Chapter 6 Requirements Engineering Process Ian Sommerville

Chapter 6 Requirements Engineering Process Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 1

Requirements Engineering Processes Processes used to discover, analyse and validate system requirements Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 2

Objectives To describe the principal requirements engineering activities To introduce techniques for requirements elicitation and analysis To describe requirements validation To discuss the role of requirements management in support of other requirements engineering processes Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 3

Topics covered Feasibility studies Requirements elicitation and analysis Requirements validation Requirements management Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 4

Requirements engineering processes The processes used for RE vary widely depending on the application domain, the people involved and the organisation developing the requirements However, there are a number of generic activities common to all processes Requirements elicitation Requirements analysis Requirements validation Requirements management Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 5

The requirements engineering process Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 6

Feasibility studies A feasibility study decides whether or not the proposed system is worthwhile A short focused study that checks If the system contributes to organisational objectives If the system can be engineered using current technology and within budget If the system can be integrated with other systems that are used Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 7

Feasibility study implementation Based on information assessment (what is required), information collection and report writing Questions for people in the organisation What if the system wasn’t implemented? What are current process problems? How will the proposed system help? What will be the integration problems? Is new technology needed? What skills? What facilities must be supported by the proposed system? Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 8

Elicitation and analysis Sometimes called requirements elicitation or requirements discovery 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 Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 9

Problems of requirements analysis 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 Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 10

The requirements analysis process Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 11

Process activities Domain understanding Requirements collection Classification Conflict resolution Prioritisation Requirements checking Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 12

System models Different models may be produced during the requirements analysis activity Requirements analysis may involve three structuring activities which result in these different models Partitioning. Identifies the structural (part-of) relationships between entities Abstraction. Identifies generalities among entities Projection. Identifies different ways of looking at a problem System models covered in Chapter 7 Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 13

Viewpoint-oriented elicitation Stakeholders represent different ways of looking at a problem or problem viewpoints This multi-perspective analysis is important as there is no single correct way to analyse system requirements Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 14

Banking ATM system The example used here is an auto-teller system which provides some automated banking services I use a very simplified system which offers some services to customers of the bank who own the system and a narrower range of services to other customers Services include cash withdrawal, message passing (send a message to request a service), ordering a statement and transferring funds Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 15

Autoteller viewpoints Bank customers Representatives of other banks Hardware and software maintenance engineers Marketing department Bank managers and counter staff Database administrators and security staff Communications engineers Personnel department Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 16

Types of viewpoint Data sources or sinks Representation frameworks Viewpoints are responsible for producing or consuming data. Analysis involves checking that data is produced and consumed and that assumptions about the source and sink of data are valid Viewpoints represent particular types of system model. These may be compared to discover requirements that would be missed using a single representation. Particularly suitable for real-time systems Receivers of services Viewpoints are external to the system and receive services from it. Most suited to interactive systems Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 17

External viewpoints Natural to think of end-users as receivers of system services Viewpoints are a natural way to structure requirements elicitation It is relatively easy to decide if a viewpoint is valid Viewpoints and services may be sued to structure non-functional requirements Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 18

Method-based analysis Widely used approach to requirements analysis. Depends on the application of a structured method to understand the system Methods have different emphases. Some are designed for requirements elicitation, others are close to design methods A viewpoint-oriented method (VORD) is used as an example here. It also illustrates the use of viewpoints Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 19

The VORD method Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 20

VORD process model Viewpoint identification Viewpoint structuring Group related viewpoints into a hierarchy. Common services are provided at higher-levels in the hierarchy Viewpoint documentation Discover viewpoints which receive system services and identify the services provided to each viewpoint Refine the description of the identified viewpoints and services Viewpoint-system mapping Transform the analysis to an object-oriented design Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 21

VORD standard forms Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 22

Viewpoint identification Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 23

Viewpoint service information Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 24

Viewpoint data/control Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 25

Viewpoint hierarchy Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 26

Customer/cash withdrawal templates Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 27

Scenarios Scenarios are descriptions of how a system is used in practice They are helpful in requirements elicitation as people can relate to these more readily than abstract statement of what they require from a system Scenarios are particularly useful for adding detail to an outline requirements description Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 28

Scenario descriptions System state at the beginning of the scenario Normal flow of events in the scenario What can go wrong and how this is handled Other concurrent activities System state on completion of the scenario Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 29

Event scenarios Event scenarios may be used to describe how a system responds to the occurrence of some particular event such as ‘start transaction’ VORD includes a diagrammatic convention for event scenarios. Data provided and delivered Control information Exception processing The next expected event Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 30

Event scenario - start transaction Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 31

Notation for data and control analysis Ellipses. data provided from or delivered to a viewpoint Control information enters and leaves at the top of each box Data leaves from the right of each box Exceptions are shown at the bottom of each box Name of next event is in box with thick edges Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 32

Exception description Most methods do not include facilities for describing exceptions In this example, exceptions are Timeout. Customer fails to enter a PIN within the allowed time limit Invalid card. The card is not recognised and is returned Stolen card. The card has been registered as stolen and is retained by the machine Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 33

