Software Requirements Definition - Jones The statement of needs by a user that triggers the development of a program or system - Jones 1994

Software Requirements Definition – Davis A user need or necessary feature, function, or attribute of a system that can be sensed from a position external to that system - Alan Davis 1993

Software Requirements - Definition Requirements are... A specification of what should be implemented. They are descriptions of how the system should behave, or of a system property or attribute. They may be a constraint on the development process of the system. - Sommerville 1997

Levels of Requirements Business Requirements User Requirements Functional Requirements Non-Functional Requirements

Business Requirements High-level statements of the goals, objectives, or needs of an organization. They usually describe opportunities that an organization wants to realize or problems that they want to solve. Often stated in a business case.business case

User Requirements Mid-level statements of the needs of a particular stakeholder or group of stakeholders. They usually describe how someone wants to interact with the intended solution. Often acting as a mid-point between the high-level business requirements and more detailed solution requirements.

Functional Requirements Usually detailed statements of capabilities, behaviour, and information that the solution will need. Examples include formatting text, calculating a number, modulating a signal. They are also known as capabilities.

Non-Functional Requirements Usually detailed statements of the conditions under which the solution must remain effective, qualities that the solution must have, or constraints within which it must operate. Other terms for non-functional requirements are "constraints", "quality attributes", "quality goals", "quality of service requirements" and "non-behavioral requirements". Informally these are sometimes called the "ilities", from attributes like stability and portability.

Qualities, that is non-functional requirements, can be divided into two main categories: Execution qualities, such as security and usability, which are observable at run time. Evolution qualities, such as testability, maintainability, extensibility and scalability, which are embodied in the static structure of the software system.

Example A system may be required to present the user with a display of the number of records in a database. This is a functional requirement. How up-to-date this number needs to be is a non-functional requirement. If the number needs to be updated in real time, the system architects must ensure that the system is capable of updating the displayed record count within an acceptably short interval of the number of records changing.

Sufficient network bandwidth may be a non-functional requirement of a system. Other examples include: Accessibility, Audit and control, Availability (see service level agreement), Backup, Capacity, Current forecast. certification, compliance,configuration,dependency on other parties, Deployment, Documentation, Disaster recovery, Efficiency (resource consumption for given load), Effectiveness (resulting performance in relation to effort),Emotional factors (like fun or absorbing or has "Wow! Factor"), Environmental protection, Escrow

Non Functional Exploitability, Extensibility (adding features, and carry-forward of customizations at next major version upgrade), Failure management Fault tolerance (e.g. Operational System Monitoring, Measuring, and Management), Legal and licensing issues or patent-infringement- avoidability, Interoperability, Maintainability, Modifiability Network topology, Open source, Operability, Performance / response time (performance engineering),Platform compatibility, Price Privacy, Portability, Quality (e.g. faults discovered, faults delivered, fault removal efficacy), Recovery / recoverability (e.g. mean time to recovery - MTTR), Reliability (e.g. mean time between failures - MTBF, or availability), Reporting, Resilience, Resource constraints (processor speed, memory, disk space, network bandwidth, etc.), Response time, Reusability, Robustness, Safety or Factor of safety, Scalability (horizontal, vertical), Security, Software, tools, standards etc. Compatibility, Stability, Supportability, Testability, Usability by target user community, User Friendliness

Business Requirements Vision and Scope document User Requirements System Requirements Functional Requirements Other Non-functional Requirements Constraints Quality Attributes Functional Specification Documents Use Case document Relationship of Several components of Software Requirements

Importance of the Software Requirement Process The hardest single part of building a software system is deciding precisely what to build. No other part of the conceptual work is as difficult as establishing the detailed technical requirements, including all the interfaces to people, to machines, and to other software systems. No other part of the work so cripples the system if done wrong. No other part is more difficult to rectify later. Fred Brooks - No Silver Bullet: Essence and Accidents of Software Engineering, 1987.

Role of Requirements Software Requirements Project Planning Project Tracking Change Control System Testing User Documentation Construction Process

Some Risks From Inadequate Requirement Process Insufficient user involvement leads to unacceptable products. Creeping user requirements contribute to overruns and degrade product quality. Ambiguous requirements lead to ill-spent time and rework. Gold-plating by developers and users adds unnecessary features. Minimal specifications lead to missing key requirements. Overlooking the needs of certain user classes (stake holders) leads to dissatisfied customers. Incompletely defined requirements make accurate project planning and tracking impossible.

