软件工程：Computer-BasedSystemEngineering.ppt

Chapter2 Computer BasedSystemEngineering SystemsEngineering Designing implementing deployingandoperatingsystemswhichincludehardware softwareandpeople Objectives ToexplainwhysystemsoftwareisaffectedbybroadersystemengineeringissuesTointroducetheconceptofemergentsystempropertiessuchasreliabilityandsecurityToexplainwhythesystemsenvironmentmustbeconsideredinthesystemdesignprocessToexplainsystemengineeringandsystemprocurementprocesses Topicscovered EmergentsystempropertiesSystemsandtheirenvironmentSystemmodellingThesystemengineeringprocessSystemprocurement Whatisasystem Apurposefulcollectionofinter relatedcomponentsworkingtogethertowardssomecommonobjective Asystemmayincludesoftware mechanical electricalandelectronichardwareandbeoperatedbypeople SystemcomponentsaredependentonothersystemcomponentsThepropertiesandbehaviourofsystemcomponentsareinextricablyinter mingled Problemsofsystemsengineering Largesystemsareusuallydesignedtosolve wicked problemsSystemsengineeringrequiresagreatdealofco ordinationacrossdisciplinesAlmostinfinitepossibilitiesfordesigntrade offsacrosscomponentsMutualdistrustandlackofunderstandingacrossengineeringdisciplinesSystemsmustbedesignedtolastmanyyearsinachangingenvironment Softwareandsystemsengineering Theproportionofsoftwareinsystemsisincreasing Software drivengeneralpurposeelectronicsisreplacingspecial purposesystemsProblemsofsystemsengineeringaresimilartoproblemsofsoftwareengineeringSoftwareis unfortunately seenasaprobleminsystemsengineering Manylargesystemprojectshavebeendelayedbecauseofsoftwareproblems Emergentproperties PropertiesofthesystemasawholeratherthanpropertiesthatcanbederivedfromthepropertiesofcomponentsofasystemEmergentpropertiesareaconsequenceoftherelationshipsbetweensystemcomponentsTheycanthereforeonlybeassessedandmeasuredoncethecomponentshavebeenintegratedintoasystem Examplesofemergentproperties TheoverallweightofthesystemThisisanexampleofanemergentpropertythatcanbecomputedfromindividualcomponentproperties ThereliabilityofthesystemThisdependsonthereliabilityofsystemcomponentsandtherelationshipsbetweenthecomponents TheusabilityofasystemThisisacomplexpropertywhichisnotsimplydependentonthesystemhardwareandsoftwarebutalsodependsonthesystemoperatorsandtheenvironmentwhereitisused Typesofemergentproperty FunctionalpropertiesTheseappearwhenallthepartsofasystemworktogethertoachievesomeobjective Forexample abicyclehasthefunctionalpropertyofbeingatransportationdeviceonceithasbeenassembledfromitscomponents Non functionalemergentpropertiesExamplesarereliability performance safety andsecurity Theserelatetothebehaviourofthesysteminitsoperationalenvironment Theyareoftencriticalforcomputer basedsystemsasfailuretoachievesomeminimaldefinedlevelinthesepropertiesmaymakethesystemunusable Becauseofcomponentinter dependencies faultscanbepropagatedthroughthesystemSystemfailuresoftenoccurbecauseofunforeseeninter relationshipsbetweencomponentsItisprobablyimpossibletoanticipateallpossiblecomponentrelationshipsSoftwarereliabilitymeasuresmaygiveafalsepictureofthesystemreliability Systemreliabilityengineering HardwarereliabilityWhatistheprobabilityofahardwarecomponentfailingandhowlongdoesittaketorepairthatcomponent SoftwarereliabilityHowlikelyisitthatasoftwarecomponentwillproduceanincorrectoutput Softwarefailureisusuallydistinctfromhardwarefailureinthatsoftwaredoesnotwearout OperatorreliabilityHowlikelyisitthattheoperatorofasystemwillmakeanerror Influencesonreliability Reliabilityrelationships