Architectural Design
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 1
Objectives




To introduce architectural design and to
discuss its importance
To explain the architectural design decisions
that have to be made
To introduce three complementary
architectural styles covering organisation,
decomposition and control
To discuss reference architectures are used
to communicate and compare architectures
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 2
Topics covered






Introduction
Architectural design decisions
System organisation
Decomposition styles
Control styles
Reference architectures
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 3
The software design process
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 4
Software architecture


The design process for identifying the subsystems making up a system and the
framework for sub-system control and
communication is architectural design.
The output of this design process is a
description of the software architecture.
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 5
Architectural design




An early stage of the system design process.
Represents the link between specification
and design processes.
Often carried out in parallel with some
specification activities.
It involves identifying major system
components and their communications.
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 6
Subsystem decomposition



Concerned with decomposing the system
into interacting sub-systems.
The architectural design is normally
expressed as a block diagram presenting an
overview of the system structure.
More specific models showing how subsystems share data, are distributed and
interface with each other may also be
developed.
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 7
Packing robot control system
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 8
Box and line diagrams


Very abstract - they do not show the nature
of component relationships nor the externally
visible properties of the sub-systems.
However, useful for communication with
stakeholders and for project planning.
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 9
Advantages of explicit architecture

Stakeholder communication
•

Large-scale reuse
•

Architecture may be used as a focus of
discussion by system stakeholders.
The architecture may be reusable across a
range of systems.
System analysis
•
Means that analysis of whether the system can
meet its non-functional requirements is
possible.
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 10
Architecture and system characteristics

Performance
•

Security
•

Localise safety-critical features in a small number of subsystems.
Availability
•

Use a layered architecture with critical assets in the inner
layers.
Safety
•

Localise critical operations and minimise communications.
Use large rather than fine-grain components.
Include redundant components and mechanisms for fault
tolerance.
Maintainability
•
Use fine-grain, replaceable components.
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 11
Architectural conflicts



Using large-grain components improves
performance but reduces maintainability.
Introducing redundant data improves
availability but makes security more difficult.
Localising safety-related features usually
means more communication so degraded
performance.
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 12
Topics covered






Introduction
Architectural design decisions
System organisation
Decomposition styles
Control styles
Reference architectures
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 13
Architectural design decisions


Architectural design is a creative process so
the process differs depending on the type of
system being developed.
However, a number of common decisions
span all design processes.
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 14
Architectural design decisions

Is there a generic application architecture that can
be reused? How will the subsystems be distributed?
•
•
•
•

What architectural styles are appropriate? What
approach will be used to structure the system?
•

Distributed architectures (ch. 12)
Application architectures (ch. 13)
Product line architectures (ch. 18)
Reference architectures (11.5)
Repository, client-server, layered (11.2)
How will the system be decomposed into modules?
•
•
Object-oriented decomposition (11.3, ch. 14)
Functional/dataflow decomposition (11.3)
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 15
Architectural design decisions
(con’t)

What control strategy should be used?
•


Centralized, event-driven (11.4)
How will the architectural design be
evaluated?
How should the architecture be
documented?
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 16
Architecture evaluation:
“Good” architectures

Well-defined modules.
•
•

Separation of concerns.
•
•

Allocate functionalities based on information hiding and
separation of concerns.
Parallel-processing systems should have well-defined
processes that may not necessarily mirror the static structure.
Allow relatively independent module development.
Separate modules that produce data from modules that
consume data.
Information-hiding.
•
•
•
Encapsulate idiosyncracies of platform.
Do not depend on a particular version of a product or tool.
Make it easy to reallocate processes to different processors.
Bass, L., Clements, P., Kazman, R.
Software
Architecture
in Practice originals
Modified
from Sommerville’s
Software Engineering, 7th edition. Chapter 11
Slide 19
Best practices


The architecture should be the product of a single
architect or team.
The architect should have the technical
requirements of the system and prioritized list of
quality attributes.
•
•


