Module 1 - Requirements Engineering PDF

MODULE 1 – REQUIREMENTS ENGINEERING 9/3/24 Chapter 4 Requirements Engineering 1 GOSPEL OF THE DAY 9/3/24 Chapter 4 Requirements Engineering 2 COURSE LEARNING OUTCOME Evaluate software requirements and appraise the processes involved in discovering and documenting these requi...

MODULE 1 – REQUIREMENTS ENGINEERING 9/3/24 Chapter 4 Requirements Engineering 1 GOSPEL OF THE DAY 9/3/24 Chapter 4 Requirements Engineering 2 COURSE LEARNING OUTCOME Evaluate software requirements and appraise the processes involved in discovering and documenting these requirements. 9/3/24 Chapter 4 Requirements Engineering 3 TOPIC LEARNING OUTCOMES 1. Evaluate the concepts of user and system requirements and why these requirements should be written in different ways. 2. Value the differences between functional and nonfunctional software requirements. 3. Appraise the main requirements engineering activities of elicitation, analysis, and validation, and the relationships between these activities. 4. Weigh why requirements management is necessary and how it supports other requirements engineering activities. 9/3/24 Chapter 4 Requirements Engineering 4 Topics covered Functional and non-functional requirements Requirements engineering processes Requirements elicitation Requirements specification Requirements validation Requirements change 9/3/24 Chapter 4 Requirements Engineering 5 Requirements engineering The process of establishing the services that a customer requires from a system and the constraints under which it operates and is developed. The system requirements are the descriptions of the system services and constraints that are generated during the requirements engineering process. 9/3/24 Chapter 4 Requirements Engineering 6 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. May be the basis for a bid for a contract - therefore must be open to This is inevitable as interpretation; requirements may serve a May be the basis for the contract itself - therefore must be defined in detail; dual function Both these statements may be called requirements. 9/3/24 Chapter 4 Requirements Engineering 7 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.” 9/3/24 Chapter 4 Requirements Engineering 8 Types of requirement User 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. 9/3/24 Chapter 4 Requirements Engineering 9 User and system requirements User requirements definition 1. The Mentcare system shall generate monthly management reports showing the cost of drugs prescribed by each clinic during that month. System requirements specification 1.1 On the last working day of each month, a summary of the drugs prescribed, their cost and the prescribing clinics shall be generated. 1.2 The system shall generate the report for printing after 17.30 on the last working day of the month. 1.3 A report shall be created for each clinic and shall list the individual drug names, the total number of prescriptions, the number of doses prescribed and the total cost of the prescribed drugs. 1.4 If drugs are available in different dose units (e.g. 10mg, 20mg, etc) separate reports shall be created for each dose unit. 1.5 Access to drug cost reports shall be restricted to authorized users as listed on a management access control list. 9/3/24 Chapter 4 Requirements Engineering 10 Readers of different types of requirements specification Client managers System end-users User Client engineers requirements Contractor managers System architects System end-users System Client engineers requirements System architects Software developers 9/3/24 Chapter 4 Requirements Engineering 11 System stakeholders Any person or organization who is affected by the system in some way and so who has a legitimate interest End users System managers Stakeholder types System owners External stakeholders 9/3/24 Chapter 4 Requirements Engineering 12 Stakeholders in the Mentcare system Patients 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. 9/3/24 Chapter 4 Requirements Engineering 13 Agile methods and requirements Many agile methods argue that producing detailed system requirements is a waste of time as requirements change so quickly. The requirements document is therefore always out of date. Agile methods usually use incremental requirements engineering and may express requirements as ‘user stories’ (discussed in Chapter 3). This is practical for business systems but problematic for systems that require pre-delivery analysis (e.g. critical systems) or systems developed by several teams. 9/3/24 Chapter 4 Requirements Engineering 14 Stakeholders in the Mentcare system A 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. 9/3/24 Chapter 4 Requirements Engineering 15  FUNCTIONAL AND NON- FUNCTIONAL REQUIREMENTS 9/3/24 Chapter 4 Requirements Engineering 16 Functional and non-functional requirements Functional 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 9/3/24 Chapter 4 Requirements Engineering 17 Functional requirements Describe 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. 9/3/24 Chapter 4 Requirements Engineering 18 Mentcare system: functional requirements A 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. 9/3/24 Chapter 4 Requirements Engineering 19 Requirements imprecision Problems arise when functional 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. 9/3/24 Chapter 4 Requirements Engineering 20 Requirements completeness and consistency In principle, requirements should be both complete and consistent. They should include Complete descriptions of all facilities required. There should be no conflicts or Consistent contradictions in the descriptions of the system facilities. In practice, because of system and environmental complexity, it is impossible to produce a complete and consistent requirements document. 