©ian sommerville 2006software engineering, 8th edition. chapter 18 slide 1 software reuse

©Ian Sommerville 2006 Software Engineering, 8th edition. Chapter 18 Slide 1

Software Reuse


Objectives

l To explain the benefits of software reuse and some reuse problems

l To discuss several different ways to implement software reuse

l To explain how reusable concepts can be represented as patterns or embedded in program generators

l To discuss COTS reusel To describe the development of software

product lines


Topics covered

l The reuse landscapel Design patternsl Generator based reusel Application frameworksl Application system reuse


Software reuse

l In most engineering disciplines, systems are designed by composing existing components that have been used in other systems.

l 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 software reuse.


Reuse-based software engineering

l 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.

l Component reuse• Components of an application from sub-systems to

single objects may be reused. Covered in Chapter 19.l Object and function reuse

• Software components that implement a single well-defined object or function may be reused.


Reuse benefits 1

Increased dependability Reused software, that has been tried and tested in working systems,should be m ore dependable than new software. The initial use of thesoftware reveals any design and implementation faults. These are thenfixed, thus reducing the number of failures when the software is reused.

Reduced process risk If software exists, there is less uncertainty in the costs of reusing thatsoftware than in the costs of development. This is an important factorfor project management as it reduces the margin of error in project costestimation. This is particularly true when relatively large softwarecomponents such as sub-systems are reused.

Effective use of specialists Instead of application specialists doing the same work on differentprojects, these specialists can develop reusable software thatencapsulate their knowledge.


Reuse benefits 2

Standards compliance Some standards, such as user interface standards, can beimplemented as a set of standard reusable components. Forexample, if menus in a user interfaces are implemented usingreusable components, all applications present the same menuformats to users. The use of standard user interfaces improvesdependability as users are less likely to make mistakes whenpresented with a familiar interface.

Accelerated development Bringing a system to market as early as possible is o ften moreimportant than overall development costs. Reusing software canspeed up system production because both development andvalidation time should be reduced.


Reuse problems 1

Increased maintenancecosts

If the source code of a reused software system or component is notavailable then maintenance costs may be increased as the reusedelements of the system may become increasingly incompatible withsystem changes.

Lack of tool support CASE toolsets may not support development with reuse. It may bedifficult or impossible to integrate these tools with a componentlibrary system. The software process assumed by these tools may nottake reuse into account.

Not-invented-heresyndrome

Some software engineers sometimes prefer to re-write components asthey believe that they can improve on the reusable component. This ispartly to do with trust and partly to do with the fact that writingoriginal software is s een as more challenging than reusing otherpeopleÕs software.


Reuse problems 2

Creating and maintaining acomponent library

Populating a reusable component library and ensuring the softwaredevelopers can use this library can be expensive. Our current techniquesfor classifying, cataloguing and retrieving software components areimmature.

Finding, understanding andadapting reusable components

Software components have to be discovered in a library, understood and,sometimes, adapted to work in a n ew environment. Engineers must bereasonably confident of finding a component in the library before they willmake routinely include a component search as part of their normaldevelopment process.


The reuse landscape

l Although reuse is often simply thought of as the reuse of system components, there are many different approaches to reuse that may be used.

l Reuse is possible at a range of levels from simple functions to complete application systems.

l The reuse landscape covers the range of possible reuse techniques.


The reuse landscape

Designpatterns

Component-baseddevelopment

Componentframeworks

Service-orientedsystems

COTSintegration

Applicationproduct lines

Legacy systemwrapping

Programlibraries

Programgenerators

Aspect-orientedsoftware development

Configurable verticalapplications


Reuse approaches 1

Design patterns Generic abstractions that occur across applications arerepresented as design patterns that show abstract and concreteobjects and interactions.

Component-baseddevelopment

Systems are developed by integrating components(collections of objects) that conform to component-modelstandards. This is covered in Chapter 19.

Applicationframeworks

Collections of abstract and concrete classes that can beadapted and extended to create application systems.

Legacy systemwrapping

Legacy systems (see Chapter 2) that can be ŌwrappedÕ bydefining a set of interfaces and providing access to theselegacy systems through these interfaces.

Service-orientedsystems

Systems are developed by linking shared services that may beexternally provided.


Reuse approaches 2

Application productlines

An application type is generalised around a commonarchitecture so that it can be adapted in different ways fordifferent customers.

COTS integration Systems are developed by integrating existing applicationsystems.

Configurable verticalapplications

A generic system is designed so that it can be configured tothe needs of specific system customers.

Program libraries Class and function libraries implementing commonly-usedabstractions are available for reuse.

Program generators A generator system embeds knowledge of a particular typesof application and can generate systems or system fragmentsin that domain.

Aspect-orientedsoftware development

Shared components are woven into an application at differentplaces when the program is compiled.


Reuse planning factors

l The development schedule for the software.• Off-the-shelf

l The expected software lifetime.• Maintainability , Source code

l The background, skills and experience of the development team.

• Reuse technologies are complex and time consuming

l The criticality of the software and its non-functional requirements.

• Source code

l The application domain.l The execution platform for the software.


