Transcript Lecture 1 for Chapter 6, System Design

Design
“There are two ways of constructing a software
design: One way is to make it so simple that there are
obviously no deficiencies, and the other way is to
make it so complicated that there are no obvious
deficiencies.”
- C.A.R. Hoare
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Version 1
Version 2
Stairs
Stairs
Entrance
door
Bath
Hallway
Dining
Master
Bedroom
Study
Hallway
Bedroom2
Bath
Bedroom2
Kitchen
Kitchen
Study
Dining
Entrance
door
Master
Bedroom
Hallway
Dining
Version 3
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Bedroom2
Study
Bath
Kitchen
Stairs
Entrance
door
N
Master
Bedroom
Figure 6-1. Example of iterative
floor plan design. Three successive
versions show how we minimize
walking distance and take
advantage of sunlight.
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Overview
System Design I (chap6_1)
0. Overview of System Design
1. Design Goals
2. Subsystem Decomposition
System Design II (chap6_2)
3. Concurrency
4. Hardware/Software Mapping
5. Persistent Data Management
6. Global Resource Handling and Access Control
7. Software Control
8. Boundary Conditions
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Figure 6-2. The activities of system design (UML activity diagram).
Define
design goals
Define
subsystems
Implement
subsystems
Map subsystems
to hardware/
software platform
Manage
persistent data
Define access
control policies
Select a
global
control flow
Describe boundary
conditions
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Why is Design so Difficult?


Analysis: Focuses on the application domain
Design: Focuses on the implementation domain
 Design knowledge is a moving target
 The reasons for design decisions are changing very rapidly




Halftime knowledge in software engineering: About 3-5 years
What I teach today (specific technology) will be out of date in 3-5 years
Cost of hardware rapidly sinking
“Design window”:
 Time in which design decisions have to be made
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The Purpose of System Design
Problem


Bridging the gap between
desired and existing system
in a manageable way
Use Divide and Conquer
New
System
 We model the new system to
be developed as a set of
subsystems
Existing System
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System Design
System Design
1. Design Goals
8. Boundary
Conditions
Definition
Trade-offs
Initialization
Termination
Failure
2. System
Decomposition
Layers/Partitions
Coherence/Coupling
7. Software
Control
3. Concurrency
Identification of
Threads
4. Hardware/
Software
Mapping
5. Data
Management
Special purpose
Buy or Build Trade-off
Allocation
Connectivity
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Persistent Objects
Files
Databases
Data structure
Monolithic
Event-Driven
Threads
Conc. Processes
6. Global
Resource Handling
Access control
Security
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How to use the results from the Requirements
Analysis for System Design

Nonfunctional requirements =>
 Activity 1: Design Goals Definition

Use Case model =>
 Activity 2: System decomposition (Selection of subsystems based on
functional requirements, coherence, and coupling)

Object model =>
 Activity 4: Hardware/software mapping
 Activity 5: Persistent data management

Dynamic model =>




Activity 3: Concurrency
Activity 6: Global resource handling
Activity 7: Software control
Activity 8: Boundary conditions
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Section 1. Design Goals














Reliability
Modifiability
Maintainability
Understandability
Adaptability
Reusability
Efficiency
Portability
Traceability of requirements
Fault tolerance
Backward-compatibility
Cost-effectiveness
Robustness
High-performance
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












Good documentation
Well-defined interfaces
User-friendliness
Reuse of components
Rapid development
Minimum # of errors
Readability
Ease of learning
Ease of remembering
Ease of use
Increased productivity
Low-cost
Flexibility
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Relationship Between Design Goals
End User
Low cost
Increased Productivity
Backward-Compatibility
Traceability of requirements
Rapid development
Flexibility
Runtime
Efficiency
Functionality
User-friendliness
Ease of Use
Ease of learning
Fault tolerant
Robustness
Reliability
Client
(Customer,
Sponsor)
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Portability
Good Documentation
Minimum # of errors
Modifiability, Readability
Reusability, Adaptability
Well-defined interfaces
Object-Oriented Software Engineering: Conquering Complex and Changing Systems
Developer/
Maintainer
10
Typical Design Trade-offs






