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Institutt for datateknikk og informasjonsvitenskap. Inah Omoronyia and Tor Stålhane Requirements Specification and Testing An introduction. TDT 4242. Challenges in Requirements Engineering. What is a requirement? What a system must do (functional): System requirements

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Inah Omoronyia and Tor Stålhane Requirements Specification and Testing An introduction


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    1. Institutt for datateknikk og informasjonsvitenskap Inah Omoronyia and Tor Stålhane Requirements Specification and Testing An introduction TDT 4242 TDT 4242

    2. Challenges in Requirements Engineering • What is a requirement? • What a system must do (functional): System requirements • How well the system will perform its functions (non-functional): System quality attributes The RE process: defined operational capabilities Ultimately: satisfy business needs TDT 4242

    3. Challenges in Requirements Engineering TDT 4242

    4. Challenges in Requirements Engineering Importance of getting requirements right: 1/3 budget to correct errors originate from requirements Source: Benoy R Nair (IBS software services) TDT 4242

    5. Challenges in Requirements Engineering Factors that make a software project challenging: Source: Benoy R Nair (IBS software services) TDT 4242

    6. Challenges in Requirements Engineering Why projects are cancelled: Source: Benoy R Nair (IBS software services) TDT 4242

    7. Requirements Development - 1 • Requirements Elicitation: • The process of discovering the requirements for a system by communication with customers, system users and others who have a stake in the system development. • Requirements gathering techniques • Methodical extraction of concrete requirements from high level goals • Requirements quality metrics TDT 4242

    8. Requirements Development – 2 • Effects of Inadequate Requirements development – Ariane 5: • (An expendable launch system used to deliver payloads into geostationary transfer orbit or low Earth orbit) • Ariane 5 succeeded Ariane 4. Wrong implicit assumptions about the parameters, in particular the horizontal velocity that were safe for Ariane 4 but not Ariane 5. • horizontal velocity exceeded the maximum value for a 16 bit unsigned integer when it was converted from it's signed 64 bit representation. • Ariane 5: component (requirements) should have been designed for reuse – but the context of reuse was not specified. • Cost of poor requirements in Ariane 5 • Data overflow on launch • Self-destruction of the complete system • Loss > 500 Million EUR TDT 4242

    9. Requirements Development – 3 Effects of Inadequate Requirements development – Airbus: Requirement: Reverse thrust may only be used, when the airplane is landed. Translation: Reverse thrust may only be used while the wheels are rotating. Implementation: Reverse thrust may only be used while the wheels are rotating fast enough. Situation: Rainstorm – aquaplaning Result: Crash due to overshooting the runway! Problem: erroneous modeling in the requirement phase TDT 4242

    10. Problem world and machine solution • The problem to be solved is rooted in a complex organizational, technical or physical world. • The aim of a software project is to improve the world by building some machine expected to solve the problem. • Problem world and machine solution each have their own phenomena while sharing others. • The shared phenomena defines the interface through which the machine interacts with the world. E-commerce world Requirements engineering is concerned with the machine’s effect on the surrounding world and the assumption we make about that world. TDT 4242

    11. Formulation of requirements statements • Statement scope: • Phenomenon of train physically moving is owned by environment. It cannot be directly observed by software phenomenon • The phenomenon of train measured speed being non-null is shared by software and environment. It is measured by a speedometer in the environment and observed by the software. TDT 4242

    12. Two types of requirements statements • Descriptive statements: state properties about the system that holds regardless of how the system behaves. E.g. If train doors are open, they are not closed. • Prescriptive statements: States desirable properties about the system that may hold or not depending on how the system behaves • Need to be enforced by system components • E.g. Train doors shall always remain closed when the train is moving TDT 4242

    13. Formulation of system requirement • A prescriptive statement enforced by the software-to-be. • Possibly in cooperation with other system components • Formulated in terms of environment phenomena • Example: • All train doors shall always remain closed while the train is moving • In addition to the software-to-be we also requires the cooperation of other components: • Train controller being responsible for the safe control of doors. • The passenger refraining from opening doors unsafely • Door actuators working properly TDT 4242

    14. Formulation of software requirement A prescriptive statement enforced solely by the software-to-be. Formulated in terms of phenomena shared between the software and environment. The software “understand” or “sense” the environment through input data Example: The doorState output variable shall always have the value ‘closed’ when the measuredSpeed input variable has a non-null value TDT 4242

    15. Domain properties • A domain property: • Is a descriptive statement about the problem world • Should hold invariably regardless of how the system behaves • Usually corresponds to some physical laws • Example: • A train is moving if and only if its physical speed is non-null. TDT 4242

