Ch4Requirements

nullTopics coveredTopics coveredFunctional and non-functional requirements The software requirements document Requirements specification Requirements engineering processes Requirements elicitation and analysis Requirements validation Requirements management *Chapter 4 Requirements engineeringRequirements engineeringRequirements engineeringThe process of establishing the services that the customer requires from a system and the constraints under which it operates and is developed. The requirements themselves are the descriptions of the system services and constraints that are generated during the requirements engineering process.*Chapter 4 Requirements engineeringWhat is a requirement?What is a requirement?It may range from a high-level abstract statement of a service or of a system constraint to a detailed mathematical functional specification. This is inevitable as requirements may serve a dual function May be the basis for a bid for a contract - therefore must be open to interpretation; May be the basis for the contract itself - therefore must be defined in detail; Both these statements may be called requirements.*Chapter 4 Requirements engineeringRequirements abstraction (Davis)Requirements abstraction (Davis)“If a company wishes to let a contract for a large software development project, it must define its needs in a sufficiently abstract way that a solution is not pre-defined. The requirements must be written so that several contractors can bid for the contract, offering, perhaps, different ways of meeting the client organization’s needs. Once a contract has been awarded, the contractor must write a system definition for the client in more detail so that the client understands and can validate what the software will do. Both of these documents may be called the requirements document for the system.”*Chapter 4 Requirements engineeringTypes of requirementTypes of requirementUser requirements Statements in natural language plus diagrams of the services the system provides and its operational constraints. Written for customers. System requirements A structured document setting out detailed descriptions of the system’s functions, services and operational constraints. Defines what should be implemented so may be part of a contract between client and contractor.*Chapter 4 Requirements engineeringUser and system requirements User and system requirements *Chapter 4 Requirements engineeringReaders of different types of requirements specification Readers of different types of requirements specification *Chapter 4 Requirements engineeringFunctional and non-functional requirementsFunctional and non-functional requirementsFunctional requirements Statements of services the system should provide, how the system should react to particular inputs and how the system should behave in particular situations. May state what the system should not do. Non-functional requirements Constraints on the services or functions offered by the system such as timing constraints, constraints on the development process, standards, etc. Often apply to the system as a whole rather than individual features or services. Domain requirements Constraints on the system from the domain of operation*Chapter 4 Requirements engineeringFunctional requirementsFunctional requirementsDescribe functionality or system services. Depend on the type of software, expected users and the type of system where the software is used. Functional user requirements may be high-level statements of what the system should do. Functional system requirements should describe the system services in detail.*Chapter 4 Requirements engineeringFunctional requirements for the MHC-PMSFunctional requirements for the MHC-PMSA user shall be able to search the appointments lists for all clinics. The system shall generate each day, for each clinic, a list of patients who are expected to attend appointments that day. Each staff member using the system shall be uniquely identified by his or her 8-digit employee number. *Chapter 4 Requirements engineeringRequirements imprecisionRequirements imprecisionProblems arise when requirements are not precisely stated. Ambiguous requirements may be interpreted in different ways by developers and users. Consider the term ‘search’ in requirement 1 User intention – search for a patient name across all appointments in all clinics; Developer interpretation – search for a patient name in an individual clinic. User chooses clinic then search.*Chapter 4 Requirements engineeringRequirements completeness and consistencyRequirements completeness and consistencyIn principle, requirements should be both complete and consistent. Complete They should include descriptions of all facilities required. Consistent There should be no conflicts or contradictions in the descriptions of the system facilities. In practice, it is impossible to produce a complete and consistent requirements document.*Chapter 4 Requirements engineeringNon-functional requirementsNon-functional requirementsThese define system properties and constraints e.g. reliability, response time and storage requirements. Constraints are I/O device capability, system representations, etc. Process requirements may also be specified mandating a particular IDE, programming language or development method. Non-functional requirements may be more critical than functional requirements. If these are not met, the system may be useless.*Chapter 4 Requirements engineeringTypes of nonfunctional requirement Types of nonfunctional requirement *Chapter 4 Requirements engineeringNon-functional requirements implementationNon-functional requirements implementationNon-functional requirements may affect the overall architecture of a system rather than the individual components. For example, to ensure that performance requirements are met, you may have to organize the system to minimize communications between components. A single non-functional requirement, such as a security requirement, may generate a number of related functional requirements that define system services that are required. It may also generate requirements that restrict existing requirements. *Chapter 4 Requirements engineeringNon-functional classificationsNon-functional classificationsProduct requirements Requirements which specify that the delivered product must behave in a particular way e.g. execution speed, reliability, etc. Organisational requirements Requirements which are a consequence of organisational policies and procedures e.g. process standards used, implementation requirements, etc. External requirements Requirements which arise from factors which are external to the system and its development process e.g. interoperability requirements, legislative requirements, etc.