9/3/24 Chapter 4 Requirements Engineering 21 Non-functional requirements These 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. 9/3/24 Chapter 4 Requirements Engineering 22 Types of nonfunctional requirement Non-functional requirements Product Organizational External requirements requirements requirements Efficiency Dependability Security Regulatory Ethical requirements requirements requirements requirements requirements Usability Environmental Operational Development Legislative requirements requirements requirements requirements requirements Performance Space Accounting Safety/security requirements requirements requirements requirements 9/3/24 Chapter 4 Requirements Engineering 23 Non-functional requirements implementation Non-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. 9/3/24 Chapter 4 Requirements Engineering 24 Non-functional classifications Product 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. 9/3/24 Chapter 4 Requirements Engineering 25 Examples of nonfunctional requirements in the Mentcare system Product requirement The Mentcare system shall be available to all clinics during normal working hours (Mon–Fri, 0830–17.30). Downtime within normal working hours shall not exceed five seconds in any one day. Organizational requirement Users of the Mentcare system shall authenticate themselves using their health authority identity card. External requirement The system shall implement patient privacy provisions as set out in HStan-03-2006-priv. 9/3/24 Chapter 4 Requirements Engineering 26 Goals and requirements Non-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. 9/3/24 Chapter 4 Requirements Engineering 27 Usability requirements The 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) 9/3/24 Chapter 4 Requirements Engineering 28 Metrics for specifying nonfunctional requirements Property Measure Speed Processed transactions/second User/event response time Screen refresh time Size Mbytes Number of ROM chips Ease of use Training time Number of help frames Reliability Mean time to failure Probability of unavailability Rate of failure occurrence Availability Robustness Time to restart after failure Percentage of events causing failure Probability of data corruption on failure Portability Percentage of target dependent statements Number of target systems 9/3/24 Chapter 4 Requirements Engineering 29 REQUIREMENTS ENGINEERING PROCESSES 9/3/24 Chapter 4 Requirements Engineering 30 Requirements engineering processes The processes used for RE vary widely depending on the application domain, the people involved and the organisation developing the requirements. Requirements elicitation; However, there are a Requirements analysis; number of generic activities Requirements validation; common to all processes Requirements management. In practice, RE is an iterative activity in which these processes are interleaved. 9/3/24 Chapter 4 Requirements Engineering 31 A spiral view of the requirements engineering process Requirements specification System requirements specification and modeling User requirements specification Business requirements specification Start Feasibility System study Requirements req. Requirements elicitation elicitation User validation requirements elicitation Prototyping Reviews System requirements document 9/3/24 Chapter 4 Requirements Engineering 32  REQUIREMENTS ELICITATION 9/3/24 Chapter 4 Requirements Engineering 33 Requirements elicitation and analysis Sometimes 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. 9/3/24 Chapter 4 Requirements Engineering 34  REQUIREMENTS ELICITATION 9/3/24 Chapter 4 Requirements Engineering 35 Requirements elicitation Software 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. Requirements discovery, Requirements classification and organization, Stages include: Requirements prioritization and negotiation, Requirements specification. 9/3/24 Chapter 4 Requirements Engineering 36 Problems of requirements elicitation Stakeholders 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 may change. 9/3/24 Chapter 4 Requirements Engineering 37 The requirements elicitation and analysis process 1. Requirements discovery 4. Requirements 2. Requirements specification classification and organization 3. Requirements prioritization and negotiation 9/3/24 Chapter 4 Requirements Engineering 38 Process activities Requirements 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. 9/3/24 Chapter 4 Requirements Engineering 39 Requirements discovery The 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. 9/3/24 Chapter 4 Requirements Engineering 40 Interviewing Formal or informal interviews with stakeholders are part of most RE processes. Closed interviews based on pre- determined list of questions Types of interview Open interviews where various issues are explored with stakeholders. Be open-minded, avoid pre-conceived ideas about the requirements and are willing to listen to stakeholders. Effective interviewing Prompt the interviewee to get discussions going using a springboard question, a requirements proposal, or by working together on a prototype system. 9/3/24 Chapter 4 Requirements Engineering 41 Interviews in practice Normally 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. Interviewers need to be open-minded without pre-conceived ideas of what the system should do You need to prompt the use to talk about the system by suggesting requirements rather than simply asking them what they want. 9/3/24 Chapter 4 Requirements Engineering 42 Problems with interviews Application specialists may use language to describe their work that isn’t easy for the requirements engineer to understand. Requirements engineers cannot Interviews are not good understand specific domain terminology; for understanding domain Some domain knowledge is so familiar that people find it hard to requirements articulate or think that it isn’t worth articulating. 