The resulting architecture should be analyzed against
these attributes.
The resulting architecture should clearly set resource
budgets.
The architecture should be well-documented and
circulated to stakeholders.
The architecture should have an infrastructure that
can be implemented first.
Bass, L., Clements, P., Kazman, R.
Software
Architecture
in Practice originals
Modified
from Sommerville’s
Software Engineering, 7th edition. Chapter 11
Slide 20
Architecture evaluation techniques

Architecture reviews
•
•
Useful for evaluating functional completeness,
dependability, maintainability.
Scenario-based
• Work through several scenarios to understand system
behavior.
•
Questionnaire-based
• Architects answer questions about how the architecture was
derived, level of coupling between subsystems, etc.
•
Checklist-based
• Derived from past experiences and proven architecture
design techniques for a particular domain.

Simulation and prototypes
•
Good for evaluating performance and usability
Bass, L., Clements, P., Kazman, R.
Software
Architecture
in Practice originals
Modified
from Sommerville’s
Software Engineering, 7th edition. Chapter 11
Slide 21
Documenting architectures:
Architectural models

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
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Used to document an architectural design.
Static structural model that shows the major system
components.
Dynamic process model that shows the process
structure of the system.
Interface model that defines sub-system interfaces.
Relationships model such as a data-flow model that
shows sub-system relationships.
Distribution model that shows how sub-systems are
distributed across computers.
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 22
Example: structural model
(Hierarchical decomposition)
Produce
design repor ts
names
design
name
sorted
entity
data
names
Get design
entity names
sorted
entity
data
Collate
entities
entity
names
sor ted
entity
data
names
sor ted
names
Get design
name
Get entity
names
design
name
entity
names
Sor t entities
by name
Modified from Sommerville’s originals
Generate
repor t
sor ted
entity
data
entity
data
Get entity
data
Integrated
repor t
Sor t entities
by type
Produce
integrated repor t
entity
data
Software Engineering, 7th edition. Chapter 11
Print
report
repor t
Slide 23
Example: structural model
(Object packages)
PlanningSubsystem
RoutingSubsystem
RouteAssistant
PlanningService
Trip
Location
TripProxy
Destination
Direction
Crossing
SegmentProxy
Segment
CommunicationSubsystem
Message
Connection
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 24
Example: process model
(Linux kernel)
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 25
Example: process model
(A temperature control system)
500 Hz
Sensor
process
Sensor
values
500 Hz
Thermostat
process
Switch command
Room n umber
500 Hz
Heater contr ol
process
Modified from Sommerville’s originals
Thermostat pr ocess
Furnace
control pr ocess
Software Engineering, 7th edition. Chapter 11
Slide 26
Example: interface model
(A data collector component)
Requires interface
Provides interface
sensorManagement
Data collector
sensorData
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
addSensor
removeSensor
startSensor
stopSensor
testSensor
initialise
report
listAll
Slide 27
Example: relationship model
(Salary payment DFD)
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 28
Example: distribution model
(An internet banking system)
Client
HTTP interactio n
Client
Database server
Web s erv er
SQL q u ery
Acco u nt service
p rov is ion
SQL
Cus tomer
acco un t
d atabase
Client
Client
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 29
Topics covered






Introduction
Architectural design decisions
System organisation
Decomposition styles
Control styles
Reference architectures
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 30
Architectural styles



The architectural model of a system may
conform to a generic architectural model or
style.
An awareness of these styles can simplify
the problem of defining system architectures.
However, most large systems are
heterogeneous and do not follow a single
architectural style.
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 31
System organisation


Reflects the basic strategy that is used to
organize a system.
Three architectural styles are widely used:
•
•
•
A shared data repository style;
A shared services and servers style;
An abstract machine or layered style.
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 32
The repository model

Sub-systems must exchange data. This may
be done in two ways:
•
•

Shared data is held in a central database or
repository and may be accessed by all subsystems;
Each sub-system maintains its own database
and passes data explicitly to other sub-systems.
When large amounts of data are to be
shared, the repository model of sharing is
most commonly used.
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 33
CASE toolset architecture
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 34
Repository model characteristics

