with Design Reuse
Building software from reusable components.
Computer Science
CS425/CS625
8/23/2001
1
Objectives
To explain the benefits of software reuse and some reuse problems To describe different types of reusable component and processes for reuse To introduce application families as a route to reuse To describe design patterns as highlevel abstractions that promote reuse
Computer Science
CS425/CS625
8/23/2001
2
Topics covered
Component-based development Application families Design patterns
Computer Science
CS425/CS625
8/23/2001
3
Software reuse
In most engineering disciplines, systems are designed by composing existing components that have been used in other systems Software engineering has been more focused on original development but it is now recognised that to achieve better software, more quickly and at lower cost, we need to adopt a design process that is based on systematic reuse
Computer Science
CS425/CS625
8/23/2001
4
Reuse-based software engineering
Application system reuse
The whole of an application system may be reused either by incorporating it without change into other systems (COTS reuse) or by developing application families
Component reuse
Components of an application from sub-systems to single objects may be reused
Function reuse
Software components that implement a single welldefined function may be reused
Computer Science
CS425/CS625
8/23/2001
5
Reuse practice
Application system reuse
Widely practised as software systems are implemented as application families. COTS reuse is becoming increasingly common
Component reuse
Now seen as the key to effective and widespread reuse through component-based software engineering. However, it is still relatively immature
Function reuse
Common in some application domains (e.g. engineering) where domain-specific libraries of reusable functions have been established
Computer Science
CS425/CS625
8/23/2001
6
Benefits of reuse
Increased reliability
Components exercised in working systems
Reduced process risk
Less uncertainty in development costs
Effective use of specialists
Reuse components instead of people
Standards compliance
Embed standards in reusable components
Accelerated development
Avoid original development and hence speed-up production
Computer Science
CS425/CS625
8/23/2001
7
Requirements for design with reuse
It must be possible to find appropriate reusable components The re-user of the component must be confident that the components will be reliable and will behave as specified The components must be documented so that they can be understood and, where appropriate, modified
Computer Science
CS425/CS625
8/23/2001
8
Reuse problems
Increased maintenance costs Lack of tool support Not-invented-here syndrome Maintaining a component library Finding and adapting reusable components
Computer Science
CS425/CS625
8/23/2001
9
Generator-based reuse
Program generators involve the reuse of standard patterns and algorithms These are embedded in the generator and parameterised by user commands. A program is then automatically generated Generator-based reuse is possible when domain abstractions and their mapping to executable code can be identified A domain specific language is used to compose and control these abstractions
Computer Science
CS425/CS625
8/23/2001
10
Types of program generator
Types of program generator
Application generators for business data processing Parser and lexical analyser generators for language processing Code generators in CASE tools
Generator-based reuse is very cost-effective but its applicability is limited to a relatively small number of application domains It is easier for end-users to develop programs using generators compared to other component-based approaches to reuse CS425/CS625 8/23/2001 11
Computer Science
Reuse through program generation
A p c ti n p li a o d sc i t n e rpio Po r m e e a r r ga g n r to Gn r te po r m e e a d r ga
A p c ti nd min p li a o o a k o le g nw d e
Dta a a b se
Computer Science
CS425/CS625
8/23/2001
12
Component-based development
Component-based software engineering (CBSE) is an approach to software development that relies on reuse It emerged from the failure of object-oriented development to support effective reuse. Single object classes are too detailed and specific Components are more abstract than object classes and can be considered to be standalone service providers
Computer Science
CS425/CS625
8/23/2001
13
Components
Components provide a service without regard to where the component is executing or its programming language
A component is an independent executable entity that can be made up of one or more executable objects The component interface is published and all interactions are through the published interface
Components can range in size from simple functions to entire application systems
Computer Science
CS425/CS625
8/23/2001
14
Component interfaces
R uires in eq terface
C p en om on t
Prov es interface id
Computer Science
CS425/CS625
8/23/2001
15
Component interfaces
Provides interface
Defines the services that are provided by the component to other components
Requires interface
Defines the services that specifies what services must be made available for the component to execute as specified
Computer Science
CS425/CS625
8/23/2001
16
Printing services component
Requires interface PrintService Print GetQueue PrinterInt Remove Transfer Register Unregister Provides interface
GetPDfile
Computer Science
CS425/CS625
8/23/2001
17
Component abstractions
Functional abstraction
The component implements a single function such as a mathematical function
Casual groupings
The component is a collection of loosely related entities that might be data declarations, functions, etc.
