Software Processes
Software EngineeringCOMP 2011COMP201 - Software EngineeringLecturer: Sebastian CoopeAshton Building, Room G.18E-mail: [email protected]
COMP 201 web-page:http://www.csc.liv.ac.uk/~coopes/comp201
Lecture 2 Software Processes
Processes (building a house)COMP201 - Software Engineering2
TasksWhat to build?Where to build?How much money?Get the moneyDesign the build (Detailed plans, timescales etc.)Get permissions?Start with foundationsBuild up from foundationsFully test everythingWhen basic structure complete, make sure it is looks rightShow to customerRe-adjust to customers feedback
COMP201 - Software Engineering3Questions?Does the previous list apply to Software?COMP201 - Software Engineering4Task order (early fix is easy)COMP201 - Software Engineering5
What is a Process ?When we provide a service or create a product we always follow a sequence of steps to accomplish a set of tasksYou do not usually put up the drywall before the wiring for a house is installed orbake a cake before all the ingredients are mixed togetherWe can think of a series of activities as a processDuring this lecture we shall see some examples of software development processes that are used to ensure software is developed in a systematic way using tried and tested techniques
6COMP201 - Software EngineeringWhat is a Process ?Any process has the following characteristicsIt prescribes all of the major activitiesIt uses resources and produces intermediate and final productsIt may include sub-processes and has entry and exit criteriaThe activities are organized in a sequenceConstraints or controls may apply to activities (budget constraints, availability of resources , etc.) 7COMP201 - Software EngineeringProcessBuilding developmentCOMP201 - Software Engineering8SecurefundingDesignhouseGetPlanning permissionSite survey(re)Draw plans(re)Specify buildArchitectPlans/specificationsSoftware Processes Coherent sets of activities for Specifying, Designing, Implementing and Testing software systemsWhen the process involves the building of some product we refer to the process as a life cycleSoftware development process software life cycle9COMP201 - Software EngineeringProcesses and softwareSoftware (unlike buildings/bridges etc.)Can be changed at anytimeIs often required to change often after constructionBenefitsSoftware can be improved almost without limit Leading to problemsSoftware often gets faults as it evolvesSoftware cost is hard to manageProblems with users experience and expectations
COMP201 - Software Engineering10The Software ProcessThe Software Process is a structured set of activities required to develop a software system consisting ofSpecification Design and implementationValidationEvolutionA software process model is an abstract representation of a process It presents a description of a process from some particular perspective11COMP201 - Software EngineeringGeneric Software Process ModelsThe Waterfall Model (classic engineering, example bridge building) Separate and distinct phases of specification and developmentEvolutionary Development (more like product engineering)Specification and development are interleavedFormal Systems Development (example - ASML)A mathematical system model is formally transformed to an implementationReuse-Based DevelopmentThe system is assembled from existing components12COMP201 - Software EngineeringWaterfall Model
13COMP201 - Software EngineeringThe drawback of the waterfall model is the difficulty of accommodating change after the process is underway
Waterfall Model ProblemsInflexible partitioning of the project into distinct stagesThis makes it difficult to respond to changing customer requirementsTherefore, this model is only appropriate when the (final) requirements are well-understood (rare in software)Waterfall model describes a process of stepwise refinementBased on hardware engineering models Widely used in military and aerospace industries14COMP201 - Software EngineeringReality check!Practically no one in industry follows the waterfall method as shown here to produce softwareWhy bother, then?Each stage is an important step in software developmentIts easy to rememberThe sequence is importantSpec. before DesignDesign before coding etc.Many industry practises could do with improvement!
COMP201 - Software Engineering15Why Not a Waterfall But software is different from hardware :No fabrication step Program code is another design level Hence, no commit step software can always be changed!No body of experience for design analysis (yet) Most analysis (testing) is done on program code Hence, problems are often not detected until late in the processWaterfall model takes a static view of requirements It ignores changing needs Lack of user involvement once specification is written Unrealistic separation of specification from the design
Doesnt accommodate prototyping, reuse, etc.16COMP201 - Software EngineeringEvolutionary DevelopmentRather than using the waterfall model we may use evolutionary development which is based upon the idea of developing an initial implementation , exposing it to the user and refining it based upon their response. Exploratory development Objective is to work with customers and to evolve a final system from an initial outline specification. Should start with well-understood requirements. The system evolves by adding new features as they are proposed by customer.