Advantages
•
•
•
•

Efficient way to share large amounts of data;
Sub-systems need not be concerned with how data is
produced;
Centralised management e.g. backup, security, etc.
Sharing model is published as the repository schema.
Disadvantages
•
•
•
•
Sub-systems must agree on a repository data model.
Inevitably a compromise;
Data evolution is difficult and expensive;
No scope for specific management policies;
Difficult to distribute efficiently.
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 35
Client-server model




Distributed system model which shows how
data and processing is distributed across a
range of components.
Set of stand-alone servers which provide
specific services such as printing, data
management, etc.
Set of clients which call on these services.
Network which allows clients to access
servers.
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 36
Film and picture library
Client 1
Client 2
Client 3
Client 4
Internet
Catalogue
server
Video
server
Picture
server
Web server
Library
catalogue
Film clip
files
Digitised
photo graphs
Film and
photo info.
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 37
Client-server characteristics

Advantages
•
•
•

Distribution of data is straightforward;
Makes effective use of networked systems. May require
cheaper hardware;
Easy to add new servers or upgrade existing servers.
Disadvantages
•
•
•
No shared data model so sub-systems use different data
organisation. Data interchange may be inefficient;
Redundant management in each server;
No central register of names and services - it may be hard
to find out what servers and services are available.
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 38
Abstract machine (layered) model


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Used to model the interfacing of sub-systems.
Organises the system into a set of layers (or abstract
machines) each of which provide a set of services.
Supports the incremental development of subsystems in different layers. When a layer interface
changes, only the adjacent layer is affected.
However, often difficult to structure systems in this
way. New services may require changes that cut
across multiple layers.
Performance can be a problem as a request may
have to go through several layers before being
processed.
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 39
Version management system
Configuration management system layer
Object management system layer
Database system layer
Operating system layer
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 40
General application layers
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 41
Topics covered






Introduction
Architectural design decisions
System organisation
Decomposition styles
Control styles
Reference architectures
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 43
Modular decomposition styles


Styles of decomposing sub-systems into
modules.
No rigid distinction between system
organisation and modular decomposition.
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 44
Sub-systems and modules


A sub-system is a system in its own right
whose operation is independent of the
services provided by other sub-systems.
A module is a system component that
provides services to other components but
would not normally be considered as a
separate system.
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 45
Modular decomposition


Another structural level where sub-systems are
decomposed into modules.
Two modular decomposition models covered
•
•

An object model where the system is decomposed into
interacting object;
A pipeline or data-flow model where the system is
decomposed into functional modules which transform
inputs to outputs.
If possible, decisions about concurrency should be
delayed until modules are implemented.
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 46
Object models



Structure the system into a set of loosely
coupled objects with well-defined interfaces.
Object-oriented decomposition is concerned
with identifying object classes, their attributes
and operations.
When implemented, objects are created from
these classes and some control model used
to coordinate object operations.
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 47
Invoice processing system
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 48
Object model advantages and
disadvantages




Objects are loosely coupled so their
implementation can be modified without
affecting other objects.
The objects may reflect real-world entities.
OO implementation languages are widely
used.
However, object interface changes may
cause problems and complex entities may
be hard to represent as objects.
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 49
Function-oriented pipelining




Functional transformations process their
inputs to produce outputs.
May be referred to as a pipe and filter model
(as in UNIX shell).
Variants of this approach are very common.
When transformations are sequential, this is
a batch sequential model which is
extensively used in data processing systems.
Not really suitable for interactive systems.
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 50
Invoice processing system
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 51
Pipeline model advantages and
disadvantages





Supports transformation reuse.
Intuitive organisation for stakeholder
communication.
Easy to add new transformations.
Relatively simple to implement as either a
concurrent or sequential system.
However, requires a common format for data
transfer along the pipeline and difficult to
support event-based interaction.
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 52
Topics covered






