Formal SpecificationObjectivesTopics coveredFormal methodsAcceptance of formal methodsUse of formal methodsSpecification in the software processSpecification and designSlide 9Use of formal specificationCost profileDevelopment costs with formal specificationSpecification techniquesFormal specification languagesInterface specificationSub-system interfacesThe structure of an algebraic specificationSpecification componentsSystematic algebraic specificationSpecification operationsOperations on a list ADTList specificationRecursion in specificationsInterface specification in critical systemsA sector objectPrimitive operationsSector specification (1)Sector specification (2)Specification commentaryBehavioural specificationThe structure of a Z schemaModelling the insulin pumpSchema invariantInsulin pump schemaState invariantsThe dosage computationRUN schema (1)RUN schema (2)Sugar OK schemaKey pointsSlide 41©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 10 Slide 1Formal Specification©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 10 Slide 2ObjectivesTo explain why formal specification techniques help discover problems in system requirementsTo describe the use of algebraic techniques for interface specificationTo describe the use of model-based techniques for behavioural specification©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 10 Slide 3Topics coveredFormal specification in the software processSub-system interface specificationBehavioural specification©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 10 Slide 4Formal 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.©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 10 Slide 5Acceptance 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.©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 10 Slide 6Use 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.©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 10 Slide 7Specification 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.©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 10 Slide 8Specification and designIncreasing contractor involvementDecreasing client involvementSpecificationDesignUserrequirementsdefinitionSystemrequirementsspecificationArchitecturaldesignFormalspecificationHigh-leveldesign©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 10 Slide 9Specification in the software processSystemrequirementsspecificationFormalspecificationHigh-leveldesignUserrequirementsdefinitionSystemmodellingArchitecturaldesign©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 10 Slide 10Use 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.©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 10 Slide 11Cost 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.©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 10 Slide 12Development costs with formal specificationSpecificationSpecificationDesign andimplementationDesign andimplementationValidationValidationCost©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 10 Slide 13Specification 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.©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 10 Slide 14Formal specification languages©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 10 Slide 15Interface specificationLarge systems are decomposed into subsystems with well-defined interfaces between these subsystems.Specification of subsystem interfaces allows independent development of the different subsystems.Interfaces may be defined as abstract data types or object classes.The algebraic approach to formal specification is particularly well-suited to interface specification as it is focused on the defined operations in an object.©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 10 Slide 16Sub-system