Data abstractions
The component represents a data abstraction or class in an object-oriented language
Cluster abstractions
8/23/2001 18 The component isCS425/CS625 of related classes that a group Computer Science work together
CBSE processes
Component-based development can be integrated into a standard software process by incorporating a reuse activity in the process However, in reuse-driven development, the system requirements are modified to reflect the components that are available CBSE usually involves a prototyping or an incremental development process with components being `glued together' using a scripting language
Computer Science
CS425/CS625
8/23/2001
19
An opportunistic reuse process
D esign system aachitecture
Spec ify components
Search for reusable components
Incorpora te disc overed components
Computer Science
CS425/CS625
8/23/2001
20
Development with reuse
O utline system requirements Search for reusable components Modify requirements according to disc overed components
A rchitectural de sign
Search for reusable components
Spec system ify components ba on reusable sed components
Computer Science
CS425/CS625
8/23/2001
21
CBSE problems
Component incompatibilities may mean that cost and schedule savings are less then expected Finding and understanding components Managing evolution as requirements change in situations where it may be impossible to change the system components
Computer Science
CS425/CS625
8/23/2001
22
Application frameworks
Frameworks are a sub-system design made up of a collection of abstract and concrete classes and the interfaces between them The sub-system is implemented by adding components to fill in parts of the design and by instantiating the abstract classes in the framework Frameworks are moderately large entities that can be reused
Computer Science
CS425/CS625
8/23/2001
23
Framework classes
System infrastructure frameworks
Support the development of system infrastructures such as communications, user interfaces and compilers
Middleware integration frameworks
Standards and classes that support component communication and information exchange
Enterprise application frameworks
Support the development of specific types of application such as telecommunications or financial systems
Computer Science
CS425/CS625
8/23/2001
24
Extending frameworks
Frameworks are generic and are extended to create a more specific application or subsystem Extending the framework involves
Adding concrete classes that inherit operations from abstract classes in the framework Adding methods that are called in response to events that are recognised by framework
the Problem with frameworks is their complexity and the time it takes to use them effectively
Computer Science
CS425/CS625
8/23/2001
25
Model-view controller
System infrastructure framework for GUI design Allows for multiple presentations of an object and separate interactions with these presentations MVC framework involves the instantiation of a number of patterns (discussed later)
Computer Science
CS425/CS625
8/23/2001
26
Model-view controller
V w state ie V w m od ie eth s view m dificatio o n m essag es C tro state on ller U in ts ser pu C tro m od on ller eth s
M el q od ueries an up d dates
M el ed od its M el state od
M el m ds od etho
Computer Science
CS425/CS625
8/23/2001
27
COTS product reuse
COTS - Commercial Off-The-Shelf systems COTS systems are usually complete application systems that offer an API (Application Programming Interface) Building large systems by integrating COTS systems is now a viable development strategy for some types of system such as E-commerce systems
Computer Science
CS425/CS625
8/23/2001
28
COTS system integration problems
Lack of control over functionality and performance
COTS systems may be less effective than they appear
Problems with COTS system inter-operability
Different COTS systems may make different assumptions that means integration is difficult
No control over system evolution
COTS vendors not system users control evolution
Support from COTS vendors
COTS vendors may not offer support over the lifetime of the product CS425/CS625 8/23/2001 29 Computer Science
Component development for reuse
Components for reuse may be specially constructed by generalising existing components Component reusability Should reflect stable domain abstractions Should hide state representation Should be as independent as possible Should publish exceptions through the component interface There is a trade-off between reusability and usability. The more general the interface, the greater the reusability but it is then more complex and hence less usable
Computer Science
CS425/CS625
8/23/2001
30
Reusable components
The development cost of reusable components is higher than the cost of specific equivalents. This extra reusability enhancement cost should be an organization rather than a project cost Generic components may be less space-efficient and may have longer execution times than their specific equivalents
Computer Science
CS425/CS625
8/23/2001
31
Reusability enhancement
Name generalisation
Names in a component may be modified so that they are not a direct reflection of a specific application entity
Operation generalisation
Operations may be added to provide extra functionality and application specific operations may be removed
Exception generalisation
Application specific exceptions are removed and exception management added to increase the robustness of the component
Component certification
Computer Science
Component is certified as reusable
CS425/CS625
8/23/2001
32
Reusability enhancement process
In itial co po t m nen
R sab eu le com nen po t
N e am g aliza ener tion
O peration g eneraliza n tio
E xcep tion g eneraliza n tio
Com en pon t certification
Computer Science
CS425/CS625
8/23/2001
33
Application families
An application family or product line is a related set of applications that has a common, domain-specific architecture The common core of the application family is reused each time a new application is required Each specific application is specialised in some way
Computer Science
CS425/CS625
8/23/2001
34
Application family specialisation
Platform specialisation
Different versions of the application are developed for different platforms
Configuration specialisation
Different versions of the application are created to handle different peripheral devices
Functional specialisation
Different versions of the application are created for customers with different requirements
Computer Science
CS425/CS625
8/23/2001
35
A resource management system
User access Program access
Add
Delete
Query
Browse
Admin
Report
Resource desc.
Screen spec.
Report spec.
Resource database
Computer Science
CS425/CS625
8/23/2001
36
Inventory management systems
Resource database
Maintains details of the things that are being managed
I/O descriptions
Describes the structures in the resource database and input and output formats that are used
Query level
Provides functions implementing queries over the resources
Access interfaces
A user interface and an application programming interface CS425/CS625 8/23/2001 37
Computer Science
Application family architectures
Architectures must be structured in such a way as to separate different subsystems and to allow them to be modified The architecture should also separate entities and their descriptions and the higher levels in the system access entities through descriptions rather than directly
Computer Science
CS425/CS625
8/23/2001
38
A library system
Library user access
Add
Delete
Query
Browse
Admin
Report
Issue
Return
User...