Topic2 Requirements Engineering.pdf

Chapter 4 Requirements engineering REQUIREMENTS ENGINEERING 1 Chapter 4- by Ian Sommerville TOPICS COVERED  Functional and non-functional requirements  The software requirements document Chapt...

Chapter 4 Requirements engineering REQUIREMENTS ENGINEERING 1 Chapter 4- by Ian Sommerville TOPICS COVERED  Functional and non-functional requirements  The software requirements document Chapter 4 Requirements engineering  Requirements specification  Requirements engineering processes  Requirements elicitation and analysis  Requirements validation  Requirements management 2 REQUIREMENTS ENGINEERING  The process of establishing the services that the customer requires from a system and the constraints under which it operates and is Chapter 4 Requirements engineering developed.  The requirements themselves are the descriptions of the system services and constraints that are generated during the requirements engineering process. 3 WHAT IS A REQUIREMENT?  A software requirement is a rule the software must conform to: what it must do, how well, and within what constraints or limits. Chapter 4 Requirements engineering  Types of Requirements  Functional requirements  Nonfunctional requirements.  Domain Requirements 4 FUNCTIONAL AND NON-FUNCTIONAL REQUIREMENTS  Functional requirements  Statements of services the system should provide, how the Chapter 4 Requirements engineering 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 5 FUNCTIONAL REQUIREMENTS  Describe functionality or system services.  Depend on the type of software, expected users Chapter 4 Requirements engineering 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. 6 NON-FUNCTIONAL REQUIREMENTS  Non-functional requirements focus on quality attributes and constraints rather than specific functionalities. It specifies how the system Chapter 4 Requirements engineering performs a particular function, rather than what the system performs  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. 7 TYPES OF NONFUNCTIONAL REQUIREMENT Chapter 4 Requirements engineering 8 NON-FUNCTIONAL CLASSIFICATIONS  Product requirements  Requirements which specify that the delivered product must behave in a particular way e.g. execution speed, Chapter 4 Requirements engineering 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 DOMAIN REQUIREMENTS  Domain Requirements are industry vertical specific. It can be either functional or non- functional. They include terminology, rules, and Chapter 4 Requirements engineering standards relevant to that domain. - For example, Google Play and the IOS app have requirements that must be met when uploading apps.  Domain requirements can be new functional requirements, constraints on existing requirements or define specific computations.  If domain requirements are not satisfied, the system may be unworkable. 10 REQUIREMENTS ENGINEERING PROCESSES  The processes used for RE vary widely depending on the application domain, the people involved and the organisation developing the Chapter 4 Requirements engineering 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. 11 REQUIREMENTS ELICITATION AND ANALYSIS  Sometimes called requirements elicitation or requirements discovery. Chapter 4 Requirements engineering  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. 12 PROBLEMS OF REQUIREMENTS ANALYSIS  Stakeholders don’t know what they really want.  Stakeholders express requirements in their own Chapter 4 Requirements engineering 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. 13 REQUIREMENTS ELICITATION AND ANALYSIS  Software engineers work with a range of system stakeholders to find out about the application domain, the services that the system should Chapter 4 Requirements engineering provide, the required system performance, hardware constraints, other systems, etc.  Stages include:  Requirements discovery,  Requirements classification and organization,  Requirements prioritization and negotiation,  Requirements specification. 14 THE REQUIREMENTS ELICITATION AND ANALYSIS PROCESS Chapter 4 Requirements engineering 15 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. 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 change. THE SOFTWARE REQUIREMENTS DOCUMENT  The software requirements document is the official statement of what is required of the system developers. Chapter 4 Requirements engineering  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. 18 USERS OF A REQUIREMENTS DOCUMENT Chapter 4 Requirements engineering 19 REQUIREMENTS SPECIFICATION o The process of writing put on the user and system requirements in a requirements document. Chapter 4 Requirements engineering  User requirements have to be understandable by end-users and customers who do not have a technical background.  System requirements are more detailed requirements and may include more technical information.  The requirements may be part of a contract for the system development  It is therefore important that these are as complete as possible. 20 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. Chapter 4 Requirements engineering 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. 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 21 REQUIREMENTS VALIDATION  Concerned with demonstrating that the requirements define the system that the customer really wants. Chapter 4 Requirements engineering  Requirements error costs are high so validation is very important  Fixing a requirements error after delivery may cost up to 100 times the cost of fixing an implementation error. 22 REQUIREMENTS CHECKING  Validity. Does the system provide the functions which best support the customer’s needs? Chapter 4 Requirements engineering  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? 23 REQUIREMENTS VALIDATION TECHNIQUES  Requirements reviews  Systematic manual analysis of the requirements. Prototyping Chapter 4 Requirements engineering   Using an executable model of the system to check requirements. Covered in Chapter 2.  Test-case generation  Developing tests for requirements to check testability. 24 REQUIREMENTS MANAGEMENT  Requirements management is the process of managing changing requirements during the requirements engineering process and system Chapter 4 Requirements engineering 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. 25 KEY POINTS  Requirements for a software system set out what the system should do and define constraints on its operation and implementation. Chapter 4 Requirements engineering  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. 26 KEY POINTS  The software requirements document is an agreed statement of the system requirements. It should be organized so that both system customers and software Chapter 4 Requirements engineering developers can use it.  The requirements engineering process is an iterative process including requirements elicitation, specification and validation.  Requirements elicitation and analysis is an iterative process– requirements discovery, requirements classification and organization, requirements negotiation and requirements documentation. 27 KEY POINTS  You can use a range of techniques for requirements elicitation including interviews, scenarios, use-cases and ethnography. Chapter 4 Requirements engineering  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. 28