Software Models (Cont.)

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- Component-based software engineering - Formal Development Model

Objectives

• To explain that CBSE is concerned with developing standardised components and composing these into applications

• To describe components and component models

• To show the principal activities in the CBSE process

• To discuss approaches to component composition and problems that may arise

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Topics covered

• Components and component models• The CBSE process• Component composition

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Component-based development

• Component-based software engineering (CBSE) is an approach to software development that relies on software 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 stand-alone service providers.

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CBSE essentials

• Independent components specified by their interfaces.

• Component standards to facilitate component integration.

• Middleware that provides support for component inter-operability.

• A development process that is geared to reuse.

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CBSE and design principles

• Apart from the benefits of reuse, CBSE is based on sound software engineering design principles:– Components are independent so do not interfere

with each other;– Component implementations are hidden;– Communication is through well-defined

interfaces;– Component platforms are shared and reduce

development costs.

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CBSE problems

• Component trustworthiness - how can a component with no available source code be trusted?

• Component certification - who will certify the quality of components?

• Emergent property prediction - how can the emergent properties of component compositions be predicted?

• Requirements trade-offs - how do we do trade-off analysis between the features of one component and another?

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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;

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Component definitions

• Councill and Heinmann:– A software component is a software element that conforms to a

component model and can be independently deployed and composed without modification according to a composition standard.

• Szyperski:– A software component is a unit of composition with contractually

specified interfaces and explicit context dependencies only. A software component can be deployed independently and is subject to composition by third-parties.

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Component as a service provider

• The component is an independent, executable entity. It does not have to be compiled before it is used with other components.

• The services offered by a component are made available through an interface and all component interactions take place through that interface.

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Component characteristics 1

Standardised Component standardisation means that a component that isused in a CBSE process has to conform to some standardisedcomponent model. This model may define componentinterfaces, component meta-data, documentation, compositionand deployment.

Independent A component should be independent Ğ it should be possible tocompose and deploy it without having to use other specificcomponents. In situations where the component needsexternally provided services, these should be explicitly set outin a ÔrequiresÕ interface specification.

Composable For a component to be composable, all external interactionsmust take place through publicly defined interfaces. Inaddition, it must provide external access to information aboutitself such as its methods and attributes.

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Component characteristics 2

Deployable To be deployable, a component has to be self-contained andmust be able to operate as a stand-alone entity on somecomponent platform that implements the component model.This usually means that the component is a binary componentthat does not have to be compiled before it is deployed.

Documented Components have to be fully documented so that potentialusers of the component can decide whether or not they meettheir needs. The syntax and, ideally, the semantics of allcomponent interfaces have to be specified.

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Formal Specification

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Objectives

• To explain why formal specification techniques help discover problems in system requirements

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Topics covered

• Formal specification in the software process

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Formal methods

• Formal specification is part of a more general collection of techniques that are known as ‘formal methods’.

• These are all based on mathematical representation and analysis of software.

• Formal methods include– Formal specification;– Specification analysis and proof;– Transformational development;– Program verification.

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Acceptance of formal methods• Formal methods have not become mainstream software

development techniques as was once predicted– Other software engineering techniques have been successful at

increasing system quality. Hence the need for formal methods has been reduced;

– Market changes have made time-to-market rather than software with a low error count the key factor. Formal methods do not reduce time to market;

– The scope of formal methods is limited. They are not well-suited to specifying and analysing user interfaces and user interaction;

– Formal methods are still hard to scale up to large systems.

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Use of formal methods

• The principal benefits of formal methods are in reducing the number of faults in systems.

• Consequently, their main area of applicability is in critical systems engineering. There have been several successful projects where formal methods have been used in this area.

• In this area, the use of formal methods is most likely to be cost-effective because high system failure costs must be avoided.

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Specification in the software process

• Specification and design are inextricably intermingled.

• Architectural design is essential to structure a specification and the specification process.

• Formal specifications are expressed in a mathematical notation with precisely defined vocabulary, syntax and semantics.

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Specification and design

Increasing contractor involvement

Decreasing client involvement

Specification

Design

Userrequirements

definition

Systemrequirementsspecification

Architecturaldesign

Formalspecification

High-leveldesign

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Specification in the software process

Systemrequirementsspecification

Formalspecification

High-leveldesign

Userrequirements

definition

Systemmodelling

Architecturaldesign

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Use of formal specification

• Formal specification involves investing more effort in the early phases of software development.

• This reduces requirements errors as it forces a detailed analysis of the requirements.

• Incompleteness and inconsistencies can be discovered and resolved.

• Hence, savings as made as the amount of rework due to requirements problems is reduced.

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Cost profile

• The use of formal specification means that the cost profile of a project changes– There are greater up front costs as more time and

effort are spent developing the specification;– However, implementation and validation costs

should be reduced as the specification process reduces errors and ambiguities in the requirements.

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Development costs with formal specification

Specification

Specification

Design andimplementation

Design andimplementation

Validation

Validation

Cost

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Specification techniques

• Algebraic specification– The system is specified in terms of its operations

and their relationships.

• Model-based specification– The system is specified in terms of a state model

that is constructed using mathematical constructs such as sets and sequences. Operations are defined by modifications to the system’s state.

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Formal specification languages

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