Functionality vs. Usability
Cost vs. Robustness
Efficiency vs. Portability
Rapid development vs. Functionality
Cost vs. Reusability
Backward Compatibility vs. Readability
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Nonfunctional Requirements give a clue for the use of
Design Patterns



Read the problem statement again
Use textual clues (similar to Abbot’s technique in Analysis) to
identify design patterns
Text: “manufacturer independent”, “device independent”,
“must support a family of products”
 Abstract Factory Pattern

Text: “must interface with an existing object”
 Adapter Pattern

Text: “must deal with the interface to several systems, some of
them to be developed in the future”, “ an early prototype must
be demonstrated”
 Bridge Pattern
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Textual Clues in Nonfunctional Requirements

Text: “complex structure”, “must have variable depth and
width”
 Composite Pattern

Text: “must interface to an set of existing objects”
 Façade Pattern

Text: “must be location transparent”
 Proxy Pattern

Text: “must be extensible”, “must be scalable”
 Observer Pattern

Text: “must provide a policy independent from the mechanism”
 Strategy Pattern
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Section 2. System Decomposition

Subsystem (UML: Package)
 Collection of classes, associations, operations, events and constraints
that are interrelated
 Seed for subsystems: UML Objects and Classes.

Service:
 Group of operations provided by the subsystem
 Seed for services: Subsystem use cases

Service is specified by Subsystem interface:
 Specifies interaction and information flow from/to subsystem
boundaries, but not inside the subsystem.
 Should be well-defined and small.
 Often called API: Application programmer’s interface, but this
term should used during implementation, not during System
Design
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Services and Subsystem Interfaces

Service: A set of related operations that share a common
purpose
 Notification subsystem service:




LookupChannel()
SubscribeToChannel()
SendNotice()
UnscubscribeFromChannel()
 Services are defined in System Design

Subsystem Interface: Set of fully typed related operations. Also
called application programmer interface (API)
 Subsystem Interfaces are defined in Object Design
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FieldOfficerInterface
DispatcherInterface
Notification
IncidentManagement
Figure 6-4. Subsystem decomposition for an accident management system (UML class
diagram, collapsed view). Subsystems are shown as UML packages. Dashed arrows indicate
dependencies between subsystems.
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System
*
Class
Part *
S u b s y s t e mp a r t s
Figure 6-3. Subsystem decomposition (UML class diagram).
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Choosing Subsystems

Criteria for subsystem selection: Most of the interaction should
be within subsystems, rather than across subsystem boundaries
(High coherence).
 Does one subsystem always call the other for the service?
 Which of the subsystems call each other for service?

Primary Question:
 What kind of service is provided by the subsystems (subsystem
interface)?

Secondary Question:
 Can the subsystems be hierarchically ordered (layers)?

What kind of model is good for describing layers and
partitions?
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Definition: Subsystem Interface Object

A Subsystem Interface Object provides a service
 This is the set of public methods provided by the
subsystem
 The Subsystem interface describes all the methods of the
subsystem interface object

Use a Facade pattern for the subsystem interface
object
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RouteAssistant
PlanningService
Trip
Location
Direction
Destination
Crossing
Segment
Figure 6-28. Analysis model for the MyTrip route planning and execution.
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RoutingSubsystem
PlanningSubsystem
RouteAssistant
PlanningService
Trip
Location
Direction
Destination
Crossing
Segment
Figure 6-29. Initial subsystem decomposition for MyTrip (UML class diagram).
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PlanningSubsystem
RoutingSubsystem
RouteAssistant
PlanningService
Trip
Location
TripProxy
Destination
Direction
Crossing
SegmentProxy
Segment
CommunicationSubsystem
Message
Connection
Figure 6-32. Revised design model for MyTrip (UML Class diagram, associations omitted for
clarity).
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RoutingSubsystem
PlanningSubsystem
CommunicationSubsystem
TripFileStoreSubsystem
MapDBStoreSubsystem
Figure 6-35. Subsystem decomposition of MyTrip after deciding on the issue of data stores
(UML class diagram, packages collapsed for clarity).
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Coupling and Coherence


Goal: Reduction of complexity
Coherence measures the dependence among classes
 High coherence: The classes in the subsystem perform similar tasks
and are related to each other (via associations)
 Low coherence: Lots of misc and aux objects, no associations

Coupling measures dependencies between subsystems
 High coupling: Modifications to one subsystem will have high
impact on the other subsystem (change of model, massive
recompilation, etc.)

