University of Toronto University of Toronto Department of Computer Science Lecture 7 Requirements Modeling III What are Non functional Requirements Last Last Week Week Modeling ModelingII II Information Information Structure Structure Behaviour Behaviour Functional vs Non Functional Functional requirements describe what the system should do things that can be captured in use cases things that can be analyzed by drawing interaction diagrams statecharts etc Functional requirements will probably trace to individual chunks of a program Non functional requirements are global constraints on a software system e g development costs operational costs performance reliability maintainability portability robustness etc Often known as the ilities Usually cannot be implemented in a single module of a program This This Week Week Modeling Modeling System System Qualities Qualities Non functional Non functionalRequirements Requirements Satisficing Softgoals Satisficing Softgoals Quality Qualitymeasures measures often contradictory difficult to enforce during development difficult to evaluate for the customer prior to delivery Specification Specificationand andValidation Validation Specification SpecificationLanguages Languages Documentation Standards Documentation Standards Reviews Reviewsand andInspections Inspections Hard to make them measurable requirements We d like to state them in a way that we can measure how well they ve been met 2000 2003 Steve Easterbrook 1 University of Toronto The challenge of NFRs Hard to model Usually stated informally and so are Next Next Week Week 2000 2003 Steve Easterbrook Interface requirements how will the new system interface with its environment User interfaces and user friendliness Interfaces with other systems Performance requirements time space bounds workloads response time throughput and available storage space e g the system must handle 1 000 transactions per second E g permissible information flows or who can do what E g system will need to survive fire natural catastrophes etc 2000 2003 Steve Easterbrook Focus on system or software quality Aim is to have a way of measuring the product once it s built Process oriented Approaches Focus on how NFRs can be used in the design process Aim is to have a way of making appropriate design decisions Lifecycle requirements Find measurable scales for the quality attributes Calculate degree to which a design meets the quality targets Qualitative Approaches E g development time limitations resource availability methodological standards etc Quantitative vs Qualitative Quantitative Approaches limits on development security Product vs Process Product oriented Approaches Maintainability Enhanceability Portability expected market or product lifespan the availability of components integrity of information maintained and supplied to the system e g system must have less than 1hr downtime per three months survivability physical constraints size weight personnel availability skill level accessibility for maintenance environmental conditions etc Future proofing reliability Department of Computer Science Approaches to NFRs Operating requirements 2 University of Toronto Department of Computer Science Example NFRs Department of Computer Science Study various relationships between quality goals Reason about trade offs etc Economic requirements e g restrictions on immediate and or long term costs 3 2000 2003 Steve Easterbrook 4 1 University of Toronto University of Toronto Department of Computer Science Software Qualities Fitness Source Budgen 1994 pp58 9 Think of an everyday object e g a chair How would you measure it s quality All quality measures are relative there is no absolute scale we can sometimes say A is better than B but it is usually hard to say how much better Software quality is all about fitness to purpose does it do what is needed does it do it in the way that its users need it to does it do it reliably enough fast enough safely enough securely enough will it be affordable will it be ready when its users need it can it be changed as the needs change construction quality e g strength of the joints aesthetic value e g elegance fit for purpose e g comfortable But this means quality is not a measure of software in isolation it is a measure of the relationship between software and its application domain For software might not be able to measure this until you place the software into its environment and the quality will be different in different environments construction quality software is not manufactured aesthetic value during design we need to be able to predict how well the software will fit its purpose but most of the software is invisible aesthetic value matters for the user interface but is only a marginal concern we need to understand that purpose requirements analysis we need to look for quality predictors design analysis fit for purpose Need to understand the purpose 2000 2003 Steve Easterbrook 5 University of Toronto Department of Computer Science Factors vs Criteria Department of Computer Science 2000 2003 Steve Easterbrook 6 University of Toronto Department of Computer Science Boehm s NFR list device independence Source See Blum 1992 p176 Quality Factors portability These are customer related concerns Examples efficiency integrity reliability correctness survivability usability reliability Design Criteria These are technical development oriented concerns such as anomaly management completeness consistency traceability visibility General utility efficiency As is utility Quality Factors and Design Criteria are related usability Each factor depends on a number of associated criteria testability There are some standard mappings to help you robustness integrity consistency accountability device efficiency communicativeness self descriptiveness During Analysis Maintainability understandability Identify the relative importance of each quality factor structuredness conciseness From the customer s point of view modifiability Identify the design criteria on which these factors depend Make the requirements measurable 2000 2003 Steve Easterbrook completeness accessibility E g correctness depends on completeness consistency traceability E g verifiability depends on modularity self descriptiveness and simplicity self containedness accuracy legibility augmentability 7 2000 2003 Steve Easterbrook 8 2 University of Toronto McCall s NFR list Source See van Vliet 2000 pp111 3 Product operation University of Toronto