9/3/24 Chapter 4 Requirements Engineering 43 Ethnography A social scientist spends a People do not considerable have to explain or time observing articulate their and analysing work. how people actually work. Ethnographic Social and studies have shown that work is organisational usually richer and factors of more complex importance may than suggested by be observed. simple system models. 9/3/24 Chapter 4 Requirements Engineering 44 Scope of ethnography Requirements that are derived from the way that people actually work rather than the way I which process definitions suggest that they ought to work. Requirements that are derived Awareness of what other people from cooperation and awareness are doing leads to changes in the of other people’s activities. ways in which we do things. Ethnography is effective for understanding existing processes but cannot identify new features that should be added to a system. 9/3/24 Chapter 4 Requirements Engineering 45 Focused ethnography Developed in a project studying the air traffic control process Combines ethnography with prototyping Prototype development results in unanswered questions which focus the ethnographic analysis. The problem with ethnography is that it studies existing practices which may have some historical basis which is no longer relevant. 9/3/24 Chapter 4 Requirements Engineering 46 Ethnography and prototyping for requirements analysis Ethnographic Debriefing Focused analysis meetings ethnography Prototype evaluation Generic system System development protoyping 9/3/24 Chapter 4 Requirements Engineering 47 Stories and scenarios Scenarios and user stories are real-life examples of how a system can be used. Stories and scenarios are a description of how a system may be used for a particular task. Because they are based on a practical situation, stakeholders can relate to them and can comment on their situation with respect to the story. 9/3/24 Chapter 4 Requirements Engineering 48 Photo sharing in the classroom (iLearn) ² Jack is a primary school teacher in Ullapool (a village in northern Scotland). He has decided that a class project should be focused around the fishing industry in the area, looking at the history, development and economic impact of fishing. As part of this, pupils are asked to gather and share reminiscences from relatives, use newspaper archives and collect old photographs related to fishing and fishing communities in the area. Pupils use an iLearn wiki to gather together fishing stories and SCRAN (a history resources site) to access newspaper archives and photographs. However, Jack also needs a photo sharing site as he wants pupils to take and comment on each others’ photos and to upload scans of old photographs that they may have in their families. Jack sends an email to a primary school teachers group, which he is a member of to see if anyone can recommend an appropriate system. Two teachers reply and both suggest that he uses KidsTakePics, a photo sharing site that allows teachers to check and moderate content. As KidsTakePics is not integrated with the iLearn authentication service, he sets up a teacher and a class account. He uses the iLearn setup service to add KidsTakePics to the services seen by the pupils in his class so that when they log in, they can immediately use the system to upload photos from their mobile devices and class computers. 9/3/24 Chapter 4 Requirements Engineering 49 Scenarios A structured form of user story Scenarios should include A description of the starting situation; A description of the normal flow of events; A description of what can go wrong; Information about other concurrent activities; A description of the state when the scenario finishes. 9/3/24 Chapter 4 Requirements Engineering 50 Uploading photos iLearn Initial assumption: A user or a group of users have one or more digital photographs to be uploaded to the picture sharing site. These are saved on either a tablet or laptop computer. They have successfully logged on to KidsTakePics. Normal: The user chooses upload photos and they are prompted to select the photos to be uploaded on their computer and to select the project name under which the photos will be stored. They should also be given the option of inputting keywords that should be associated with each uploaded photo. Uploaded photos are named by creating a conjunction of the user name with the filename of the photo on the local computer. On completion of the upload, the system automatically sends an email to the project moderator asking them to check new content and generates an on-screen message to the user that this has been done. 9/3/24 Chapter 4 Requirements Engineering 51 Uploading photos What can go wrong: No moderator is associated with the selected project. An email is automatically generated to the school administrator asking them to nominate a project moderator. Users should be informed that there could be a delay in making their photos visible. Photos with the same name have already been uploaded by the same user. The user should be asked if they wish to re-upload the photos with the same name, rename the photos or cancel the upload. If they chose to re-upload the photos, the originals are overwritten. If they chose to rename the photos, a new name is automatically generated