Use cases Use-cases are a scenario based technique in the UML which identify the actors in an interaction and which describe the interaction itself A set of use cases should describe all possible interactions with the system Sequence diagrams may be used to add detail to use-cases by showing the sequence of event processing in the system Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 34

Lending use-case Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 35

Library use-cases Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 36

Catalogue management Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 37

Social and organisational factors Software systems are used in a social and organisational context. This can influence or even dominate the system requirements Social and organisational factors are not a single viewpoint but are influences on all viewpoints Good analysts must be sensitive to these factors but currently no systematic way to tackle their analysis Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 38

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. This may limit the type of system interface used Managerial responsibilities. Managers may have no uninterrupted time where they can learn to use the system Organisational resistance. Middle managers who will be made redundant may deliberately provide misleading or incomplete information so that the system will fail Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 39

Ethnography A social scientists spends a considerable time observing and analysing how people actually work People do not have to explain or articulate their work Social and organisational 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, 6th edition. Chapter 6 Slide 40

Focused ethnography Developed in a project studying the air traffic control process Combines ethnography with prototyping Prototype development results in unanswered questions which focus the ethnographic analysis Problem with ethnography is that it studies existing practices which may have some historical basis which is no longer relevant Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 41

Ethnography and prototyping Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 42

Scope of ethnography Requirements that are derived from the way that people actually work rather than the way I which process definitions suggest that they ought to work Requirements that are derived from cooperation and awareness of other people’s activities Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 43

Requirements validation Concerned with demonstrating that the requirements define the system that the customer really wants 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 Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 44

Requirements checking Validity. Does the system provide the functions which best support the customer’s needs? 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? Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 45

Requirements validation techniques Requirements reviews Prototyping Using an executable model of the system to check requirements. Covered in Chapter 8 Test-case generation Systematic manual analysis of the requirements Developing tests for requirements to check testability Automated consistency analysis Checking the consistency of a structured requirements description Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 46

Requirements reviews Regular reviews should be held while the requirements definition is being formulated Both client and contractor staff should be involved in reviews Reviews may be formal (with completed documents) or informal. Good communications between developers, customers and users can resolve problems at an early stage Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 47

Review checks Verifiability. Is the requirement realistically testable? Comprehensibility. Is the requirement properly understood? Traceability. Is the origin of the requirement clearly stated? Adaptability. Can the requirement be changed without a large impact on other requirements? Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 48

Automated consistency checking Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 49

Requirements management Requirements management is the process of managing changing requirements during the requirements engineering process and system development Requirements are inevitably incomplete and inconsistent New requirements emerge during the process as business needs change and a better understanding of the system is developed Different viewpoints have different requirements and these are often contradictory Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 50

Requirements change The priority of requirements from different viewpoints changes during the development process System customers may specify requirements from a business perspective that conflict with end-user requirements The business and technical environment of the system changes during its development Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 51

Requirements evolution Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 52

Enduring and volatile requirements Enduring requirements. Stable requirements derived from the core activity of the customer organisation. E.g. a hospital will always have doctors, nurses, etc. May be derived from domain models Volatile requirements. Requirements which change during development or when the system is in use. In a hospital, requirements derived from health-care policy Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 53

Classification of requirements Mutable requirements Emergent requirements Requirements that emerge as understanding of the system develops Consequential requirements Requirements that change due to the system’s environment Requirements that result from the introduction of the computer system Compatibility requirements Requirements that depend on other systems or organisational processes Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 54

Requirements management planning During the requirements engineering process, you have to plan: Requirements identification » How requirements are individually identified A change management process » The process followed when analysing a requirements change Traceability policies » The amount of information about requirements relationships that is maintained CASE tool support » The tool support required to help manage requirements change Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 55

Traceability Traceability is concerned with the relationships between requirements, their sources and the system design Source traceability Requirements traceability Links from requirements to stakeholders who proposed these requirements Links between dependent requirements Design traceability Links from the requirements to the design Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 56

A traceability matrix Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 57

CASE tool support Requirements storage Change management Requirements should be managed in a secure, managed data store The process of change management is a workflow process whose stages can be defined and information flow between these stages partially automated Traceability management Automated retrieval of the links between requirements Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 58

Requirements change management Should apply to all proposed changes to the requirements Principal stages Problem analysis. Discuss requirements problem and propose change Change analysis and costing. Assess effects of change on other requirements Change implementation. Modify requirements document and other documents to reflect change Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 59

Requirements change management Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 60

Key points The requirements engineering process includes a feasibility study, requirements elicitation and analysis, requirements specification and requirements management Requirements analysis is iterative involving domain understanding, requirements collection, classification, structuring, prioritisation and validation Systems have multiple stakeholders with different requirements Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 61

Key points Social and organisation factors influence system requirements Requirements validation is concerned with checks for validity, consistency, completeness, realism and verifiability Business changes inevitably lead to changing requirements Requirements management includes planning and change management Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 6 Slide 62