Requirements Engineering RE processes l l l Processes

Requirements Engineering (RE) processes l l l Processes used to discover, analyse and validate system requirements RE vary widely depending on the application domain, the people involved and the organization developing the requirements However, there a number of generic activities common to all processes • • ©Ian Sommerville Requirements elicitation Requirements analysis Requirements validation Requirements management Software Engineering Slide 1

Feasibility studies l l A feasibility study decides whether or not the proposed system is worthwhile A short focused study that checks • • • ©Ian Sommerville If the system contributes to organizational 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 Software Engineering Slide 2

Feasibility study questions: l l l 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 Software Engineering Slide 3

Elicitation and analysis l l l 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 Software Engineering Slide 4

Problems of requirements analysis l l l Stakeholders don’t know what they really want Stakeholders express requirements in their own terms Different stakeholders may have conflicting requirements Organizational 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 Software Engineering Slide 5

Process activities for elicitation & analysis l l l Domain understanding (e. g. if application for a supermarket, must understand all supermarket functions) Requirements collection (interact with stakeholders) Classification (organize data) Conflict resolution Prioritisation Requirements checking (check for completeness & consistency) ©Ian Sommerville Software Engineering Slide 6

System models l l Different models may be produced during the requirements analysis activity Requirements analysis may involve three structuring activities which result in these different models • • • l Partitioning. Identifies the structural relationships between entities Abstraction. Identifies generalities among entities Projection. Identifies different ways of looking at a problem System models covered in Chapter 8 ©Ian Sommerville Software Engineering Slide 7

Viewpoint-oriented elicitation l l 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 Software Engineering Slide 8

ATM viewpoints l l l l 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 Software Engineering Slide 9

Types of viewpoint l Data sources or sinks • l Representation frameworks • l 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 (e. g. State machine representation). Particularly suitable for real-time systems Receivers of services • ©Ian Sommerville Viewpoints are external to the system and receive services from it. Most suited to interactive systems Software Engineering Slide 10

The VORD method A Viewpoint-Oriented Requirements Definition (VORD) method has been designed as a serviceoriented framework for requirements elicitation and analysis. Viewpoint Identification ©Ian Sommerville Viewpoint Structuring Viewpoint Documentation Software Engineering Viewpoint System mapping Slide 11

VORD process model l Viewpoint identification • l Viewpoint structuring • l Group related viewpoints into a hierarchy. Common services are provided at higher-levels in the hierarchy Viewpoint documentation • l 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 • ©Ian Sommerville Transform the analysis to an object-oriented design Software Engineering Slide 12

VORD standard forms Viewpoint template Service template Reference: The view point name Reference: The service name Attributes: Attributes providing viewpoint information Rationale: Reason why the service is provided. Events: A reference to a set of event scenarios describing how the system reacts to viewpoint events Specification: Reference to a list of service specifications. These may be expressed in different notations. Services: A reference to a set of service descriptions Viewpoints: A List of viewpoint names receiving the service Sub-VPs: The names of sub-viewpoints Non-functional requirements: Reference to a set of non-functional requirements which constrain the service. Provider: Reference to a list of system objects which provide the service. ©Ian Sommerville Software Engineering Slide 13

Viewpoint service information ACCOUNT HOLDER FOREIGN CUSTOMER BANK TELLER Service list Withdraw cash Run diagnostics Query balance Add cash Order checks Add paper Send message Transaction list Order statement Transfer funds ©Ian Sommerville Software Engineering Slide 14

Viewpoint data/control ACCOUNT HOLDER ©Ian Sommerville Control Input Start transaction Card details Cancel transaction PIN End transaction Amount required Select service Message Software Engineering Slide 15

Viewpoint hierarchy All Viewpoints Services Query balance Withdraw cash Customer Bank staff Services Order checks Send message Transaction list Account holder Teller Manager Engineer Order statement Transfer funds ©Ian Sommerville Software Engineering Slide 16

Customer/cash withdrawal templates Reference: Customer Reference: Cash withdrawal Attributes: Account number; PIN; Start transaction Rationale: To improve customer service and reduce paperwork Events: Select service; Specification: Users choose this service by pressing the cash withdrawal button. They then enter the amount required. This is confirmed and, if funds allow, the balance is delivered. Cancel transaction; End transaction Services: Cash withdrawal Balance inquiry Sub-Viewpoints: Account holder Foreign customer Viewpoints: Customer Non-functional requirements: Deliver cash within 1 minute of amount being confirmed Provider: Filled in later ©Ian Sommerville Software Engineering Slide 17

Scenarios l l l 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 Software Engineering Slide 18

Scenario descriptions l l l 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 Software Engineering Slide 19

Event scenarios l l 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. • • ©Ian Sommerville Data provided and delivered Control information Exception processing The next expected event Software Engineering Slide 20

Event scenario - start transaction Card present Valid card Card PIN Validate user Request PIN Timeout Return card User OK Account number PIN Select Service Account number Incorrect PIN Re-enter PIN Invalid card Return card Incorrect PIN Stolen card Re-enter PIN Retain card ©Ian Sommerville Software Engineering Slide 21

Notation for data and control analysis l l l 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 Software Engineering Slide 22

Use cases l l l 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 Software Engineering Slide 23

Lending use-case Lending services Actor ©Ian Sommerville Software Engineering Slide 24

Library use-cases Lending services Library User administration Library Staff Supplier ©Ian Sommerville Catalog services Software Engineering Slide 25

Ethnography l l l Ethnography is an observational technique that can be used to understand social and organizational requirements. Developed in a project studying the air traffic control process Problem with ethnography is that it studies existing practices which may have some historical basis which is no longer relevant ©Ian Sommerville Software Engineering Slide 26

Requirements validation l l Concerned with demonstrating that the requirements define the system that the customer really wants Requirements error costs are high so validation is very important • ©Ian Sommerville Fixing a requirements error after delivery may cost up to 100 times the cost of fixing an implementation error Software Engineering Slide 27

Requirements checking l l l 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 Software Engineering Slide 28

Requirements validation techniques l Requirements reviews • l Prototyping • l Using an executable model of the system to check requirements. Test-case generation • l Systematic manual analysis of the requirements Developing tests for requirements to check testability Automated consistency analysis • ©Ian Sommerville Checking the consistency of a structured requirements description Software Engineering Slide 29

Requirements management l l Requirements management is the process of managing changing requirements during the requirements engineering process and system development Requirements are inevitably incomplete and inconsistent • • ©Ian Sommerville 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 Software Engineering Slide 30

Enduring and volatile requirements l l 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 Software Engineering Slide 31

Classification of requirements l Mutable requirements • l Emergent requirements • l Requirements that emerge as understanding of the system develops Consequential requirements • l Requirements that change due to the system’s environment Requirements that result from the introduction of the computer system Compatibility requirements • ©Ian Sommerville Requirements that depend on other systems or organisational processes Software Engineering Slide 32