Chapter 7 Requirements Engineering Process Ian Sommerville 2000

Chapter 7 Requirements Engineering Process ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 1

Objectives l l To describe the principal RE activities. To introduce techniques for requirements elicitation and analysis. To describe requirements validation. To discuss the role of requirements management in support of other RE processes. ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 2

RE processes… l Vary widely depending on: § § § l Application domain People involved Organization developing the requirements Generic activities common to most: § § Feasibility study Requirements elicitation and analysis Requirements specification Requirements validation ©Ian Sommerville 2000 Software Engineering. Chapter 7 core, iterative activities Slide 3

RE Process Model ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 4

Spiral RE Process Model Emphasizes iterative nature of core activities ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 5

Feasibility Study Feasibility study issues ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 6

Feasibility study l Aims to answer three basic questions: § § § Would the system contribute to overall organizational objectives? Could the system be engineered using current technology and within budget? Could the system be integrated with other systems already in use? ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 7

Feasibility study issues (a highlevel checklist) l l l How would the organization cope if the system wasn’t implemented? What are the current process problems and how would the system help with these? What will the integration problems be? Is new technology needed? New skills? What must be supported by the system, and what need not be supported? ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 8

Elicitation and Analysis Problems Process activities Viewpoint-oriented elicitation Method-based RE Interviewing Scenarios Social and organizational factors Ethnography & focused ethnography ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 9

Elicitation and analysis l l Involves working with customers to learn about the application domain, the services needed and the system’s operational constraints, etc. May also involve end-users, managers, maintenance personnel, domain experts, trade unions, etc. (That is, other stakeholders. ) ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 10

Problems of elicitation and analysis l Getting all, and only, the right people involved l Stakeholders often: § § express requirements in their own terms. § l don’t know what they really want have conflicting or competing requirements. Requirements naturally change as insight improves. (Should this really be thought of as a problem? ) (cont'd) ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 11

Problems of elicitation and analysis (cont’d) l l l New stakeholders may emerge. Political or organizational factors may affect requirements. (Examples? ) The environment may evolve during the RE process. ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 12

Elicitation and analysis process activities l Requirements discovery § l Requirements classification and organization § l Grouping and organizing requirements to facilitate analysis Prioritization and negotiation § l Interacting with stakeholders to discover product and domain requirements Prioritizing requirements and resolving requirements conflicts. Requirements documentation § Requirements are documented and input into the next round of the spiral. ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 13

Elicitation and Analysis spiral ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 14

Viewpoint-oriented elicitation l l l Stakeholders represent different ways of looking at a problem (“viewpoints”). A multi-perspective analysis is important as there is no single correct way to analyze system requirements. Provides a natural way to structure the elicitation process and organize requirements. ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 15

Types of viewpoints l l l Interactor viewpoints § People or other systems that interact directly with the system. Indirect viewpoints § Stakeholders who do not use the system themselves but who influence the requirements. Domain viewpoints § Domain characteristics and constraints that affect the requirements. ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 16

Method-based RE l l “Structured methods” to elicit, analyze, and document requirements. Suzanne & James part of “SA/SD” Robertson, Atlantic Examples include: Systems Guild § § § Ross’ Structured Analysis (SA), Volere Requirements Process (www. volere. co. uk) Knowledge Aquisition and Sharing for Requirement Engineering (KARE) Esprit project (http: //cordis. europa. eu/esprit/home. html), Sommerville’s Viewpoint-Oriented Requirements Definition (VORD), and Thebaut’s Scenario-Based Requirements Engineering (SBRE) ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 17

Volere Requirements Process Start here ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 18

Volere requirement shell ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 19

KARE workbench architecture ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 20

Sommerville’s VORD method ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 21

VORD standard forms two points of reference ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 22

Interviewing l l RE’s meet with stakeholders to discuss the system currently in place and the system to be developed. May be: § § l formal or informal closed (with a pre-defined agenda), open (no pre-defined agenda), or a mix Useful for learning how stakeholders might affect or be affected by the system. (cont'd) ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 23

Interviewing (cont’d) l Less useful for learning about domain requirements since: § § l RE’s may not understand domain-specific terminology; stakeholders may not communicate such requirements because they are so obvious (to the stakeholders) Gause & Weinberg (“Exploring Requirements: Quality Before Design, ” Dorset House, 1989) describe many useful interviewing techniques. ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 24

Scenarios l l Depict examples or scripts of possible system behavior People often relate to these more readily than to abstract statements of requirements “Give me an example to help tie the parts together” (into a coherent whole. ) l Particularly useful in elucidating fragmentary, incomplete, or conflicting requirements ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 25

Scenario elements 1. 2. 3. 4. System state at the beginning of the scenario (if relevant) Sequence of events for a specific case of some generic task the system is required to accomplish. Any relevant concurrent activities. System state at the completion of the scenario. ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 26

A simple scenario t 0: The user enters values for input array A. The values are [1, 23, -4, 7, 19]. t 1: The user executes program MAX. t 2: The value of variable BIG is 23 and the values of A are [1, 23, -4, 7, 19]. (Compare this to the interface and operational specification examples from the Chap. 6 lecture notes. ) ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 27

Scenario-Based Requirements Engineering (Thebaut) l l l A CASE tool supports the rapid construction of an operational specification of the desired system and its environment. Utilizes a forward chaining, parallel, rulebased language. An interpreter executes the specification to produce natural language based scenarios of system behavior. ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 28

Scenario representation in VORD (Sommerville) l VORD supports the graphical description of multi-threaded “event scenarios” to document system behavior: § § l Data provided and delivered Control information Exception processing The next expected event Multi-threading supports description of exceptions. (blurs the distinction between scenarios and operational specifications) ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 29