Subsystems should have as maximum coherence and minimum
coupling as possible:
 How can we achieve loose coupling?
 Which subsystems are highly coupled?
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Binary tree representation
add1:OpNode
a:ArgNode
add2:OpNode
b:ArgNode
Sharing through attributes
class OpNode {
ArgNode left;
ArgNode right;
String name;
}
class ArgNode {
String name;
}
c:ArgNode
Sharing through operations
class OpNode {
Enumeration getArguments();
String getName();
}
class ArgNode {
String getName();
}
Figure 6-5. Example of coupling reduction (UML object diagram and Java declarations). This figure shows a parse tree for the
expression “a + b + c”. The left column shows the interface of the OpNode class with sharing through attributes. The right
column shows the interface of OpNode with sharing through operations. Figure 6-6 shows the changes for each case when a
linked list is selected instead.
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Linked list representation
add:OpNode
a:ArgNode
Sharing through attributes
b:ArgNode
c:ArgNode
Sharing through operations
class OpNode {
class OpNode {
ArgNode first;
Enumeration getArguments();
ArgNode left;
String getName();
ArgNode right;
}
String name;
}
class ArgNode {
class ArgNode {
String name;
String getName();
ArgNode next;
}
}
Figure 6-6. Example of coupling reduction (UML object diagram and Java declarations). This
figure shows the impact of changing the parse tree representation of Figure 6-5 to a linked list.
In the left column, with sharing through attributes, four attributes need to change (changes
indicated in italics). In the right column, with sharing through operations, the interface
remains unchanged.
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DecisionSubsystem
assesses
Criterion
Alternativ
*
*
*
ya s e d
D e s i g n P r o b ls
eo
ml v a b l e Bb
resolvedBy
SubTask
*
Decision
ActionItem
Task
subtasks
implemente
Figure 6-7. Decision tracking system (UML class diagram). The DecisionSubsystem has a
low coherence: The classes Criterion, Alternative, and DesignProblem have no
relationships with Subtask, ActionItem, and Task.
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RationaleSubsystem
Figure 6-8. Alternative subsystem
decomposition for the decision tracking
system of Figure 6-7 (UML class diagram).
The coherence of the
RationaleSubsystem and the
PlanningSubsystem is higher than the
coherence of the original
DecisionSubsystem. Note also that we
also reduced the complexity by decomposing
the system into smaller subsystems.
Criterion
assesses
Alternative
*
*
DesignProblem
solvableBy
*
based-on
resolvedBy
Decision
PlanningSubsystem
implementedBy
SubTask
*
ActionItem
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Task
subtasks
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Partitions and Layers
A large system is usually decomposed into subsystems using
both, layers and partitions.
 Partitions vertically divide a system into several independent
(or weakly-coupled) subsystems that provide services on the
same level of abstraction.
 A layer is a subsystem that provides services to a higher level
of abstraction
 A layer can only depend on lower layers
 A layer has no knowledge of higher layers
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Properties of Layered Systems





Layered systems are hierarchical. They are desirable because
hierarchy reduces complexity.
Closed architectures are more portable.
Open architectures are more efficient.
If a subsystem is a layer, it is often called a virtual machine.
Layered systems often have a chicken-and egg problem
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Subsystem Decomposition into Layers
A: Subsystem
Layer 1
s t 2e m
B : S u b s y s t e mC : S u b s y s tDe:mS u b s yLayer
E : S u b s y s tFe:mS u b s y s t e m


G : S u b s yLayer
ste
3 m
Subsystem Decomposition Heuristics:
No more than 7+/-2 subsystems
 More subsystems increase coherence but also complexity (more
services)

No more than 5+/-2 layers
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Layer and Partition Relationships between
Subsystems

Layer relationship
 Layer A “Calls” Layer B (runtime)
 Layer A “Depends on” Layer B (“make” dependency, compile time)

