System Design

©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 1
System Design
Architectural Design
Distributed System Architectures
Object Oriented Design
Real-time Software Design
Design with reuse
User Interface Design
Lecture 4

Software Architecture
Identifying the sub-systems and making a framework for
these sub-system control and communication is called
architectural design.
Output of design process = software architecture.
Output of architectural design process = architectural
design document
This document contains e.g. DFD, ERD etc.

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.

Advantages of explicit architecture
Stakeholder Communication
• Architecture may be used as a focus of discussion by system
stakeholders[involved].
System Analysis
• Means that analysis of whether the system can meet its non-
functional requirements is possible.
• [What a system should do = non functional requirements]
• [How a system should do = functional requirements]
Large-Scale Reuse
• The architecture may be reusable across a range of systems.

System Architecture affects:
Performance
Security
• Use a layered architecture with critical assets in the inner
layers.
Safety
• Use safety-critical features in a small number of sub-systems.
Availability
• Include redundant components and mechanisms for fault
tolerance.
Maintainability
• Provides support, resilency

System Structuring
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 sub-systems share
data, are distributed and interface with each other may
also be developed.

Block Diagram of Computer

1- 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.

Film and picture library
Catalogue
server
Library
catalogue
Video
server
Film clip
files
Picture
server
Digitised
photo graphs
Web server
Film and
photo info.
Client 1 Client 2 Client 3 Client 4
Internet

2- Abstract machine (layered) model
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 sub-systems in different
layers. When a layer interface changes, only the adjacent layer is
affected.
However, often artificial to structure systems in this way.

Version Management System
Configuration management systemlayer
Database systemlayer
Operating systemlayer
Object management systemlayer

3- Object Oriented 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.

Invoice processing system
issue ()
sendReminder ()
acceptPayment ()
sendReceipt ()
invoice#
date
amount
customer
invoice#
date
amount
customer#
invoice#
date
amount
customer#
customer#
name
address
credit period
Customer
Payment
Invoice
Receipt

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.

Real-time system control
System
contr oller
User
inter face
Fault
handler
Computa tion
pr ocesses
Actuator
processes
Sensor
pr ocesses

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 sub-
system 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.

Selective broadcasting
Sub-system
1
Event and messa ge handler
Sub-system
2
Sub-system
3
Sub-system
4

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.

Interrupt-driven control
Handler
1
Handler
2
Handler
3
Handler
4
Process
1
Process
2
Process
3
Process
4
Interrupts
Interrupt
vector

Reference architectures
Architectural models may be specific to some application
domain.
Two types of domain-specific model
• Generic models.
• Reference models.
Generic models are usually bottom-up models; Reference
models are top-down models.

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.

OSI reference model
Presenta tion
Session
Transpor t
Network
Data link
Physical
7
6
5
4
3
2
1
Comm unica tions medium
Networ k
Data link
Physical
Applica tion
Presenta tion
Session
Transpor t
Networ k
Data link
Physical
Applica tion

System Design

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