*Chapter 4 Requirements engineeringExamples of nonfunctional requirements in the MHC-PMS Examples of nonfunctional requirements in the MHC-PMS *Chapter 4 Requirements engineeringGoals and requirementsGoals and requirementsNon-functional requirements may be very difficult to state precisely and imprecise requirements may be difficult to verify. Goal A general intention of the user such as ease of use. Verifiable non-functional requirement A statement using some measure that can be objectively tested. Goals are helpful to developers as they convey the intentions of the system users.*Chapter 4 Requirements engineeringUsability requirementsUsability requirementsThe system should be easy to use by medical staff and should be organized in such a way that user errors are minimized. (Goal) Medical staff shall be able to use all the system functions after four hours of training. After this training, the average number of errors made by experienced users shall not exceed two per hour of system use. (Testable non-functional requirement) *Chapter 4 Requirements engineeringMetrics for specifying nonfunctional requirementsMetrics for specifying nonfunctional requirements*Chapter 4 Requirements engineeringDomain requirementsDomain requirementsThe system’s operational domain imposes requirements on the system. For example, a train control system has to take into account the braking characteristics in different weather conditions. Domain requirements be new functional requirements, constraints on existing requirements or define specific computations. If domain requirements are not satisfied, the system may be unworkable.*Chapter 4 Requirements engineeringTrain protection systemTrain protection systemThis is a domain requirement for a train protection system: The deceleration of the train shall be computed as: Dtrain = Dcontrol + Dgradient where Dgradient is 9.81ms2 * compensated gradient/alpha and where the values of 9.81ms2 /alpha are known for different types of train. It is difficult for a non-specialist to understand the implications of this and how it interacts with other requirements.*Chapter 4 Requirements engineeringDomain requirements problemsDomain requirements problemsUnderstandability Requirements are expressed in the language of the application domain; This is often not understood by software engineers developing the system. Implicitness Domain specialists understand the area so well that they do not think of making the domain requirements explicit.*Chapter 4 Requirements engineeringThe software requirements documentThe software requirements documentThe software requirements document is the official statement of what is required of the system developers. Should include both a definition of user requirements and a specification of the system requirements. It is NOT a design document. As far as possible, it should set of WHAT the system should do rather than HOW it should do it.*Chapter 4 Requirements engineeringAgile methods and requirementsAgile methods and requirementsMany agile methods argue that producing a requirements document is a waste of time as requirements change so quickly. The document is therefore always out of date. Methods such as XP use incremental requirements engineering and express requirements as ‘user stories’ (discussed in Chapter 3). This is practical for business systems but problematic for systems that require a lot of pre-delivery analysis (e.g. critical systems) or systems developed by several teams.*Chapter 4 Requirements engineeringUsers of a requirements document Users of a requirements document *Chapter 4 Requirements engineeringRequirements document variabilityRequirements document variabilityInformation in requirements document depends on type of system and the approach to development used. Systems developed incrementally will, typically, have less detail in the requirements document. Requirements documents standards have been designed e.g. IEEE standard. These are mostly applicable to the requirements for large systems engineering projects.Chapter 4 Requirements engineering*The structure of a requirements document The structure of a requirements document *Chapter 4 Requirements engineeringThe structure of a requirements document The structure of a requirements document *Chapter 4 Requirements engineeringRequirements specificationRequirements specificationThe process of writing don the user and system requirements in a requirements document. User requirements have to be understandable by end-users and customers who do not have a technical background. System requirements are more detailed requirements and may include more technical information. The requirements may be part of a contract for the system development It is therefore important that these are as complete as possible.Chapter 4 Requirements engineering*Ways of writing a system requirements specification Ways of writing a system requirements specification *Chapter 4 Requirements engineeringRequirements and designRequirements and designIn principle, requirements should state what the system should do and the design should describe how it does this. In practice, requirements and design are inseparable A system architecture may be designed to structure the requirements; The system may inter-operate with other systems that generate design requirements; The use of a specific architecture to satisfy non-functional requirements may be a domain requirement. Natural language specificationNatural language specificationRequirements are written as natural language sentences supplemented by diagrams and tables. Used for writing requirements because it is expressive, intuitive and universal. This means that the requirements can be understood by users and customers.Chapter 4 Requirements engineering*Guidelines for writing requirementsGuidelines for writing requirementsInvent a standard format and use it for all requirements. Use language in a consistent way. Use shall for mandatory requirements, should for desirable requirements. Use text highlighting to identify key parts of the requirement. Avoid the use of computer jargon. Include an explanation (rationale) of why a requirement is necessary.Problems with natural languageProblems with natural languageLack of clarity Precision is difficult without making the document difficult to read. Requirements confusion Functional and non-functional requirements tend to be mixed-up. Requirements amalgamation Several different requirements may be expressed together.Example requirements for the insulin pump software system Example