17COMP201 - Software EngineeringEvolutionary DevelopmentThrow-away prototypingObjective is to understand the system requirements. Should start with poorly understood requirementsDevelop quick and dirty system quickly;Expose to user comment;Refine; Until an adequate system is developed.Particularly suitable where:detailed requirements not possible;powerful development tools (e.g. GUI) available18COMP201 - Software EngineeringEvolutionary Development
19COMP201 - Software EngineeringEvolutionary DevelopmentProblemsLack of process visibilitySystems are sometimes poorly structuredSpecial skills (e.g. in languages prototyping) may be requiredApplicabilityAll types of system but rare in safety critical20COMP201 - Software EngineeringReality checkIn reality all modern development has a degree of evolutionary development BUT is hybrid (see SCRUM later)Sometimes the specification is mostly completed at the start and then added toThe evolution cycles pre-determined100 functionsIn phase 1 develop functions 1-30In phase 2 develop functions 31-80In phase 3 develop functions 81-100
COMP201 - Software Engineering21Formal Systems DevelopmentBased on the transformation of a mathematical specification through different representations to an executable programTransformations are correctness-preserving so it is straightforward to show that the program conforms to its specificationEmbodied in the Cleanroom approach (which was originally developed by IBM) to software development22COMP201 - Software EngineeringFormal Systems Development
23COMP201 - Software EngineeringFormal Transformations
24COMP201 - Software EngineeringExample code (in Z)COMP201 - Software Engineering25
Formal Systems DevelopmentProblemsNeed for specialised skills and training to apply the technique (Higher initial cost)Difficult to formally specify some aspects of the system such as the user interfaceCan be more time consuming than other approaches (increased time to market)Many stake holders cannot understand the specificationApplicabilityCritical systems especially those where a safety or security case must be made before the system is put into operation26COMP201 - Software EngineeringReuse-Oriented DevelopmentBased on systematic reuse where systems are integrated from existing components or COTS (Commercial-off-the-shelf) systemsProcess stagesComponent analysisRequirements modificationSystem design with reuseDevelopment and integration
27COMP201 - Software EngineeringProcess IterationModern development processes take iteration as fundamental, and try to provide ways of managing, rather than ignoring, the riskSystem requirements ALWAYS evolve in the course of a project so process iteration where earlier stages are reworked is always part of the process for large systemsIteration can be applied to any of the generic process modelsThere are two (related) approaches:Incremental developmentSpiral development28COMP201 - Software EngineeringIncremental Development(example Scrum)Rather than deliver the system as a single delivery, the development and delivery is broken down into increments with each increment delivering part of the required functionalityUser requirements are prioritised and the highest priority requirements are included in early incrementsOnce the development of an increment is started, the requirements are frozen though requirements for later increments can continue to evolve29COMP201 - Software EngineeringIncremental Development AdvantagesCustomer value can be delivered with each increment so system functionality is available earlierEarly increments act as a prototype to help elicit requirements for later incrementsLower risk of overall project failureThe highest priority system services tend to receive the most testing
30COMP201 - Software EngineeringIn RealityMost software processes involvePrototypingIterative buildingWhyIt reduces risk of making the wrong productIt allows the software to undergo more testingIt produces working product as we go along, so less chance of inventory lossCOMP201 - Software Engineering31Final questionThe specification is by a long marginThe MOST critical phase of any software engineering projectWhy?COMP201 - Software Engineering32Lecture Key PointsSoftware processes are the activities involved in producing and evolving a software system. They are represented in a software process modelGeneral activities are specification, design and implementation, validation and evolutionGeneric process models describe the organisation of software processesIterative process models describe the software process as a cycle of activities33COMP201 - Software Engineering