Concept reuse

l When you reuse program or design components, you have to follow the design decisions made by the original developer of the component.

l This may limit the opportunities for reuse.l However, a more abstract form of reuse is concept

reuse when a particular approach is described in an implementation independent way and an implementation is then developed.

l The two main approaches to concept reuse are:• Design patterns;• Generative programming.


Design patterns

l A design pattern is a way of reusing abstract knowledge about a problem and its solution.

l A pattern is a description of the problem and the essence of its solution.

l It should be sufficiently abstract to be reused in different settings.

l Patterns often rely on object characteristics such as inheritance and polymorphism.


Pattern elements

l Name• A meaningful pattern identifier.

l Problem description.l Solution description.

• Not a concrete design but a template for a design solution that can be instantiated in different ways.

l Consequences• The results and trade-offs of applying the pattern.


Multiple displays

A: 40B: 25C: 15D: 20

Observer 1

A

B

C

D

Observer 2

Subject

0

50

25

A B C D


The Observer pattern

l Name• Observer.

l Description• Separates the display of object state from the object itself.

l Problem description• Used when multiple displays of state are needed.

l Solution description• See slide with UML description.

l Consequences• Optimisations to enhance display performance are impractical.


The Observer pattern

Subject Observer

Attach (Observer)Detach (Observer)Notify ()

Update ()

ConcreteSubject

GetState ()

subjectState

ConcreteObserver

Update ()

observerState

observerState = subject -> GetState ()return subjectState

for all o in observers o -> Update ()


Generator-based reuse

l Program generators involve the reuse of standard patterns and algorithms.

l These are embedded in the generator and parameterised by user commands. A program is then automatically generated.

l Generator-based reuse is possible when domain abstractions and their mapping to executable code can be identified.

l A domain specific language is used to compose and control these abstractions.


Types of program generator

l Types of program generator• Application generators for business data processing;• Parser and lexical analyser generators for language

processing;• Code generators in CASE tools.

l Generator-based reuse is very cost-effective but its applicability is limited to a relatively small number of application domains.

l It is easier for end-users to develop programs using generators compared to other component-based approaches to reuse.


Reuse through program generation

Program generator Generated programApplicationdescription

Application domainknowledge

Database


Aspect-oriented development

l Aspect-oriented development addresses a major software engineering problem - the separation of concerns.

l Concerns are often not simply associated with application functionality but are cross-cutting - e.g. all components may monitor their own operation, all components may have to maintain security, etc.

l Cross-cutting concerns are implemented as aspects and are dynamically woven into a program. The concern code is reuse and the new system is generated by the aspect weaver.


Aspect-oriented development

Generated code

Aspect 1 Aspect 2

AspectWeaver

<statements 1>Aspect 1

<statements 2>Aspect 2

<statements 3>

Input source code

<statements 1>join point 1

<statements 2>join point 2

<statements 3>


Application frameworks

l Frameworks are a sub-system design made up of a collection of abstract and concrete classes and the interfaces between them.

l The sub-system is implemented by adding components to fill in parts of the design and by instantiating the abstract classes in the framework.

l Frameworks are moderately large entities that can be reused.


Framework classes

l System infrastructure frameworks• Support the development of system

infrastructures such as communications, user interfaces and compilers.

l Middleware integration frameworks• Standards and classes that support component

communication and information exchange.l Enterprise application frameworks

• Support the development of specific types of application such as telecommunications or financial systems.


Extending frameworks

l Frameworks are generic and are extended to create a more specific application or sub-system.

l 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 the framework.

l Problem with frameworks is their complexity which means that it takes a long time to use them effectively.


Model-view controller

l System infrastructure framework for GUI design.

l Allows for multiple presentations of an object and separate interactions with these presentations.

l MVC framework involves the instantiation of a number of patterns (as discussed earlier under concept reuse).


Model-view-controller

Model methods

Controller methods View methods

Userinputs

view modificationmessages

Model edits

Model queriesand updates

Controller state View state

Model state



l Model - The model represents enterprise data and the business rules that govern access to and updates of this data. Often the model serves as a software approximation to a real-world process, so simple real-world modeling techniques apply when defining the model.

l View -The view renders the contents of a model. It accesses enterprise data through the model and specifies how that data should be presented. It is the view's responsibility to maintain consistency in its presentation when the model changes. This can be achieved by using a push model, where the view registers itself with the model for change notifications, or a pull model, where the view is responsible for calling the model when it needs to retrieve the most current data.

l Controller - The controller translates interactions with the view into actions to be performed by the model. In a stand-alone GUI client, user interactions could be button clicks or menu selections, whereas in a Web application, they appear as GET and POST HTTP requests. The actions performed by the model include activating business processes or changing the state of the model. Based on the user interactions and the outcome of the model actions, the controller responds by selecting an appropriate view.


Application system reuse

l Involves the reuse of entire application systems either by configuring a system for an environment or by integrating two or more systems to create a new application.

l Two approaches covered here:• COTS product integration;• Product line development.