by adding a number to the existing file name. Other activities: The moderator may be logged on to the system and may approve photos as they are uploaded. System state on completion: User is logged on. The selected photos have been uploaded and assigned a status ‘awaiting moderation’. Photos are visible to the moderator and to the user who uploaded them. 9/3/24 Chapter 4 Requirements Engineering 52  REQUIREMENTS SPECIFICATION 9/3/24 Chapter 4 Requirements Engineering 53 Requirements specification ² The process of writing down the user and system requirements in a requirements document. ² User requirements have to be understandable by end- users and customers who do not have a technical background. ² System requirements are more detailed requirements and may include more technical information. ² The requirements may be part of a contract for the system development § It is therefore important that these are as complete as possible. 9/3/24 Chapter 4 Requirements Engineering 54 Ways of writing a system requirements specification Notation Description Natural language The requirements are written using numbered sentences in natural language. Each sentence should express one requirement. Structured natural The requirements are written in natural language on a standard form or language template. Each field provides information about an aspect of the requirement. Design description This approach uses a language like a programming language, but with more languages abstract features to specify the requirements by defining an operational model of the system. This approach is now rarely used although it can be useful for interface specifications. Graphical notations Graphical models, supplemented by text annotations, are used to define the functional requirements for the system; UML use case and sequence diagrams are commonly used. Mathematical These notations are based on mathematical concepts such as finite-state specifications machines or sets. Although these unambiguous specifications can reduce the ambiguity in a requirements document, most customers don’t understand a formal specification. They cannot check that it represents what they want and are reluctant to accept it as a system contract 9/3/24 Chapter 4 Requirements Engineering 55 Requirements and design In 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. This may be the consequence of a regulatory requirement. 9/3/24 Chapter 4 Requirements Engineering 56 Natural language specification Used for writing requirements Requirements are written as because it is expressive, natural language sentences intuitive and universal. This supplemented by diagrams and means that the requirements tables. can be understood by users and customers. 9/3/24 Chapter 4 Requirements Engineering 57 Guidelines for writing requirements Invent 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. 9/3/24 Chapter 4 Requirements Engineering 58 Problems with natural language Precision is difficult without making Lack of clarity the document difficult to read. Requirements Functional and non-functional confusion requirements tend to be mixed-up. Requirements Several different requirements may amalgamation be expressed together. 9/3/24 Chapter 4 Requirements Engineering 59 Example requirements for the insulin pump software system 3.2 The system shall measure the blood sugar and deliver insulin, if required, every 10 minutes. (Changes in blood sugar are relatively slow so more frequent measurement is unnecessary; less frequent measurement could lead to unnecessarily high sugar levels.) 3.6 The system shall run a self-test routine every minute with the conditions to be tested and the associated actions defined in Table 1. (A self-test routine can discover hardware and software problems and alert the user to the fact the normal operation may be impossible.) 9/3/24 Chapter 4 Requirements Engineering 60 Structured specifications An 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. 9/3/24 Chapter 4 Requirements Engineering 61 Form-based specifications Definition 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. 9/3/24 Chapter 4 Requirements Engineering 62 A structured specification of a requirement for an insulin pump 9/3/24 Chapter 4 Requirements Engineering 63 A structured specification of a requirement for an insulin pump 9/3/24 Chapter 4 Requirements Engineering 64 Tabular specification Used 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. 9/3/24 Chapter 4 Requirements Engineering 65 Tabular specification of computation for an insulin pump Condition Action Sugar level falling (r2 < r1) CompDose = 0 Sugar level stable (r2 = r1) CompDose = 0 Sugar level increasing and rate of increase CompDose = 0 decreasing ((r2 – r1) < (r1 – r0)) Sugar level increasing and rate of increase CompDose = stable or increasing round ((r2 – r1)/4) ((r2 – r1) ≥ (r1 – r0)) If rounded result = 0 then CompDose = MinimumDose 9/3/24 Chapter 4 Requirements Engineering 66 Use cases Use-cases are a kind of scenario that are included in the UML. Use cases identify the actors in an interaction and which describe the interaction itself. A set of use cases should describe all possible interactions with the system. High-level graphical model supplemented by more detailed tabular description (see Chapter 5). UML sequence diagrams may be used to add detail to use-cases by showing the sequence of event processing in the system. 9/3/24 Chapter 4 Requirements Engineering 67 The software requirements document The 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. 