1. Part 3 Design What does design mean in different fields?
How important is it?
What are the methods?
Are there patterns you can use?
Is it creative?
Where does it fit with requirements?
An overview of design considerations. Not very specific

2. Chapter 11: Architectural Design
What is architecture?
What does an architect do?
What skills does an architect have?
What does a software architect do?
Skills?
This will look a lot like systems engineering
Can often reuse some of it's results
Architects are the technical interface between the customer and the contractor building the system
A bad architectural design for a building cannot be rescued by good construction; the same is true for software
There are specialist types of building and software architects
There are schools or styles of building and software architecture

3. Architectural design Architectural design: the first step in design
From a system perspective, not necessarily a user perspective
Goal: decompose a system into interacting subsystems
high level: just the subsystems connected by lines
better: show how subsystems share data and interface with each other
Refine by decomposing the subsystems
11.2 System Organization (identify the pieces)
Representative models
repository
client-server
layered

4. 11.2.1 The repository model Subsystems may exchange data in two ways:
shared data is held in a central database or repository and may be accessed by all subsystems
each subsystem maintains its own database and passes data explicitly to other subsystems
Repository commonly used for large amounts of shared data
Fig 11.2, CASE toolset :
A fairly simple box and line diagram.
Strengths? Weaknesses?

5. Repository model characteristics
Advantages
efficient way to share large amounts of data
subsystems need not be concerned with how data is produced centralized management e.g. backup, security, etc.
a single sharing model published as the repository schema
Disadvantages
subsystems must agree on a data model--often a compromise
data evolution is difficult and expensive
no scope for specific management policies
difficult to distribute efficiently
(How) does the web support this model?

6. 11.2.2 Client-server architecture
Data and processing distributed across a range of components
servers provide specific services such as printing, data management
clients call on these services
network allows clients to access servers
Film and picture library:

7. Client-server characteristics
Advantages
straightforward data distribution
uses networked systems effectively
may use cheap hardware
easy to add new servers or upgrade existing servers
Disadvantages
no shared data model so subsystems use different data organization
data interchange may be inefficient
redundant management in each server
it may be hard to find out what servers and services are available

8. Layered model
Organizes the system into layers (or abstract machines)
Each layer provides a set of services
Supports incremental development in different layers
When a layer interface changes, only the adjacent layer is affected
It is often difficult to structure
systems in this way
Is there an example from CS 330?

9. 11.3 Modular decomposition
Subsystems are decomposed into modules
What models do you know?
object model: the system is decomposed into interacting objects
function oriented pipelining: the system is decomposed into functional modules which transform inputs to outputs. Also called data flow models

10. Object models
A set of loosely coupled objects with well-defined interfaces
OO decomposition: identify classes, attributes and operations
Implementation: create objects from these classes and a control model to
coordinate object operations
How does this work in C++?
Invoice processing system

11. 11.3.2 Function–oriented pipelining
Functional transformations process inputs to produce outputs
May be called a pipe and filter model (e.g. UNIX shell)
Not well suited for interactive systems
Invoice processing system:

12. 11.4 Control styles (how do the pieces communicate)
How control flows between subsystems
Distinct from the system decomposition model
Centralized control
one subsystem controls/starts/stops other subsystems
Event-driven control
each subsystem responds to external events from other subsystems or the system environment

13. Centralized control
A control subsystem is responsible for managing the execution of other subsystems
Call-return model
top-down subroutine model where control starts at the top of a subroutine hierarchy and moves downwards.
usually sequential systems

14. 11.4.1 Centralized control Manager model applies 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
E.g. this might describe a robot for selecting inventory from a warehouse.

15. Event-driven systems
Driven by external events whose timing is not controlled by the subsystems which process the event
Two models
broadcast. An event is broadcast to all subsystems. Any subsystem that can handle the event may do so
interrupt-driven. Used in real-time systems where interrupts are detected by an interrupt handler and passed to some other component for processing

16. Broadcast model
Effective for subsystems on different computers in a network
Subsystems register an interest in specific events. When these occur, the event is transferred to the subsystem to handle it
Control policy is embedded in the subsystem, not the event and message handler
What broadcast model(s) do you know?

17. Interrupt-driven systems
Used in real-time systems for fast response to an event
Known interrupt types have defined handlers
Each interrupt type is associated with a memory location and a hardware switch that causes transfer to its handler
Fast response but complex to
program and validate
Examples you know?

18. 11.5 Reference architectures
Specific to a domain
Generic models (more in Chapter 13)
abstract from real systems and encapsulate principal characteristics
usually bottom up
examples: compiler, natural language processing
Reference models
more abstract, idealized
derived from study of the application, not existing systems
may be a basis for an implementation or to compare systems
example: OSI 7 layers

19. Generic example: compiler

20. OSI reference model
Application