulin requir ement computa tion Insulin ©Ian Sommerville 2006 Pump contr ol commands Insulin pump Insulin deli very contr oller Insulin requir ement Software Engineering, 8th edition. Chapter 8 Slide 14
  15. 15. 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 2006 Software Engineering, 8th edition. Chapter 8 Slide 15
  16. 16. 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 2006 Software Engineering, 8th edition. Chapter 8 Slide 16
  17. 17. Microwave oven model Full po w er Full po wer do: set power = 600 Timer Waiting Number do: display time Half po wer Full po wer Half po wer Set time Operation do: get number exit: set time do: operate oven Door closed Tim er 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 2006 Software Engineering, 8th edition. Chapter 8 Slide 17
  18. 18. 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 ŌH powerÕ alf . Full power The oven power is set to 600 watts. The display shows ŌF powerÕ ull . Set time The cooking time is s et to the userÕ input value. The display shows the cooking time s selected and is updated as the time is set. Disabled Oven operation is disabled for safety. Interior oven light is on. Display shows ŌN ot readyÕ . Enabled Oven operation is enabled. Interior oven light is off. Display shows Ō Ready 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 Ō Cooking completeÕ w buzzer is sounding. hile ©Ian Sommerville 2006 Software Engineering, 8th edition. Chapter 8 Slide 18
  19. 19. Microwave oven stimuli ©Ian Sommerville 2006 Software Engineering, 8th edition. Chapter 8 Slide 19
  20. 20. Microwave oven operation Operation Checking OK do: check status Tim e 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 2006 Cancel Waiting Software Engineering, 8th edition. Chapter 8 Slide 20
  21. 21. 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 2006 Software Engineering, 8th edition. Chapter 8 Slide 21
  22. 22. Library semantic model Article title authors pdf file fee m published-in Source 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 1 name has-links address title publisher issue date pages Country in 1 copyright form tax rate Buyer name address e-mail billing info ©Ian Sommerville 2006 Software Engineering, 8th edition. Chapter 8 Slide 22
  23. 23. 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 2006 Software Engineering, 8th edition. Chapter 8 Slide 23
  24. 24. Data dictionary entries ©Ian Sommerville 2006 Software Engineering, 8th edition. Chapter 8 Slide 24
  25. 25. Object models Object models describe the system in terms of object classes and their associations. Combine some of the uses of Data-flow and Semantic data models 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(Object Behaviour modelling). ©Ian Sommerville 2006 Software Engineering, 8th edition. Chapter 8 Slide 25
  26. 26. 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 2006 Software Engineering, 8th edition. Chapter 8 Slide 26
  27. 27. 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 2006 Software Engineering, 8th edition. Chapter 8 Slide 27
  28. 28. 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 2006 Software Engineering, 8th edition. Chapter 8 Slide 28
  29. 29. Library class hierarchy Library item Catalo gue n umber Acquisition da te Cost T pe y Status Num ber of copies Acquire () Catalo gue () Dispose () Issue () Return () Pub lished item Recorded item Title Medium Title Pub lisher Book A uthor Edition Pub lication da te ISBN ©Ian Sommerville 2006 Magazine Y ear Issue Film Director Date of release Distrib utor Com puter pro gram V ersion Platfor m Software Engineering, 8th edition. Chapter 8 Slide 29
  30. 30. User class hierarchy Library user Name Address Phone Reg istration # Reg ister () De-reg ister () Reader Borrower Items on loan Max. loans Af filiation Staff Depar tment Depar tment phone ©Ian Sommerville 2006 Student Major subj ect Hom e ad dress Software Engineering, 8th edition. Chapter 8 Slide 30
  31. 31. 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 2006 Software Engineering, 8th edition. Chapter 8 Slide 31
  32. 32. Multiple inheritance Book Voice recording Author Edition Pub lication da te ISBN Speak er Duration Recording da te T alking book # Tapes ©Ian Sommerville 2006 Software Engineering, 8th edition. Chapter 8 Slide 32
  33. 33. 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 2006 Software Engineering, 8th edition. Chapter 8 Slide 33
  34. 34. Object aggregation Study pack Course title Number Year Instructor OHP slides Assignment Credits Exercises #Pr ob lems Description ©Ian Sommerville 2006 Slides Lecture notes T ext Videota pe T ape ids . Solutions T ext Diagrams Software Engineering, 8th edition. Chapter 8 Slide 34
  35. 35. 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 2006 Software Engineering, 8th edition. Chapter 8 Slide 35
  36. 36. Issue of electronic items Ecat: Catalog :Library Item Lib1: NetServer :Library User Lookup Display Issue Issue licence Accept licence Compress Deliver ©Ian Sommerville 2006 Software Engineering, 8th edition. Chapter 8 Slide 36
  37. 37. Structured methods Structured methods incorporate system modelling as an inherent part of the method. It is a systematic way of producing models of an existing system that is to built. 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 2006 Software Engineering, 8th edition. Chapter 8 Slide 37
  38. 38. 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 2006 Software Engineering, 8th edition. Chapter 8 Slide 38
  39. 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 2006 Software Engineering, 8th edition. Chapter 8 Slide 39
  40. 40. An analysis and design workbench Data dictionary Structur ed diag ramming tools Repor t gener ation facilities Code gener ator Centr al infor m ation repository Query langua ge facilities Form s creation tools Design, anal y sis and checking tools Import/e xpor t facilities ©Ian Sommerville 2006 Software Engineering, 8th edition. Chapter 8 Slide 40
  41. 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 2006 Software Engineering, 8th edition. Chapter 8 Slide 41
  42. 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 2006 Software Engineering, 8th edition. Chapter 8 Slide 42
  43. 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. 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 2006 Software Engineering, 8th edition. Chapter 8 Slide 43
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