COTS product reuse

l COTS - Commercial Off-The-Shelf systems.l COTS systems are usually complete application

systems that offer an API (Application Programming Interface).

l Building large systems by integrating COTS systems is now a viable development strategy for some types of system such as E-commerce systems.

l The key benefit is faster application development and, usually, lower development costs.


COTS design choices

l Which COTS products offer the most appropriate functionality?• There may be several similar products that may be

used.l How will data be exchanged?

• Individual products use their own data structures and formats.

l What features of the product will actually be used?• Most products have more functionality than is

needed. You should try to deny access to unused functionality.


E-procurement system

Client

Web browser E-mail system

Server

E-commercesystem

Ordering andinvoicing system

E-mail system

Adaptor

Adaptor


COTS products reused

l On the client, standard e-mail and web browsing programs are used.

l On the server, an e-commerce platform has to be integrated with an existing ordering system.• This involves writing an adaptor so that they can

exchange data.• An e-mail system is also integrated to generate e-

mail for clients. This also requires an adaptor to receive data from the ordering and invoicing system.


COTS system integration problems

l Lack of control over functionality and performance• COTS systems may be less effective than they appear

l Problems with COTS system inter-operability• Different COTS systems may make different

assumptions that means integration is difficultl No control over system evolution

• COTS vendors not system users control evolutionl Support from COTS vendors

• COTS vendors may not offer support over the lifetime of the product


Software product lines

l Software product lines or application families are applications with generic functionality that can be adapted and configured for use in a specific context.

l Adaptation may involve:• Component and system configuration;

• Adding new components to the system;

• Selecting from a library of existing components;

• Modifying components to meet new requirements.


COTS product specialisation

l Platform specialisation• Different versions of the application are developed for

different platforms.l Environment specialisation

• Different versions of the application are created to handle different operating environments e.g. different types of communication equipment.

l Functional specialisation• Different versions of the application are created for

customers with different requirements.l Process specialisation

• Different versions of the application are created to support different business processes.


COTS configuration

l Deployment time configuration• A generic system is configured by embedding

knowledge of the customer’s requirements and business processes. The software itself is not changed.

l Design time configuration• A common generic code is adapted and

changed according to the requirements of particular customers.


ERP system organisation

Configurationdatabase

System database

Generic ERP system

Configurationplanning tool


ERP systems

l An Enterprise Resource Planning (ERP) system is a generic system that supports common business processes such as ordering and invoicing, manufacturing, etc.

l These are very widely used in large companies - they represent probably the most common form of software reuse.

l The generic core is adapted by including modules and by incorporating knowledge of business processes and rules.


Design time configuration

l Software product lines that are configured at design time are instantiations of generic application architectures as discussed in Chapter 13.

l Generic products usually emerge after experience with specific products.


Product line architectures

l Architectures must be structured in such a way to separate different sub-systems and to allow them to be modified.

l The architecture should also separate entities and their descriptions and the higher levels in the system access entities through descriptions rather than directly.


A resource management system

User interface

Resourcemanagement

Resource policycontrol

Resourceallocation

Userauthentication

Querymanagement

Resource database

Resourcedelivery

Transaction management


Vehicle despatching

l A specialised resource management system where the aim is to allocate resources (vehicles) to handle incidents.• Police ~ fire service

l Adaptations include:• At the UI level, there are components for operator display and

communications;

• At the I/O management level, there are components that handle authentication, reporting and route planning;

• At the resource management level, there are components for vehicle location and despatch, managing vehicle status and incident logging;

• The database includes equipment, vehicle and map databases.


A despatching system

User interface

Vehicle statusmanager

Incidentlogger

Operatorauthentication

Querymanager

Equipmentdatabase

Map and routeplanner

Transaction management

Vehicle database

Incident log

Map database

Vehicledespatcher

Equipmentmanager

Vehiclelocator

Reportgenerator

Comms systeminterface


Product instance development

l How to create a specific version of the system?



Elicitstakeholder

requirements

Chooseclosest-fit

family memberDeliver new

family member

Renegotiaterequirements

Adapt existingsystem



l Elicit stakeholder requirements• Use existing family member as a prototype

l Choose closest-fit family member• Find the family member that best meets the requirements

l Re-negotiate requirements• Adapt requirements as necessary to capabilities of the

softwarel Adapt existing system

• Develop new modules and make changes for family memberl Deliver new family member

• Document key features for further member development


l Advantages of reuse are lower costs, faster software development and lower risks.

l Design patterns are high-level abstractions that document successful design solutions.

l Program generators are also concerned with software reuse - the reusable concepts are embedded in a generator system.

l Application frameworks are collections of concrete and abstract objects that are designed for reuse through specialisation.

Key points


Key points

l COTS product reuse is concerned with the reuse of large, off-the-shelf systems.

l Problems with COTS reuse include lack of control over functionality, performance, and evolution and problems with inter-operation.

l ERP systems are created by configuring a generic system with information about a customer’s business.

l Software product lines are related applications developed around a common core of shared functionality.

©ian sommerville 2006software engineering, 8th edition. chapter 18 slide 1 software reuse

Documents

software reuse slide

reuse landscape slide

benefits of software

systematic software

application system reuse

reuse problems

reuse benefits

reuse approaches