ࡱ> rtqE@ Ybjbj (XYzzzzzzzv$v$v$v$,$Br%r%r%r%r%r%r%r%XBZBZBZBZBZBZB$DRG~Bzr%r%r%r%r%~Bzzr%r%B>(>(>(r%jzr%zr%XB.(r%XB>(6>(t(x4hzz5r%f% Zcv$%j4,>,B0B4GF&G 5zzzz50Gz=Hr%r%(r%r%r%r%r%~B~BD (Quality Attribute Scenarios The six Quality Attributes we will discuss follow: 1. Availability is concerned with system failure and duration of system failures. System failure means when the system does not provide the service for which it was intended. 2. Modifiability is about the cost of change, both in time and money. 3. Performance is about time. Events occur and the system must respond in a timely fashion. 4. Security is the ability of the system to prevent or resist unauthorized access while providing access to legitimate users. An attack is an attempt to breach security. 5. Testability refers to the ease with which the software can be made to demonstrate its faults or lack thereof. To be testable the system must control inputs and be able to observe outputs. 6. Usability is how easy it is for the user to accomplish tasks and what support the system provides for the user to accomplish this. Dimensions: Learning system features Using the system efficiently Minimizing the impact of errors Adapting the system to the users needs Increasing confidence and satisfaction A Quality Attribute Scenario is a quality-attribute-specific requirement. There are 6 parts: Source of stimulus (e.g., human, computer system, etc.) Stimulus a condition that needs to be considered Environment - what are the conditions when the stimulus occurs? Artifact what elements of the system are stimulated. Response the activity undertaken after arrival of the stimulus Response measure when the response occurs it should be measurable so that the requirement can be tested. Table 1 Availability General Scenario. Scenario Portion Possible ValuesSourceInternal to system or external to systemStimulusCrash, omission, timing, no response, incorrect response ArtifactSystems processors, communication channels, persistent storageEnvironmentNormal operation; degraded (failsafe) mode ResponseLog the failure, notify users/operators, disable source of failure, continue (normal/degraded)RespMeasureTime interval available, availability%, repair time, unavailability time interval Table 2 Modifiability General Scenario. Scenario Portion Possible ValuesSourceEnd-user, developer, system-administratorStimulusAdd/delete/modify functionality or quality attr. ArtifactSystem user interface, platform, environmentEnvironmentAt runtime, compile time, build time, design-time ResponseLocate places in architecture for modifying, modify, test modification, deploys modificationRespMeasureCost in effort, money, time, extent affects other system functions or qualities Table 3 Performance General Scenario. Scenario Portion Possible ValuesSourceA number of sources both external and internalStimulusPeriodic events, sporadic events, stochastic eventsArtifactSystem, or possibly a componentEnvironmentNormal mode; overload mode ResponseProcess stimuli; change level of serviceRespMeasureLatency, deadline, throughput, jitter, miss rate, data loss Table 4 Security General Scenario. Scenario Portion Possible ValuesSourceUser/system who is legitimate/imposter/unknown with full/limited accessStimulusAttempt to display/modify data; access services ArtifactSystem services, dataEnvironmentNormal operation; degraded (failsafe) mode ResponseAuthenticate user; hide identity of user; grant/block access; encrypt data; detect excessive demandRespMeasureTime /effort/resources to circumvent security measures with probability of success Table 5 Testability General Scenario. Scenario Portion Possible ValuesSourceUnit developer, increment integrator, system verifier, client acceptance tester, system userStimulusAnalysis, architecture, design, class, subsystem integration, system deliveredArtifactPiece of design, piece of code, complete systemEnvironmentAt design time, at development time, at compile time, at deployment timeResponseProvide access to state data values, observes results, comparesRespMeasure% coverage; prob. of failure; time to perform tests; length of time to prepare test environment Table 6 Usability General Scenario. 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