    16. Goal orientation in requirements engineering – 1 • A goal is an objective that the system under consideration shall achieve. • Ranges from high-level strategic to low-level technical concerns over a system • System consist of both the software and its environment. Interaction between active components, i.e. devices, humans, software etc also called Agents TDT 4242

    17. Goal orientation in requirements engineering – 2 • Goals can be stated at different levels of granularity: • High-level goal: A goal that requires the cooperation of many agents. They are normally stating strategic objective related to the business, e.g. The system’s transportation capacity shall be increased by 50% • Requirement: A goal under the responsibility of a single agent in the software-to-be. • Assumption (expectation): A goal under the responsibility of a single agent in the environment of the software-to-be. Assumptions cannot be enforced by the software-to-be TDT 4242

    18. Goal statement typology TDT 4242

    19. Goal types TDT 4242

    20. Behavioral goal specialization TDT 4242

    21. Goal categorization – 1 Goal categories are similar to requirements categories: TDT 4242

    22. Goal categorization – 2 • Functional goal: States the intent underpinning a system service • Satisfaction: Functional goals concerned with satisfying agent request • Information: Functional goals concerned with keeping agents informed about important system states • Stimulus-response: Functional goals concerned with providing appropriate response to specific event • Example: The on-board controller shall update the train’s acceleration to the commanded one immediately on receipt of an acceleration command from the station computer TDT 4242

    23. Goal categorization – 3 • Non-functional goal: States a quality or constraint on service provision or development. • Accuracy goal: Non-functional goals requiring the state of variables controlled by the software to reflect the state of corresponding quantities controlled by environment agent • E.g: The train’s physical speed and commanded speed may never differ by more than X miles per hour • Soft goals are different from non-functional goals. Soft goals are goals with no clear-cut criteria to determine their satisfaction. • E.g: The ATM interface should be more user friendly TDT 4242

    24. Goal refinement A mechanism for structuring complex specifications at different levels of concern. A goal can be refined in a set of sub-goals that jointly contribute to it. Each sub-goal is refined into finer-grained goals until we reach a requirement on the software and expectation (assumption) on the environment. NB: Requirements on software are associated with a single agent and they are testable TDT 4242

    25. Goal refinement: Example TDT 4242

    26. Goal refinement tree – 1 Refinement links are two way links: One showing goal decomposition, the other showing goal contribution TDT 4242

    27. Goal refinement tree – 2 Goal feature annotation TDT 4242

    28. Requirements quality metrics – 1 • Qualitative Goal-Requirements tracing: • An approach to requirements refinement/abstraction that makes it less likely to generate trace links that are ambiguous, inconsistent, opaque, noisy, incomplete or with forward referencing items TDT 4242

    29. Requirements quality metrics – 2 • Ambiguity: Requirement with terms or statements that can be interpreted in different ways. Sub-class concept reasoning: • Inconsistency: Requirement items that are not compatible with other requirement nodes. Predefined semantic reasoning • Cc TDT 4242

    30. Requirements quality metrics – 3 • Forward Referencing: Requirement items that make use of problem world domain features that are not yet defined.E, C and D need to be mapped to a requirement item TDT 4242

    31. Requirements quality metrics – 4 • Opacity: Requirement items for which rational or dependencies are invisible. • Multiple unrelated concept mapping. A is not related to B TDT 4242

    32. Requirements quality metrics – 5 • Noise: Requirement items that yield no information on problem world features. X refers to a concept undefined in the domain TDT 4242

    33. Requirements quality metrics – 6 • Completeness: The needs of a prescribed system are fully covered by requirement items without any undesirable outcome.No requirement item mentions the goal concept Z TDT 4242

    34. Requirements quality metrics Quality metrics on a requirements set provides useful understanding, tracking and control of requirements improvement process.

    35. Where do the goals come from? • We get goals from: • Preliminary analysis of the current system. • Systematically by searching intentional keywords in documents provided, interview transcripts etc. E.g. ‘objective’, ‘purpose’, ‘in order to’. • Iterative refinement and abstraction of high-level goals: By asking the how and why question. Results in a goal refinement tree • Approaches: KAOS – Goal driven requirements acquisition. TDT 4242

    36. Summary Goals can be defined at different levels of abstraction There are two types of goals: Behavioral or soft goal There are several categories of goals, e.g. • Functional and non-functional • Goal refinement provides a natural mechanism for structuring complex specifications at different levels of concern: • Goal refinement graph TDT 4242