Scenario for a “start transaction” event different scenarios ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 30

UML use-cases and sequence diagrams l Graphical notations for representing abstract scenarios in the UML. (UML is the de facto standard for OO Analysis & Design) l l l Identify actors in an interaction and describe the interaction itself. A set of use-cases should describe all types of interactions with the system. Sequence diagrams show the sequence of event processing. ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 31

Library use-cases ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 32

Catalogue management sequence diagram time ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 33

Social and organizational factors l l All software systems are used in a social and organizational context. This can influence or even dominate system requirements. Good analysts must be sensitive to these factors, but there is currently no systematic way to tackle their analysis. ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 34

Example • Consider a system which allows senior management to access information without going through middle managers. § § § Managerial status – Senior managers may feel that they are too important to use a keyboard. Managerial responsibilities – Managers may not have time to learn how to use the system Organizational resistance – Middle managers who will be made redundant may deliberately provide misleading or incomplete information so the system will fail. ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 35

Ethnography l l A social scientist observes and analyzes how people actually work. Subjects do not have to explain or otherwise articulate what they do. Social and organizational factors of importance may be observed. Ethnographic studies have shown that work is usually richer and more complex than suggested by simple system models. ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 36

Focused ethnography l l Developed during a project studying the air traffic control process. Combines ethnography with prototyping. Prototype development raises issues which focus the ethnographic analysis. Problem with ethnography alone: it studies existing practices which may not be relevant when a new system is put into place. ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 37

Requirements Validation attributes techniques ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 38

Requirements validation l l Concerned with whether or not the requirements define a system that the customer really wants. (as opposed to needs? ) Requirements error costs are high, so early validation is very important. (Fixing a requirements error after delivery may cost 100 times that of fixing an error during implementation. ) ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 39

Requirements attributes l l l Validity: Does the system provide the functions opposed which best support the customer’s needs? (aswants? ) to 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 tested? (More precisely, can the system be tested to determine whether or not the requirements are met? ) ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 40

Requirements validation techniques l Requirements reviews / inspections – systematic manual analysis of the requirements. l Prototyping – using an executable model of the system to check requirements. Covered in Chapter 17. l Test-case generation – developing tests for requirements to check testability. l Automated consistency analysis – checking the consistency of a structured requirements description. (CASE – e. g. , “Wisdom” tool in KARE workbench) ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 41

Requirements reviews / inspections l l Regular reviews should be held while requirements are being formulated. Both client and contractor staff should be involved in reviews. (+ other stakeholders) Reviews may be formal or informal… Good communication between developers, customers and users can resolve problems at an early stage. ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 42

Review check-list l l Verifiability: Is the requirement testable? Comprehensibility: Is the requirement understandable? Traceability: Is the origin of the requirement clearly stated? and rationale! Adaptability: Can the requirement be changed with minimum impact on other requirements? (Especially when change is anticipated!) ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 43

Requirements Management Enduring vs. volatile requirements Planning considerations Traceability CASE support Change management process ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 44

Requirements management… l l …is the process of understanding and controlling requirements change. Requirements evolve, priorities change, and new requirements emerge as § § a better understanding of the system is developed, and the business and technical environment of the system changes. ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 45

Enduring and volatile requirements l Enduring requirements: Stable requirements derived from the core activity of the customer organization. (E. g. , a hospital will always have doctors, nurses, etc. May be derived from domain models. ) l Volatile requirements: Requirements which change during development or when the system is in use. (E. g. , requirements derived from the latest health-care policy. ) ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 46

Types of volatile requirements l Mutable – those that change due to changes in the organization’s operating environment. l Emergent – those that emerge as a better understanding of the system develops. l Consequential – those that result from the introduction of the system. l Compatibility – those that change due to changing systems or processes within the organization. ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 47

Requirements management planning requires decisions on: l Requirements identification – how requirements will be individually identified l A change management process – to be followed when analyzing the impact and costs of a requirements change l Traceability policies – the amount of information about requirements relationships that is maintained l CASE tool support – the tool support required to help manage requirements change ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 48

Traceability… l l …is concerned with the relationships between requirements, their sources, and the system design. Types of traceability: § Source traceability – links from requirements to stakeholders who proposed the requirements. (or other sources) § Requirements traceability – links between dependent requirements. § Design traceability – links from the requirements to the design. ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 49

CASE tool support l Requirements storage – in a secure, managed data store l Change management – a workflow process whose stages can be defined and information flow between the stages partially automated l Traceability management – automated discovery and documentation of relationships between requirements (keyword search, common scenarios, etc. ) ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 50

Change management process l l Applied to all proposed requirements changes Principal stages: § Problem analysis – analyze identified requirements problem and propose specific change(s) § Change analysis and costing – assess effects of change on other requirements § Change implementation – modify requirements document (+ system design and implementation, as necessary) to reflect the change ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 51

Change management process (cont’d) ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 52

Key points l l The RE process includes a feasibility study, elicitation and analysis, specification, and validation. Elicitation and analysis involves requirements discovery, classification and organization, prioritization and negotiation, and documentation. (cont'd) ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 53

Key points (cont’d) l l l Systems have multiple stakeholders with different viewpoints and requirements. Social and organization factors influence system requirements. Requirements validation is concerned with checks for validity, consistency, completeness, realism, and verifiability. (cont'd) ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 54

Key points (cont’d) l l Business, organizational, and technical changes inevitably lead to changing requirements. Requirements management involves careful planning and a change management process. ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 55

Chapter 7 Requirements Engineering Process ©Ian Sommerville 2000 Software Engineering. Chapter 7 Slide 56