requirements for the insulin pump software system *Chapter 4 Requirements engineeringStructured specificationsStructured specificationsAn approach to writing requirements where the freedom of the requirements writer is limited and requirements are written in a standard way. This works well for some types of requirements e.g. requirements for embedded control system but is sometimes too rigid for writing business system requirements.Chapter 4 Requirements engineering*Form-based specificationsForm-based specificationsDefinition of the function or entity. Description of inputs and where they come from. Description of outputs and where they go to. Information about the information needed for the computation and other entities used. Description of the action to be taken. Pre and post conditions (if appropriate). The side effects (if any) of the function.A structured specification of a requirement for an insulin pump A structured specification of a requirement for an insulin pump *Chapter 4 Requirements engineeringPage 97 Tabular specificationTabular specificationUsed to supplement natural language. Particularly useful when you have to define a number of possible alternative courses of action. For example, the insulin pump systems bases its computations on the rate of change of blood sugar level and the tabular specification explains how to calculate the insulin requirement for different scenarios.Tabular specification of computation for an insulin pump Tabular specification of computation for an insulin pump *Chapter 4 Requirements engineeringRequirements engineering processesRequirements engineering processesThe processes used for RE vary widely depending on the application domain, the people involved and the organisation developing the requirements. However, there are a number of generic activities common to all processes Requirements elicitation; Requirements analysis; Requirements validation; Requirements management. In practice, RE is an iterative activity in which these processes are interleaved.*Chapter 4 Requirements engineeringA spiral view of the requirements engineering process A spiral view of the requirements engineering process *Chapter 4 Requirements engineeringRequirements elicitation and analysisRequirements elicitation and analysisSometimes called requirements elicitation or requirements discovery. Involves technical staff working with customers to find out about the application domain, the services that the system should provide and the system’s operational constraints. May involve end-users, managers, engineers involved in maintenance, domain experts, trade unions, etc. These are called stakeholders.*Chapter 4 Requirements engineeringRequirements elicitation and analysisRequirements elicitation and analysisSoftware engineers work with a range of system stakeholders to find out about the application domain, the services that the system should provide, the required system performance, hardware constraints, other systems, etc. Stages include: Requirements discovery, Requirements classification and organization, Requirements prioritization and negotiation, Requirements specification. Chapter 4 Requirements engineering*The requirements elicitation and analysis process The requirements elicitation and analysis process *Chapter 4 Requirements engineeringProcess activitiesProcess activitiesRequirements discovery Interacting with stakeholders to discover their requirements. Domain requirements are also discovered at this stage. Requirements classification and organisation Groups related requirements and organises them into coherent clusters. Prioritisation and negotiation Prioritising requirements and resolving requirements conflicts. Requirements specification Requirements are documented and input into the next round of the spiral.Problems of requirements elicitationProblems of requirements elicitationStakeholders don’t know what they really want. Stakeholders express requirements in their own terms. Different stakeholders may have conflicting requirements. Organisational and political factors may influence the system requirements. The requirements change during the analysis process. New stakeholders may emerge and the business environment change.Requirements discoveryRequirements discoveryThe process of gathering information about the required and existing systems and distilling the user and system requirements from this information. Interaction is with system stakeholders from managers to external regulators. Systems normally have a range of stakeholders.Chapter 4 Requirements engineering*Stakeholders in the MHC-PMSStakeholders in the MHC-PMSPatients whose information is recorded in the system. Doctors who are responsible for assessing and treating patients. Nurses who coordinate the consultations with doctors and administer some treatments. Medical receptionists who manage patients’ appointments. IT staff who are responsible for installing and maintaining the system. Chapter 4 Requirements engineering*Stakeholders in the MHC-PMSStakeholders in the MHC-PMSA medical ethics manager who must ensure that the system meets current ethical guidelines for patient care. Health care managers who obtain management information from the system. Medical records staff who are responsible for ensuring that system information can be maintained and preserved, and that record keeping procedures have been properly implemented. Chapter 4 Requirements engineering*InterviewingInterviewingFormal or informal interviews with stakeholders are part of most RE processes. Types of interview Closed interviews based on pre-determined list of questions Open interviews where various issues are explored with stakeholders. Effective interviewing Be open-minded, avoid pre-conceived ideas about the requirements and are willing to listen to stakeholders. Prompt the interviewee to get discussions going using a springboard question, a requirements proposal, or by working together on a prototype system. Chapter 4 Requirements engineering*Interviews in practiceInterviews in practiceNormally a mix of closed and open-ended interviewing. Interviews are good for getting an overall understanding of what stakeholders do and how they might interact with the system. Interviews are not good for understanding domain requirements Requirements engineers cannot understand specific domain terminology; Some domain knowledge is so familiar that people find it hard to articulate or think that it isn’t worth articulating.ScenariosScenariosScenarios are real-life examples of how a system can be used. They should include A description of the starting situation; A description of the normal flow of