Introduction
Architectural design decisions
System organisation
Decomposition styles
Control styles
Reference architectures
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 53
Control styles


Are concerned with the control flow between
sub-systems. Distinct from the system
decomposition model.
Centralised control
•

One sub-system has overall responsibility for
control and starts and stops other sub-systems.
Event-based control
•
Each sub-system can respond to externally
generated events from other sub-systems or the
system’s environment.
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 54
Centralised control


A control sub-system takes responsibility for
managing the execution of other sub-systems.
Call-return model
•

Top-down subroutine model where control starts at the top
of a subroutine hierarchy and moves downwards.
Applicable to sequential systems.
Manager model
•
Applicable to concurrent systems. One system
component controls the stopping, starting and
coordination of other system processes. Can be
implemented in sequential systems as a case statement.
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 55
Call-return model
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 56
Real-time system control
Sensor
pr ocesses
Actuator
processes
System
controller
Computation
pr ocesses
Modified from Sommerville’s originals
User
inter face
Fault
handler
Software Engineering, 7th edition. Chapter 11
Slide 57
Event-driven systems


Driven by externally generated events.
Two principal event-driven models
•
•

Broadcast models. An event is broadcast to all subsystems. Any sub-system which can handle the event
may do so;
Interrupt-driven models. Used in real-time systems where
interrupts are detected by an interrupt handler and passed
to some other component for processing.
Other event driven models include spreadsheets
and production-control systems.
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 58
Broadcast model




Effective in integrating sub-systems on different
computers in a network.
Sub-systems register an interest in specific events.
When these occur, control is transferred to the subsystem which can handle the event.
Control policy is not embedded in the event and
message handler. Sub-systems decide on events of
interest to them.
However, sub-systems don’t know if or when an
event will be handled.
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 59
Selective broadcasting
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 60
Interrupt-driven systems




Used in real-time systems where fast
response to an event is essential.
There are known interrupt types with a
handler defined for each type.
Each type is associated with a memory
location and a hardware switch causes
transfer to its handler.
Allows fast response but complex to program
and difficult to validate.
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 61
Interrupt-driven control
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 62
Topics covered






Introduction
Architectural design decisions
System organisation
Decomposition styles
Control styles
Reference architectures
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 63
Reference architectures


Architectural models may be specific to some
application domain.
Two types of domain-specific model
•
•

Generic models which are abstractions from a number of
real systems and which encapsulate the principal
characteristics of these systems. Covered in Chapter 13.
Reference models which are more abstract, idealised
model. Provide a means of information about that class of
system and of comparing different architectures.
Generic models are usually bottom-up models;
Reference models are top-down models.
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 64
Reference architectures



Reference models are derived from a study
of the application domain rather than from
existing systems.
May be used as a basis for system
implementation or to compare different
systems. It acts as a standard against which
systems can be evaluated.
OSI model is a layered model for
communication systems.
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 65
OSI reference model
7
Application
Application
6
Presentation
Presentation
5
Session
Session
4
Transport
Transport
3
Network
Networ k
Networ k
2
Data link
Data link
Data link
1
Physical
Physical
Physical
Comm unications medium
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 66
CASE reference model

Data repository services
•

Data integration services
•

Definition and enaction of process models.
Messaging services
•

Managing groups of entities.
Task management services
•

Storage and management of data items.
Tool-tool and tool-environment communication.
User interface services
•
User interface development.
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 67
The ECMA reference model
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 68
Key points




The software architecture is the fundamental
framework for structuring the system.
Architectural design decisions include decisions on
the application architecture, the distribution and the
architectural styles to be used.
Different architectural models such as a structural
model, a control model and a decomposition model
may be developed.
System organisational models include repository
models, client-server models and abstract machine
models.
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 69
Key points



Modular decomposition models include
object models and pipelining models.
Control models include centralised control
and event-driven models.
Reference architectures may be used to
communicate domain-specific architectures
and to assess and compare architectural
designs.
Modified from Sommerville’s originals
Software Engineering, 7th edition. Chapter 11
Slide 70