9/3/24 Chapter 4 Requirements Engineering 68 Use cases for the Mentcare system Register Export patient statistics View Manager personal info. Generate Medical receptionist report View record Nurse Doctor Edit record Setup consultation 9/3/24 Chapter 4 Requirements Engineering 69 Users of a requirements document Specify the requirements and System read them to check that they customers meet their needs. Customers specify changes to the requirements. Use the requirements Managers document to plan a bid for the system and to plan the system development process. System Use the requirements to engineers understand what system is to be developed. System test Use the requirements to engineers develop validation tests for the system. System Use the requirements to maintenance understand the system and engineers the relationships between its parts. 9/3/24 Chapter 4 Requirements Engineering 70 Requirements document variability Information 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. 9/3/24 Chapter 4 Requirements Engineering 71 The structure of a requirements document Chapter Description Preface This should define the expected readership of the document and describe its version history, including a rationale for the creation of a new version and a summary of the changes made in each version. Introduction This should describe the need for the system. It should briefly describe the system’s functions and explain how it will work with other systems. It should also describe how the system fits into the overall business or strategic objectives of the organization commissioning the software. Glossary This should define the technical terms used in the document. You should not make assumptions about the experience or expertise of the reader. User requirements Here, you describe the services provided for the user. The nonfunctional definition system requirements should also be described in this section. This description may use natural language, diagrams, or other notations that are understandable to customers. Product and process standards that must be followed should be specified. System architecture This chapter should present a high-level overview of the anticipated system architecture, showing the distribution of functions across system modules. Architectural components that are reused should be highlighted. 9/3/24 Chapter 4 Requirements Engineering 72 The structure of a requirements document Chapter Description System This should describe the functional and nonfunctional requirements in more detail. requirements If necessary, further detail may also be added to the nonfunctional requirements. specification Interfaces to other systems may be defined. System models This might include graphical system models showing the relationships between the system components and the system and its environment. Examples of possible models are object models, data-flow models, or semantic data models. System evolution This should describe the fundamental assumptions on which the system is based, and any anticipated changes due to hardware evolution, changing user needs, and so on. This section is useful for system designers as it may help them avoid design decisions that would constrain likely future changes to the system. Appendices These should provide detailed, specific information that is related to the application being developed; for example, hardware and database descriptions. Hardware requirements define the minimal and optimal configurations for the system. Database requirements define the logical organization of the data used by the system and the relationships between data. Index Several indexes to the document may be included. As well as a normal alphabetic index, there may be an index of diagrams, an index of functions, and so on. 9/3/24 Chapter 4 Requirements Engineering 73  REQUIREMENTS VALIDATION 9/3/24 Chapter 4 Requirements Engineering 74 Requirements validation Concerned with demonstrating that the requirements define the system that the customer really wants. Requirements error Fixing a requirements costs are high so error after delivery may cost up to 100 times the validation is very cost of fixing an important implementation error. 9/3/24 Chapter 4 Requirements Engineering 75 Requirements checking Validity. Does the system provide the functions which best support the customer’s needs? Consistency. Are there any requirements conflicts? Completeness. Are all functions required by the customer included? Realism. Can the requirements be implemented given available budget and technology Verifiability. Can the requirements be checked? 9/3/24 Chapter 4 Requirements Engineering 76 Requirements validation techniques Requirements Systematic manual analysis of the reviews requirements. Using an executable model of the Prototyping system to check requirements. Covered in Chapter 2. Test-case Developing tests for requirements generation to check testability. 9/3/24 Chapter 4 Requirements Engineering 77 Requirements reviews Regular reviews should be held while the requirements definition is being formulated. Both client and contractor staff should be involved in reviews. Reviews may be formal (with completed documents) or informal. Good communications between developers, customers and users can resolve problems at an early stage. 9/3/24 Chapter 4 Requirements Engineering 78 Review checks Is the requirement realistically Verifiability testable? Is the requirement properly Comprehensibility understood? Is the origin of the Traceability requirement clearly stated? Can the requirement be changed Adaptability without a large impact on other requirements? 