Transcript Lecture 1 for Chapter 6, System Design - ICAR-CNR

Conquering Complex and Changing Systems
Object-Oriented Software Engineering
Chapter 6,
System Design
Lecture 1
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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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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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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System Design Phases
1)
Design Goals
2)
System Decomposition
3)
Concurrency
4)
Hardware/Software Mapping
5)
Data Management
6)
Global Resource Handling
7)
Software Control
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
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Developer/
Maintainer
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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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System Design Phases
1)
Design Goals
2)
System Decomposition
3)
Concurrency
4)
Hardware/Software Mapping
5)
Data Management
6)
Global Resource Handling
7)
Software Control
8)
Boundary Conditions
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Section 2. System Decomposition

Subsystem (UML: Package)
 Collection of classes, associations, operations, events and constraints
that are interrelated

Service:
 A set of operations provided by the subsystem that share a common
purpose

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 be 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 (devoted to deal with
communications between the FieldOfficer and the Dispatcher):




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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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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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 (discussed later):
 How can we organize the subsystems?

Layer/partitions?
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Decision tracking system
The decision tracking system purpose is to record design problems, discussions,
alternative evaluations, decisions, and their implementations in terms of tasks
DecisionSubsystem
assesses
Criterion
Alternative
*
*
solvableBy
DesignProblem
based-on
resolvedBy
SubTask
*
ActionItem
*
Decision
Task
implementedBy
subtasks
The DecisionSubsystem has a low coherence: The classes Criterion, Alternative, and
DesignProblem have no relationships with Subtask, ActionItem, and Task.
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Alternative subsystem decomposition for the decision tracking system
RationaleSubsystem
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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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
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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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Subsystem Decomposition into Layers
Layer 1
A: Subsystem
C:Subsystem
B:Subsystem
E:Subsystem


F:Subsystem
D:Subsystem
Layer 2
G:Subsystem
Layer 3
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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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
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opr
C1
attr
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VM1
C1
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opr
VM3
VM4
Existing System
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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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Closed Architecture (Opaque Layering)


A virtual machine can only
call operations from the layer
below
Design goal: High
maintainability
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VM2
VM3
VM4
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Open Architecture (Transparent Layering)


A virtual machine can call
operations from any layers
below
Design goal: Runtime
efficiency
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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.
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Software Architectures





Client/Server Architecture
Peer-To-Peer Architecture
Repository Architecture
Model/View/Controller
Pipes and Filters Architecture
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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
*
requester
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*
provider
service1()
service2()
…
serviceN()
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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 on the
data invoke peripheral systems) or by the subsystems (locks
imposed by subsystems in the repository).
Repository
Subsystem
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createData()
setData()
getData()
searchData()
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Peer-to-Peer Architecture



Generalization of Client/Server Architecture
Clients can be servers and servers can be clients
Control Flow design is more difficult because of possibility of
deadlocks
requester
Peer
service1()
service2()
…
serviceN()
application1:DBUser
*
*
provider
1. updateData
database:DBMS
application2:DBUser
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2. changeNotification
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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 data structure, 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
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Sequence of Events for the MVC architecture
example
2.User types new filename
3. Request name change in model
:Controller
1. Views subscribe to event
5. Updated
views
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:Model
4. Notify subscribers
:InfoView
:FolderView
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Pipe and Filter Architecture



Subsystems process data received from a set of inputs and send
the results to other subsystems via a set of outputs
Subsystems are called filters
Associations between the subsystems are called pipes

Each filter is executed concurrently and synchronization is
done via the pipes

Filters can be substituded for others or reconfigured to achieve
a different purpose
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An instance of the pipe and filter architecture
(Unix command and UML activity diagram).
% 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
ps – process status
grep – search for a pattern
sort – sort input data
more – displays data one screen at a time
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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
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