HardwarefailurecangeneratespurioussignalsthatareoutsidetherangeofinputsexpectedbythesoftwareSoftwareerrorscancausealarmstobeactivatedwhichcauseoperatorstressandleadtooperatorerrorsTheenvironmentinwhichasystemisinstalledcanaffectitsreliability The shall not properties PropertiessuchasperformanceandreliabilitycanbemeasuredHowever somepropertiesarepropertiesthatthesystemshouldnotexhibitSafety thesystemshouldnotbehaveinanunsafewaySecurity thesystemshouldnotpermitunauthoriseduseMeasuringorassessingthesepropertiesisveryhard Systemsandtheirenvironment SystemsarenotindependentbutexistinanenvironmentSystem sfunctionmaybetochangeitsenvironmentEnvironmentaffectsthefunctioningofthesysteme g systemmayrequireelectricalsupplyfromitsenvironmentTheorganizationalaswellasthephysicalenvironmentmaybeimportant Systemhierarchies Humanandorganisationalfactors ProcesschangesDoesthesystemrequirechangestotheworkprocessesintheenvironment JobchangesDoesthesystemde skilltheusersinanenvironmentorcausethemtochangethewaytheywork OrganisationalchangesDoesthesystemchangethepoliticalpowerstructureinanorganisation Systemarchitecturemodelling Anarchitecturalmodelpresentsanabstractviewofthesub systemsmakingupasystemMayincludemajorinformationflowsbetweensub systemsUsuallypresentedasablockdiagramMayidentifydifferenttypesoffunctionalcomponentinthemodel Intruderalarmsystem Componenttypesinalarmsystem SensorMovementsensor doorsensorActuatorSirenCommunicationTelephonecallerCo ordinationAlarmcontrollerInterfaceVoicesynthesizer IanSommerville1995SoftwareEngineering 5thedition Chapter31 Slide ATCsystemarchitecture Functionalsystemcomponents SensorcomponentsActuatorcomponentsComputationcomponentsCommunicationcomponentsCo ordinationcomponentsInterfacecomponents Systemcomponents SensorcomponentsCollectinformationfromthesystem senvironmente g radarsinanairtrafficcontrolsystemActuatorcomponentsCausesomechangeinthesystem senvironmente g valvesinaprocesscontrolsystemwhichincreaseordecreasematerialflowinapipeComputationcomponentsCarryoutsomecomputationsonaninputtoproduceanoutpute g afloatingpointprocessorinacomputersystem Systemcomponents CommunicationcomponentsAworklinkingdistributedcomputersCo ordinationcomponentsCo ordinatetheinteractionsofothersystemcomponentse g schedulerinareal timesystemInterfacecomponentsFacilitatetheinteractionsofothersystemcomponentse g operatorinterfaceAllcomponentsarenowusuallysoftwarecontrolled Componenttypesinalarmsystem SensorMovementsensor DoorsensorActuatorSirenCommunicationTelephonecallerCoordinationAlarmcontrollerInterfaceVoicesynthesizer Thesystemengineeringprocess Usuallyfollowsa waterfall modelbecauseoftheneedforparalleldevelopmentofdifferentpartsofthesystemLittlescopeforiterationbetweenphasesbecausehardwarechangesareveryexpensive SoftwaremayhavetocompensateforhardwareproblemsInevitablyinvolvesengineersfromdifferentdisciplineswhomustworktogetherMuchscopeformisunderstandinghere Differentdisciplinesuseadifferentvocabularyandmuchnegotiationisrequired Engineersmayhavepersonalagendastofulfil Thesystemengineeringprocess Inter disciplinaryinvolvement Systemrequirementsdefinition ThreetypesofrequirementdefinedatthisstageAbstractfunctionalrequirements SystemfunctionsaredefinedinanabstractwaySystemproperties Non functionalrequirementsforthesystemingeneralaredefinedUndesirablecharacteristics UnacceptablesystembehaviourisspecifiedShouldalsodefineoverallorganisationalobjectivesforthesystem Systemobjectives