9/3/24 Chapter 4 Requirements Engineering 79  REQUIREMENTS CHANGE 9/3/24 Chapter 4 Requirements Engineering 80 Changing requirements The business and technical The people who pay for a system environment of the system always and the users of that system are changes after installation. rarely the same people. New hardware may be System customers impose introduced, it may be necessary requirements because of to interface the system with organizational and budgetary other systems, business constraints. These may conflict priorities may change (with with end-user requirements consequent changes in the and, after delivery, new features system support required), and may have to be added for user new legislation and regulations support if the system is to meet may be introduced that the its goals. system must necessarily abide by. 9/3/24 Chapter 4 Requirements Engineering 81 Changing requirements Large systems usually have a diverse user community, with many users having different requirements and priorities that may be conflicting or contradictory. Ø The final system requirements are inevitably a compromise between them and, with experience, it is often discovered that the balance of support given to different users has to be changed. 9/3/24 Chapter 4 Requirements Engineering 82 Requirements evolution Initial Changed understanding understanding of problem of problem Initial Changed requirements requirements Time 9/3/24 Chapter 4 Requirements Engineering 83 Requirements management Requirements management is the process of managing changing requirements during the requirements engineering process and system development. New requirements emerge as a system is being developed and after it has gone into use. You need to keep track of individual requirements and maintain links between dependent requirements so that you can assess the impact of requirements changes. You need to establish a formal process for making change proposals and linking these to system requirements. 9/3/24 Chapter 4 Requirements Engineering 84 Requirements management planning Establishes the level of requirements management detail that is required. Requirements management decisions: Requirements identification Each requirement must be uniquely identified so that it can be cross-referenced with other requirements. A change management process This is the set of activities that assess the impact and cost of changes. I discuss this process in more detail in the following section. Traceability policies These policies define the relationships between each requirement and between the requirements and the system design that should be recorded. Tool support Tools that may be used range from specialist requirements management systems to spreadsheets and simple database systems. 9/3/24 Chapter 4 Requirements Engineering 85 Requirements change management ² Deciding if a requirements change should be accepted § Problem analysis and change specification During this stage, the problem or the change proposal is analyzed to check that it is valid. This analysis is fed back to the change requestor who may respond with a more specific requirements change proposal, or decide to withdraw the request. § Change analysis and costing The effect of the proposed change is assessed using traceability information and general knowledge of the system requirements. Once this analysis is completed, a decision is made whether or not to proceed with the requirements change. § Change implementation The requirements document and, where necessary, the system design and implementation, are modified. Ideally, the document should be organized so that changes can be easily implemented. 9/3/24 Chapter 4 Requirements Engineering 86  Requirements change management Identified Revised problem Problem analysis and Change analysis Change requirements change specification and costing implementation 9/3/24 Chapter 4 Requirements Engineering 87 Key points Requirements for a software system set out what the system should do and define constraints on its operation and implementation. Functional requirements are statements of the services that the system must provide or are descriptions of how some computations must be carried out. Non-functional requirements often constrain the system being developed and the development process being used. They often relate to the emergent properties of the system and therefore apply to the system as a whole. 9/3/24 Chapter 4 Requirements Engineering 88 Key points The requirements engineering process is an iterative process that includes requirements elicitation, specification and validation. Requirements elicitation is an iterative process that can be represented as a spiral of activities – requirements discovery, requirements classification and organization, requirements negotiation and requirements documentation. You can use a range of techniques for requirements elicitation including interviews and ethnography. User stories and scenarios may be used to facilitate discussions. 9/3/24 Chapter 4 Requirements Engineering 89 Key points Requirements specification is the process of formally documenting the user and system requirements and creating a software requirements document. The software requirements document is an agreed statement of the system requirements. It should be organized so that both system customers and software developers can use it. 9/3/24 Chapter 4 Requirements Engineering 90 Key points Requirements validation is the process of checking the requirements for validity, consistency, completeness, realism and verifiability. Business, organizational and technical changes inevitably lead to changes to the requirements for a software system. Requirements management is the process of managing and controlling these changes. 9/3/24 Chapter 4 Requirements Engineering 91