Partition relationship
 The subsystem have mutual but not deep knowledge about each
other
 Partition A “Calls” partition B and partition B “Calls” partition A
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Application
Motif
Xt
Xlib
Figure 6-12. An example of open architecture: the OSF/Motif library (UML class diagram,
packages collapsed). Xlib provides low-level drawing facilities. Xt provides basic user
interface widget management. Motif provides a large number of sophisticated widgets. The
Application can access each of these layers independently.
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Level of abstraction
Application
Figure 6-10. An example of
closed architecture: the OSI
model (UML class diagram).
The OSI model decomposes
network services into seven
layers, each responsible for a
different level of abstraction.
Bernd Bruegge & Allen Dutoit
Presentation
Format
Session
Connection
Transport
Message
Network
Packet
DataLink
Frame
Physical
Bit
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Application
Object
Presentation
CORBA
Session
Transport
Network
Socket
TCP/IP
DataLink
Physical
Ethernet
Wire
Figure 6-11. An example of closed architecture (UML class diagram). CORBA enables the
access of objects implemented in different languages on different hosts. CORBA effectively
implements the Presentation and Session layers of the OSI stack.
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Virtual Machine



A virtual machine is an abstraction that provides a set of
attributes and operations.
A virtual machine is a subsystem connected to higher and lower
level virtual machines by "provides services for" associations.
Virtual machines can implement two types of software
architecture: closed and open architectures.
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Virtual Machine (Dijkstra, 1965)

A system should be developed by an ordered set of virtual
machines, each built in terms of the ones below it.
Problem
C1
attr
opr
C1
attr
opr
C1
attr
opr
C1
attr
opr
C1
attr
opr
VM2
C1
attr
opr
C1
attr
opr
VM1
C1
attr
opr
VM3
VM4
Existing System
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Closed Architecture (Opaque Layering)


A virtual machine can only
call operations from the layer
below
Design goal: High
maintainability
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C1
attr
C1
attr
C1
attr
op
op
op
VM1
C1
attr
C1
attr
op
op
C1
attr
C1
attr
op
op
C1
attr
C1
attr
op
op
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VM2
VM3
VM4
38
Open Architecture (Transparent Layering)


A virtual machine can call
operations from any layers
below
Design goal: Runtime
efficiency
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C1
attr
C1
attr
C1
attr
op
op
op
VM1
C1
attr
C1
attr
op
op
C1
attr
C1
attr
op
op
C1
attr
C1
attr
op
op
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VM2
VM3
VM4
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Software Architectures

Subsystem decomposition
 Identification of subsystems, services, and their relationship to each
other.







Specification of the system decomposition is critical.
Patterns for software architecture
Repository Architecture
Client/Server Architecture
Peer-To-Peer Architecture
Model/View/Controller
Pipes and Filters Architecture
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Repository Architecture



Subsystems access and modify data from a single data structure
Subsystems are loosely coupled (interact only through the
repository)
Control flow is dictated by central repository (triggers) or by
the subsystems (locks, synchronization primitives)
Repository
Subsystem
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createData()
setData()
getData()
searchData()
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Examples of Repository Architecture
Compiler
SyntacticAnalyzer
SemanticAnalyzer
Optimizer
CodeGenerator
LexicalAnalyzer



Hearsay II speech
understanding system
(“Blackboard architecture”)
Database Management
Systems
Modern Compilers
Repository
ParseTree
SourceLevelDebugger
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SymbolTable
SyntacticEditor
42
Client/Server Architecture


One or many servers provides services to instances of
subsystems, called clients.
Client calls on the server, which performs some service and
returns the result
 Client knows the interface of the server (its service)
 Server does not need to know the interface of the client


Response in general immediately
Users interact only with the client
Server
Client
Bernd Bruegge & Allen Dutoit
*
*
ervice1()
r e q u e s t e r p r o v is
sdeerrv i c e 2 ( )
…
serviceN()
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Client/Server Architecture

Often used in database systems:
 Front-end: User application (client)
 Back end: Database access and manipulation (server)

Functions performed by client:





Customized user interface
Front-end processing of data
Initiation of server remote procedure calls
Access to database server across the network
Functions performed by the database server:





Centralized data management
Data integrity and database consistency
Database security
Concurrent operations (multiple user access)
Centralized processing (for example archiving)
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diagram). Clients request services from one or
more Servers. The Server has no knowledge of
the Client. The client/server architecture is a
generalization of the repository architecture.
Server
Client
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*
requester
*
provider
service1()
service2()
…
serviceN()
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Figure 6-19. The World Wide Web as an instance of
the client/server architecture (UML object diagram).
netscape:WebBrowser
iexplorer:WebBrowser
lynx:WebBrowser
www12.in.tum.de:WebServer
www.cs.cmu.edu:WebServer
mosaic:WebBrowser
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Design Goals for Client/Server Systems

Portability
 Server can be installed on a variety of machines and operating systems
and functions in a variety of networking environments

Transparency
 The server might itself be distributed (why?), but should provide a
single "logical" service to the user

Performance
 Client should be customized for interactive display-intensive tasks
 Server should provide CPU-intensive operations

Scalability
 Server has spare capacity to handle larger number of clients

Flexibility
 Should be usable for a variety of user interfaces

Reliability
 System should survive individual node and/or communication link
problems
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Problems with Client/Server Architectures



Layered systems do not provide peer-to-peer communication
Peer-to-peer communication is often needed
Example: Database receives queries from application but also
sends notifications to application when data have changed
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Peer-to-Peer Architecture



Generalization of Client/Server Architecture
Clients can be servers and servers can be clients
More difficult because of possibility of deadlocks
requester
Peer
*
service1()
service2()
…
serviceN()
*
provider
a p p l i c a t i o n 1 : D1
B.
U su
ep
rd a t e D a t a
database:DBMS
a p p l i c a t i o n 2 :2D.B Ucshearn g e N o t i f i c a t i o n
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Model/View/Controller

Subsystems are classified into 3 different types
 Model subsystem: Responsible for application domain knowledge
 View subsystem: Responsible for displaying application domain objects
to the user
 Controller subsystem: Responsible for sequence of interactions with
the user and notifying views of changes in the model.

MVC is a special case of a repository architecture:
 Model subsystem implements the central datastructure, the
Controller subsystem explicitly dictate the control flow
Controller
initiator
1
*
repository
Model
1
View
notifier
subscriber
*
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Example of a File System based on MVC
Architecture
Bernd Bruegge & Allen Dutoit
Object-Oriented Software Engineering: Conquering Complex and Changing Systems
51
Sequence of Events
2.User types new filename
:Controller
3. Request name change in model
1. Views subscribe to event
:Model
5. Updated views
4. Notify subscribers
:InfoView
:FolderView
Bernd Bruegge & Allen Dutoit
Object-Oriented Software Engineering: Conquering Complex and Changing Systems
52
* input
output 1
Filter
Pipe
* output
input 1
Figure 6-22. Pipe and filter architecture (UML class diagram). A Filter can have many
inputs and outputs. A Pipe connects one of the outputs of a Filter to one of the inputs of
another Filter.
Bernd Bruegge & Allen Dutoit
Object-Oriented Software Engineering: Conquering Complex and Changing Systems
53
% ps auxwww | grep dutoit | sort | more
dutoit
dutoit
dutoit
19737
19858
19859
ps
0.2
0.2
0.2
1.6 1908 1500 pts/6
0.7 816 580 pts/6
0.6 812 540 pts/6
grep
O 15:24:36
S 15:38:46
O 15:38:47
sort
0:00 -tcsh
0:00 grep dutoit
0:00 sort
more
Figure 6-23. An instance of the pipe and filter architecture (Unix command and UML activity
diagram).
Bernd Bruegge & Allen Dutoit
Object-Oriented Software Engineering: Conquering Complex and Changing Systems
54
Summary

System Design
 Reduces the gap between requirements and the machine
 Decomposes the overall system into manageable parts

Design Goals Definition
 Describes and prioritizes the qualities that are important for the
system
 Defines the value system against which options are evaluated

Subsystem Decomposition
 Results into a set of loosely dependent parts which make up the
system
Bernd Bruegge & Allen Dutoit
Object-Oriented Software Engineering: Conquering Complex and Changing Systems
55