FunctionalobjectivesToprovideafireandintruderalarmsystemforthebuildingwhichwillprovideinternalandexternalwarningoffireorunauthorizedintrusionOrganisationalobjectivesToensurethatthenormalfunctioningofworkcarriedoutinthebuildingisnotseriouslydisruptedbyeventssuchasfireandunauthorizedintrusion Systemrequirementsproblems ChangingasthesystemisbeingspecifiedMustanticipatehardware communicationsdevelopmentsoverthelifetimeofthesystemHardtodefinenon functionalrequirements particularly withoutanimpressionofcomponentstructureofthesystem Thesystemdesignprocess PartitionrequirementsOrganiserequirementsintorelatedgroupsIdentifysub systemsIdentifyasetofsub systemswhichcollectivelycanmeetthesystemrequirementsAssignrequirementstosub systemsCausesparticularproblemswhenCOTSareintegratedSpecifysub systemfunctionalityDefinesub systeminterfacesCriticalactivityforparallelsub systemdevelopment Thesystemdesignprocess Systemdesignproblems Requirementspartitioningtohardware softwareandhumancomponentsmayinvolvealotofnegotiationDifficultdesignproblemsareoftenassumedtobereadilysolvedusingsoftwareHardwareplatformsmaybeinappropriateforsoftwarerequirementssosoftwaremustcompensateforthis Sub systemdevelopment Typicallyparallelprojectsdevelopingthehardware softwareandcommunicationsMayinvolvesomeCOTS CommercialOff the Shelf systemsprocurementLackofcommunicationacrossimplementationteamsBureaucraticandslowmechanismforproposingsystemchangesmeansthatthedevelopmentschedulemaybeextendedbecauseoftheneedforrework Theprocessofputtinghardware softwareandpeopletogethertomakeasystemShouldbetackledincrementallysothatsub systemsareintegratedoneatatimeInterfaceproblemsbetweensub systemsareusuallyfoundatthisstageMaybeproblemswithuncoordinateddeliveriesofsystemcomponents Systemintegration EnvironmentalassumptionsmaybeincorrectMaybehumanresistancetotheintroductionofanewsystemSystemmayhavetocoexistwithalternativesystemsforsometimeMaybephysicalinstallationproblems e g cablingproblems Operatortraininghastobeidentified Systeminstallation WillbringunforeseenrequirementstolightUsersmayusethesysteminawaywhichisnotanticipatedbysystemdesignersMayrevealproblemsintheinteractionwithothersystemsPhysicalproblemsofincompatibilityDataconversionproblemsIncreasedoperatorerrorratebecauseofinconsistentinterfaces Systemoperation Systemevolution Largesystemshavealonglifetime TheymustevolvetomeetchangingrequirementsEvolutionisinherentlycostlyChangesmustbeanalysedfromatechnicalandbusinessperspectiveSub systemsinteractsounanticipatedproblemscanariseThereisrarelyarationalefororiginaldesigndecisionsSystemstructureiscorruptedaschangesaremadetoitExistingsystemswhichmustbemaintainedaresometimescalledlegacysystems Systemdecommissioning TakingthesystemoutofserviceafteritsusefullifetimeMayrequireremovalofmaterials e g dangerouschemicals whichpollutetheenvironmentShouldbeplannedforinthesystemdesignbyencapsulationMayrequiredatatoberestructuredandconvertedtobeusedinsomeothersystem Systemprocurement AcquiringasystemforanorganizationtomeetsomeneedSomesystemspecificationandarchitecturaldesignisusuallynecessarybeforeprocurementYouneedaspecificationtoletacontractforsystemdevelopmentThespecificationmayallowyoutobuyacommercialoff the shelf COTS system Almostalwayscheaperthandevelopingasystemfromscratch Thesystemprocurementprocess Procurementissues Requirementsmayhavetobemodifiedtomatchthecapabilitiesofoff the shelfcomponentsTherequirementsspecificationmaybepartofthecontractforthedevelopmentofthesystemThereisusuallyacontractnegotiationperiodtoagreechangesafterthecontractortobuilda