CS223: Software Engineering Lecture 6: Requirement Engineering

Recap Software development process model Iterative Model Timeboxing Model

Objective After the end of the class students should be able to o Define software requirement o Identify components of software requirement o Differentiate between functional and non-functional requirement

Introduction It is concerned with o Elicitation, o Analysis, o Specification, and o Validation of software requirements Management of requirements during the whole life cycle of the software product Software projects are critically vulnerable when the requirements related activities are poorly performed

Topics of discussion Software Requirement Process Model Management Quality ElicitationAnalysis Classification Design Negotiation Analysis Specification SRS Validation Review Verification Test Considerations Change management Tracing Measurement

Definition It is a property that must be exhibited by something To solve some problem in the real world A complex combination from various people at different levels of an organization Objects connected with this feature from the environment in which the software will operate

Few important terminologies Product and process requirement Functional and Nonfunctional Requirements Emergent requirement Quantifiable requirement Avoid vague and unverifiable requirements

System and Software requirement International Council on Software and Systems Engineering (INCOSE) Software (user) requirements o Statements, in a natural language plus diagrams o Services the system is expected to provide to system users and the constraints under which it must operate. System requirements o Detailed descriptions of the software system’s functions, services, and operational constraints. o Define exactly what is to be implemented (Functional specification)

Example User requirement specification Generate monthly report System requirement specification On the last working day of the month Automatically generate after hrs. Created for each office Restricted access to valid user

Different Users User Requirements Client managersSystem end-usersClient engineersContractor managersSystem Architects System Requirements System end-usersClient engineersSystem architectsSoftware developers

Functional and Non-functional requirements Functional o statements of services the system should provide o System reaction to a particular input o System behaviour at a particular situation o What a system should not do (optional) Non-Functional o Constraints on the services or functions offered by the system  timing constraints,  constraints on the development process,  constraints imposed by standards o Applicable to the entire system, not on parts

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.

A case study 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.

Requirements imprecision Problems 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 o User intention – search for a patient name across all appointments in all clinics; o Developer interpretation – search for a patient name in an individual clinic. User chooses clinic then search.

Requirements completeness and consistency Requirements should be both complete and consistent. Complete o They should include descriptions of all facilities required. Consistent o There should be no conflicts or contradictions in the descriptions of the system facilities. It is a challenging task to meet both the conditions

Non-functional requirements These define system properties and constraints o E.g. reliability, response time and storage requirements. o 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. o If these are not met, the system may be useless.

Types of nonfunctional requirement Non-functional Requirement Product Usability Efficiency Performance Space DependabilitySecurity Organizational Environmental Operational Development External RegulatoryEthicalLegislative Accounting Safety/ Security

Non-functional requirements implementation Non-functional requirements may affect the overall architecture of a system rather than the individual components. o Minimize communications between components. A single non-functional requirement may generate a number of related functional requirements that define system services that are required. o It may also generate requirements that restrict existing requirements.

Non-functional classifications Product requirements o Delivered product must behave in a particular way o e.g. execution speed, reliability, etc. Organisational requirements o Consequence of organisational policies and procedures o e.g. process standards used, implementation requirements, etc. External requirements o Requirements which arise from factors which are external to the system and its development process o e.g. interoperability requirements, legislative requirements, etc.

Examples of nonfunctional requirements Product requirement The MHC-PMS 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 MHC-PMS system shall authenticate themselves using their health authority identity card. External requirement The system shall implement patient privacy provisions as set out in HStan priv.

Goals and requirements Non-functional requirements may be very difficult to state precisely Imprecise requirements may be difficult to verify. Goal o A general intention of the user such as ease of use. Verifiable non-functional requirement o 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 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)

Metrics for specifying nonfunctional requirements PropertyMeasure SpeedProcessed transactions/second User/event response time Screen refresh time SizeMbytes Number of ROM chips Ease of useTraining time Number of help frames ReliabilityMean time to failure Probability of unavailability Rate of failure occurrence Availability RobustnessTime to restart after failure Percentage of events causing failure Probability of data corruption on failure PortabilityPercentage of target dependent statements Number of target systems

Domain requirements The system’s operational domain imposes requirements on the system. o 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.

Train protection system This is a domain requirement for a train protection system: The deceleration of the train shall be computed as: o Dtrain = Dcontrol + Dgradient o Dgradient is 9.81ms2 * compensated gradient/ alpha o 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.

Domain requirements problems Understandability o Requirements are expressed in the language of the application domain; o This is often not understood by software engineers developing the system. Implicitness o Domain specialists understand the area so well that they do not think of making the domain requirements explicit.

The software requirements document The software requirements document is the official statement of what is required of the system developers. Include both o A definition of user requirements o 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.

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 o statements of the services that the system must provide or are descriptions of how some computations must be carried out. Non-functional requirements o 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.

Thank you Next Lecture: Requirement Engineering