Requirements Engineering: Processes, Techniques, and Challenges, Exams of Requirements Engineering

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Chapter 4 – Requirements Engineering

Lecture 1

Topics covered ² Functional and non-functional requirements ² The software requirements document ² Requirements specification ² Requirements engineering processes ² Requirements elicitation and analysis ² Requirements validation ² Requirements management

What is a requirement? ² It may range from a high-level abstract statement of a service or of a system constraint to a detailed mathematical functional specification. ² 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.

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.”

User and system requirements

Readers of different types of requirements specification

Functional requirements ² Describe functionality or system services. ² Depend on the type of software, expected users and the type of system where the software is used. ² Functional user requirements may be high-level statements of what the system should do. ² Functional system requirements should describe the system services in detail.

Functional requirements for the MHC-PMS ² A user shall be able to search the appointments lists for all clinics. ² The system shall generate each day, for each clinic, a list of patients who are expected to attend appointments that day. ² Each staff member using the system shall be uniquely identified by his or her 8-digit employee number.

Requirements completeness and consistency ² In principle, requirements should be both complete and consistent. ² Complete § They should include descriptions of all facilities required. ² Consistent § There should be no conflicts or contradictions in the descriptions of the system facilities. ² In practice, it is impossible to produce a complete and consistent requirements document.

Non-functional requirements ² These define system properties and constraints e.g. reliability, response time and storage requirements. Constraints are I/O device capability, system representations, etc. ² Process requirements may also be specified mandating a particular IDE, programming language or development method. ² Non-functional requirements may be more critical than functional requirements. If these are not met, the system may be useless.

Non-functional requirements implementation ² 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. ² 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.

Non-functional classifications ² Product requirements § Requirements which specify that the delivered product must behave in a particular way e.g. execution speed, reliability, etc. ² Organisational requirements § Requirements which are a consequence of organisational policies and procedures e.g. process standards used, implementation requirements, etc. ² 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 ² Non-functional requirements may be very difficult to state precisely and imprecise requirements may be difficult to verify. ² Goal § A general intention of the user such as ease of use. ² Verifiable non-functional requirement § A statement using some measure that can be objectively tested. ² Goals are helpful to developers as they convey the intentions of the system users.

Usability requirements ² The system should be easy to use by medical staff and should be organized in such a way that user errors are minimized. (Goal) ² 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)