Chapter 4 – Requirements Engineering Lecture 1 The hardest part of the software task is arriving at a complete and consistent specification, and much of the essence of building a program is in fact the debugging of the specification. F. Brooks Computers do not solve problems -computers carry out solutions, specified by people, to problems. D. D. Spencer 1Chapter 4 Requirements engineering

Where we are  Requirements Chapter 4 Requirements engineering2 Requirements analysis Requirements analysis though we will be following an agile process rather than a waterfall model, we will be going through these phases.

Topics covered  Functional and non-functional requirements  The software requirements document  Requirements specification  Requirements engineering processes  Requirements elicitation and analysis  Requirements validation  Requirements management 3Chapter 4 Requirements engineering

Requirements engineering  The process of establishing  the services that the customer requires from a system and  the constraints under which it operates and is developed. 4Chapter 4 Requirements engineering

What is a requirement?  description of the system services or constraints  range from  a high-level abstract statement of a service or system constraint to  a detailed mathematical functional specification.  requirements serve a dual function  the basis for a bid for a contract - therefore must be open to interpretation;  the basis for the contract itself - therefore must be defined in detail; 5Chapter 4 Requirements engineering

Types of requirement  User requirements  Statements in natural language from the users viewpoint.  Written for customers or users of the system.  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.  Written for developers. 6Chapter 4 Requirements engineering

Readers of different types of requirements specification 7Chapter 4 Requirements engineering

User and system requirements 8Chapter 4 Requirements engineering what’s the difference?

Functional and non-functional requirements  Both User and System requirements have both functional and non-functional requirements.  Functional requirements  Statements of services the system should provide,  how the system should react to particular inputs  how the system should behave in particular situations.  May state what the system should not do. 9Chapter 4 Requirements engineering

Functional and non-functional requirements  Non-functional requirements  Constraints on the services or functions offered by the system 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 user area of operation 10Chapter 4 Requirements engineering

Functional requirements  Describe functionality or system services.  Depend on  type of software,  expected users  type of system where the software is used.  Functional user requirements  high-level statements of what the system should do.  Functional system requirements  describe the system services in detail. 11Chapter 4 Requirements engineering

Functional requirements for the MHC-PMS  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. 12Chapter 4 Requirements engineering

Requirements ambiguity  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  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. 13Chapter 4 Requirements engineering

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 impossible 14Chapter 4 Requirements engineering

Non-functional requirements  These define system properties and constraints  reliability  response time  storage requirements  I/O device capability  system representations  Process requirements  mandating a particular IDE  programming language  development method 15Chapter 4 Requirements engineering

Non-functional requirements implementation  affects overall architecture of a system  example: performance may have to organize the system to minimize communications between components.  One non-functional requirement may generate many functional requirements  e.g., a security requirement many ways/places in a system to implement security 16Chapter 4 Requirements engineering

Non-functional classifications  Product requirements  the delivered product must behave in a particular way e.g. execution speed, reliability, etc.  Organisational requirements  a consequence of organisational policies and procedures  e.g. process standards used, implementation requirements, etc.  External requirements  arise from factors which are external to the system and its development process  e.g. interoperability requirements, legislative requirements, etc. 17Chapter 4 Requirements engineering

Types of nonfunctional requirement 18Chapter 4 Requirements engineering

Examples of nonfunctional requirements in the MHC-PMS 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. 19Chapter 4 Requirements engineering

Writing non-functional requirements  Even at user level, non-functional requirements must be testable.  Non-testable user requirement: The system should be easy to use by medical staff and should be organized in such a way that user errors are minimized.  Testable user requirement: 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. 20Chapter 4 Requirements engineering could the testable user requirement be used as a system requirement without changing it?

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 21Chapter 4 Requirements engineering

Domain requirements  The 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 may be new functional requirements, constraints on existing requirements or define specific computations.  If domain requirements are not satisfied, the system may be unworkable. 22Chapter 4 Requirements engineering

Train protection system  This 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. 23Chapter 4 Requirements engineering

Domain requirements problems  Understandability  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. 24Chapter 4 Requirements engineering

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. 25Chapter 4 Requirements engineering