0517 CH14 System Design

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Chapter14

System Design

Michael Blaha

James Rumbaugh

2005.5.17

Kim, Suhyun



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Contents Introduction

System design decisions Estimating performance

Reuse plan

Organizing and allocating subsystems

Handling global resources

Software control strategy

Architectural style

Summary



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Introduction System design

Organization of the system into subsystems Allocation of subsystems to H/W or S/W

Major policy decisions for class design


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Estimating performance Purpose

To determine if the system is feasible Rough performance estimation

Fast calculation, common sense

Getting within small factors Examples

Bank : data storage

Satellite-based ground imaging system : bandwidth,storage

Nuclear power plant : respond time



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Making reuse plan (1/ 2) Libraries

Collection of classes Organized about a few, well-focused themes

Complete behavior for the chosen themes

Extensibility to be defined subclass

Problems

Incompatibility with other class libraries

Impossibility to fix inconsistencies by inheritance



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Making reuse plan (2/ 2) Frameworks

Skeletal structure of a program Frameworks tend to be specific to a category of app.

User can choose the appropriate subclasses

Patterns Proven solution to a general problem

Achieving reuse by using existing patterns

Small number of classes and relationships



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Dividing into subsystem (1/ 2) Layered system

Providing implementation basis for above Reducing conceptual gap between adjoining layers

Desired system

Intermediate layer

Intermediate layer

Available resource

< open architecture > < closed architecture >

Desired system

Intermediate layer

Intermediate layer

Available resource



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Partitions

Independent or weakly coupled subsystem Similar level of abstraction

Combining layers and partitions

Application package

Computer hardware

Operating system

Window graphics

Screen graphics

Pixel

User

dialog

control

Simulation

package

Dividing into subsystem (2/ 2)



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Identifying concurrency Concurrent objects

Receive events at the same time without interaction ex. Engine and wing controls on an airplane

Be assigned to different hardware units

Defining concurrent tasks Examining state diagrams

Fold many objects onto a single thread of control

Implement threads of control as tasks



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Allocation of subsystems Steps

Estimate performance and resource Estimating required CPU processing power

Choose H/W or S/W implementation

Cost, performance Allocate S/W subsystems to processors

Logistics, communication limits, computation limits

Determine the connectivity Connection topology, repeated unit, communications



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Management of data storage

File

High volume and low information density Data that accessed sequentially

Data that can be fully read into memory

Database Accessed by multiple application programs

Require coordinated updates via transactions

Secured against unauthorized and malicious access



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Handling global resources

Avoiding conflict

Guardian

object

application

application

Physical units

Space

Logical names

Shared data

Normal application

Guardian

object

application

application

Physical units

Space

Logical names

Shared data

Time-critical application

lockGetting lock



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Software control strategy

Procedure-driven control

Event-driven control Concurrent control

Internal control



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Handling boundary conditions

Initialization

Constant data, parameters, global variables, tasks,guardian objects

Termination

Releasing any external resources

Failure

Plans for graceful exit on fatal errors



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Trade-off priorit ies

Determination relative importance of various

criteria Examples

Video game with limited memory

Conserving memory > fast execution Mathematical subroutine package

Well-conditioned numerical behavior >

fast development

Mobile device

Reliability > portability



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Common architectural style

Batch transformation

Continuous transformation Interactive interface

Dynamic simulation

Real-time system

Tight time constraints on actions (respond time)

Transaction manager To store and retrieve data



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Batch transformation

Executed once on an entire input set

No ongoing interaction with the outside Example

Compiler, payroll processing

ASCII

File

Graphics

Model

Connectivity

Model

Class

Model

Database

Code

Parse

Text

Determine

Connectivity

Abstract to

OO model

Generate

db code

Sequence of steps for a complier



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Continuous transformation

Performed continuously as inputs change

Updating outputs frequently Computing outputs incrementally

Adding intermediate objects for optimization

Example Signal processing, windowing system

Graphic

Model

Viewspace

Model

Window

Model

Screen

Image

map clip offset

Sequence of steps for a graphics application



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Interactive interface

Dominated by external interactions

Usually only part of entire application Example

Forms-base query interface, control panel for simulation

Interface

Communication

protocol

External

agent

Syntax of

possible

interactions

Flow

control

Error

handling

Human /

device



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Dynamic simulation

Simulate evolving real-world objects

Two ways for implementing control Explicit controller can simulate a state machine

Objects can exchange messages among themselves

Problem Providing adequate performance

Estimate computational cost of each update cycle

Example

Economic models, spacecraft trajectory computation



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Summary

System architecture

Decide basic approach to solution After analyzing an application

Before beginning the class design

Architectural style Hybrid of several forms

Possibly one for each major subsystem

Documents

0517 CH14 System Design