Socio-technical SystemsObjectivesTopics coveredWhat is a system?System categoriesSocio-technical system characteristicsEmergent propertiesExamples of emergent propertiesTypes of emergent propertySystem reliability engineeringInfluences on reliabilityReliability relationshipsThe ‘shall-not’ propertiesSystems engineeringThe system engineering processThe systems engineering processInter-disciplinary involvementSystem requirements definitionSystem objectivesSystem requirements problemsThe system design processSlide 22System design problemsRequirements and designSpiral model of requirements/designSystem modellingBurglar alarm systemSub-system descriptionATC system architectureSub-system developmentSystem integrationSystem installationSystem evolutionSystem decommissioningOrganisations/people/systemsHuman and organisational factorsOrganisational processesProcurement/development processesSystem procurementThe system procurement processProcurement issuesContractors and sub-contractorsContractor/Sub-contractor modelLegacy systemsSlide 45Legacy system componentsSlide 47Key pointsSlide 49©Ian Sommerville 2006 Software Engineering, 8th edition. Chapter 2 Slide 1Socio-technical Systems©Ian Sommerville 2006 Software Engineering, 8th edition. Chapter 2 Slide 2ObjectivesTo explain what a socio-technical system is and the distinction between this and a computer-based systemTo introduce the concept of emergent system properties such as reliability and securityTo explain system engineering and system procurement processesTo explain why the organisational context of a system affects its design and useTo discuss legacy systems and why these are critical to many businesses©Ian Sommerville 2006 Software Engineering, 8th edition. Chapter 2 Slide 3Topics coveredEmergent system propertiesSystems engineeringOrganizations, people and computer systems Legacy systems©Ian Sommerville 2006 Software Engineering, 8th edition. Chapter 2 Slide 4What is a system?A purposeful collection of inter-related components working together to achieve some common objective. A system may include software, mechanical, electrical and electronic hardware and be operated by people.System components are dependent on other system componentsThe properties and behaviour of system components are inextricably inter-mingled©Ian Sommerville 2006 Software Engineering, 8th edition. Chapter 2 Slide 5System categoriesTechnical computer-based systems•Systems that include hardware and software but where the operators and operational processes are not normally considered to be part of the system. The system is not self-aware.Socio-technical systems•Systems that include technical systems but also operational processes and people who use and interact with the technical system. Socio-technical systems are governed by organisational policies and rules.©Ian Sommerville 2006 Software Engineering, 8th edition. Chapter 2 Slide 6Socio-technical system characteristicsEmergent properties•Properties of the system of a whole that depend on the system components and their relationships.Non-deterministic•They do not always produce the same output when presented with the same input because the systems’s behaviour is partially dependent on human operators.Complex relationships with organisational objectives•The extent to which the system supports organisational objectives does not just depend on the system itself.©Ian Sommerville 2006 Software Engineering, 8th edition. Chapter 2 Slide 7Emergent propertiesProperties of the system as a whole rather than properties that can be derived from the properties of components of a systemEmergent properties are a consequence of the relationships between system componentsThey can therefore only be assessed and measured once the components have been integrated into a system©Ian Sommerville 2006 Software Engineering, 8th edition. Chapter 2 Slide 8Examples of emergent propertiesPropertyDescriptionVo lume The volume of a system (the total space occupied) varies depending on how thecomponent assemblies are arranged and connected.Reliability System reliability depends on component reliability but unexpected interactions cancause new types of failure and therefore afect the reliability of the system.Security The security of the system (its ability to resist attack) is a complex property thatcannot be easily measured. Attacks may be devised that were not anticipated by thesystem designers and so may defeat built-in safeguards.Repairability This property refects how easy it is to fx a problem with the system once it has beendiscovered. It depends on being able to diagnose the problem, access the componentsthat are faulty and modify or replace these components.Usability This property refects how easy it is to use the system. It depends on the technicalsystem components, its operators and its operating environment.©Ian Sommerville 2006 Software Engineering, 8th edition. Chapter 2 Slide 9Types of emergent propertyFunctional properties •These appear when all the parts of a system work together to achieve some objective. For example, a bicycle has the functional property of being a transportation device once it has been assembled from its components.Non-functional emergent properties•Examples are reliability, performance, safety, and security. These relate to the behaviour of the system in its operational environment. They are often critical for computer-based systems as failure to achieve some minimal defined level in these properties may make the system unusable.©Ian Sommerville 2006 Software Engineering, 8th edition. Chapter 2 Slide 10Because of component inter-dependencies, faults can be propagated through the system.System failures often occur because of unforeseen inter-relationships between components.It is probably impossible to anticipate all possible component relationships.Software reliability measures may give a false picture of the system reliability.System reliability engineering©Ian Sommerville 2006 Software Engineering, 8th edition. Chapter 2 Slide 11Hardware reliability •What is the probability of a hardware component failing and