Chapter 4 – Requirements Engineering PDF
Document Details
2014
Ian Sommerville
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Summary
This document provides a detailed overview of the key concepts and principles involved in requirements engineering as part of software development. It also covers various types of requirements, such as functional and non-functional requirements.
Full Transcript
# Chapter 4 – Requirements Engineering - 30/10/2014 - Chapter 4 Requirements Engineering - 1 ## Topics covered - Functional and non-functional requirements - Requirements engineering processes - Requirements elicitation - Requirements specification - Requirements validation - Requirements change...
# Chapter 4 – Requirements Engineering - 30/10/2014 - Chapter 4 Requirements Engineering - 1 ## Topics covered - Functional and non-functional requirements - Requirements engineering processes - Requirements elicitation - Requirements specification - Requirements validation - Requirements change ## 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. - It may range from a high-level abstract statement of a service or of a system constraint to a detailed mathematical functional specification. ## 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. ## System stakeholders - Any person or organization who is affected by the system in some way and so who has a legitimate interest - **Stakeholder types** - End users - System managers - System owners - External stakeholders ## 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. - 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. ## 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 ## 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. ## 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. ## Requirements completeness and consistency - In 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, because of system and environmental complexity, it is impossible to produce a complete and consistent requirements document. ## 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. ## 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. ## 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. ## 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) ## Requirements engineering processes - The 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. ## 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.