Software Reuse

Indu Sharma

HOD(CSE)CPTC,Rajsamand

Introduction Today, complex, high quality computer-

based-systems must be built in a very short time periods. This results in a organized approach to reuse.

Component-Based-Software-Engineering (CBSE) is a process that emphasizes the design and construction of computer-based systems using reusable components.

Types of Software Reuse Software component reuse, does not just mean

the reuse of code. It also includes the reuse of specifications and designs.

Types of Software reuse: Code components: functions, modules,

subsystems etc. Abstract Products: specifications, designs etc

The potential gains from reusing specifications may be greater than reusing code component. Code contains low-level detail which may specialize it to such extent that it cannot be reused. Designs or specifications are more abstract and hence more widely applicable.

Different levels of Reuse The reuse of software can be

considered at a no. of different levels:

i. Application system reuse: The whole of an application system may be reused. For this the software must be portable i.e. it should execute on different platforms.

ii. Sub-system reuse: Major sub-systems of an application may be reuse.

Different levels of Reuseiii. Module or object reuse: Components of a

system representing a collection of functions may be reuse. For eg., a C++ object implementing a binary tree may be reused in different applications.

iv. Function reuse: Software components which implement a single function, such as mathematical functions, may be reused.

Advantages of Reuse A systematic reuse in the development

process leads to the following advantages:i. System reliability is increased: These

components have been tested in operational systems and have therefore been exposed to realistic operating conditions.

ii. Overall process risk is reduced: Less uncertainty in the costs of reusing that component than in the costs of development. It reduces uncertainties in project cost estimation.

Advantages of Reuseiii. Effective use can be made of

specialists: Instead of application specialists doing the same work on different projects, these specialists can develop reusable components which encapsulate their knowledge.

iv. Standard Compliance: Organizational standards can be embodied in reusable components. Eg. user interfaces.

Advantages of Reusev. Software development time can be

reduced: Reusing components speeds up system production because both development and validation time should be reduced.

Design System Architecture

Specify Components

Search for reusable components

Incorporate discovered components

How reuse can be incorporated into the system development process?Fig: Reuse in a standard development

process

The system designer completes a high-level design and specifications of the components of that design. These specifications are used to find components to reuse. These may be incorporated at the architectural level or at more detailed design levels.

Outline System Architecture

Search for reusable components

Modify requirements according to discovered components

Architectural design

How reuse can be incorporated into the system development process?Fig: Reuse driven development process

Search for reusable components

Specify system components based on reusable components

How reuse can be incorporated into the system development process? In second case reusability drives the

design process. Rather than design then search for

reusable components, engineers first search for reusable components. They base their design on these components

Disadvantages of second approach: Requirements compromise The design may be less efficient than a

special purpose design.

Conditions for software development with reuse There are three conditions for software

development with reuse:i. It must be possible to find appropriate

reusable components. Organizations need a base of properly catalogued and documented reusable components.

ii. The reuser of the components must have confidence that the components will behave as specified and will be reliable. All components in an organization’s catalogue should be certified that they have reached some quality standards.

Conditions for software development with reuse

iii. The components must have associated documentation to help the reuser understand them and adapt them to a new application. The documentation should include information about where components have been reused and any reuse problems which have been found.

Difficulties in development with reuse The reuse costs may sometimes be greater

than the cost of re-implementing the component.

Some s/w engineers sometimes prefer to rewrite components as they believe that they can improve on the reusable components.

Maintaining a component library and ensuring that s/w developers can use this library can be expensive. Our current techniques for classifying, cataloguing, and retrieving s/w components are immature.

Difficulties in development with reuse

Finding, understanding, and adapting reusable components: Software components have to be discovered in a library, understand, and sometimes adapted to work in a new environment. Engineers must be reasonably confident of finding a component in library before they will make include a component search as part of their normal development process.

CASE tools do not support development with reuse. It may be difficult or impossible to integrate these tools with a component library system.

Reusability Enhancement

Initial component

Reusable component

Name Generaliz-ation

Operation Generaliz-ation

Exception Generaliz-ation

Component Certification

Fig: The process of reusability enhancement

Reusability Enhancement Adapting a component to make it

reusable may involve making difference types of changes:

i. Name Generalization: The names used in the component may be modified so that they are neutral rather than direct reflection of some specific application entity.

Reusability Enhancementii. Operation Generalization: Involve

adding operations to a component or removing operations which are very specific to some application domain.

iii. Exception Generalization: This may involve checking each component to see which exceptions it might generate and including these operations in the component interface.

Reusability Enhancement After the generalization the quality

of the generalized components should be checked. It requires program inspections or testing. The component may be certified as having reached the required quality standards.