Management UsersHardware Eng.System Eng. Marketing Software Requirements provides business requirements and project parameters Provides Business Requirements and Project Parameters allocate system requirements to software Specifies hardware interfaces the software must respect describe user requirements and quality attributes Stake Holders

Management Users System Eng. Marketing Software Requirements provides business requirements and project parameters Provides Business Requirements and Project Parameters allocate system requirements to software Specifies hardware interfaces the software must respect Stake Holders Hardware Eng. describe user requirements and quality attributes describe user requirements and quality attributes

Understanding Requirements Software Engineering: A Practitioner’s Approach, 7/e 24

Requirements Engineering-I Inception—ask a set of questions that establish … –basic understanding of the problem –the people who want a solution –the nature of the solution that is desired, and –the effectiveness of preliminary communication and collaboration between the customer and the developer Elicitation—elicit requirements from all stakeholders Elaboration—create an analysis model that identifies data, function and behavioral requirements Negotiation—agree on a deliverable system that is realistic for developers and customers 25

Requirements Engineering-II Specification—can be any one (or more) of the following: –A written document –A set of models –A formal mathematical –A collection of user scenarios (use-cases) –A prototype Validation—a review mechanism that looks for –errors in content or interpretation –areas where clarification may be required –missing information –inconsistencies (a major problem when large products or systems are engineered) –conflicting or unrealistic (unachievable) requirements. Requirements management 26

Inception Identify stakeholders –“who else do you think I should talk to?” Recognize multiple points of view Work toward collaboration The first questions –Who is behind the request for this work? –Who will use the solution? –What will be the economic benefit of a successful solution –Is there another source for the solution that you need? 27

Eliciting Requirements meetings are conducted and attended by both software engineers and customers rules for preparation and participation are established an agenda is suggested a "facilitator" (can be a customer, a developer, or an outsider) controls the meeting a "definition mechanism" (can be work sheets, flip charts, or wall stickers or an electronic bulletin board, chat room or virtual forum) is used the goal is –to identify the problem –propose elements of the solution –negotiate different approaches, and – specify a preliminary set of solution requirements 28

Eliciting Requirements 29

Quality Function Deployment Function deployment determines the “value” (as perceived by the customer) of each function required of the system Information deployment identifies data objects and events Task deployment examines the behavior of the system Value analysis determines the relative priority of requirements 30

Elicitation Work Products a statement of need and feasibility. a bounded statement of scope for the system or product. a list of customers, users, and other stakeholders who participated in requirements elicitation a description of the system’s technical environment. a list of requirements (preferably organized by function) and the domain constraints that apply to each. a set of usage scenarios that provide insight into the use of the system or product under different operating conditions. any prototypes developed to better define requirements. 31

Building the Analysis Model Elements of the analysis model –Scenario-based elements Functional—processing narratives for software functions Use-case—descriptions of the interaction between an “actor” and the system –Class-based elements Implied by scenarios –Behavioral elements State diagram –Flow-oriented elements Data flow diagram 32

Use-Cases A collection of user scenarios that describe the thread of usage of a system Each scenario is described from the point-of-view of an “actor”—a person or device that interacts with the software in some way Each scenario answers the following questions: –Who is the primary actor, the secondary actor (s)? –What are the actor’s goals? –What preconditions should exist before the story begins? –What main tasks or functions are performed by the actor? –What extensions might be considered as the story is described? –What variations in the actor’s interaction are possible? –What system information will the actor acquire, produce, or change? –Will the actor have to inform the system about changes in the external environment? –What information does the actor desire from the system? –Does the actor wish to be informed about unexpected changes? 33

Use-Case Diagram 34

Class Diagram 35 From the SafeHome system …

State Diagram 36 Reading Commands System status = “ready” Display msg = “enter cmd” Display status = steady Entry/subsystems ready Do: poll user input panel Do: read user input Do: interpret user input State name State variables State activities

Analysis Patterns 37 Pattern name: A descriptor that captures the essence of the pattern. Intent: Describes what the pattern accomplishes or represents Motivation: A scenario that illustrates how the pattern can be used to address the problem. Forces and context: A description of external issues (forces) that can affect how the pattern is used and also the external issues that will be resolved when the pattern is applied. Solution: A description of how the pattern is applied to solve the problem with an emphasis on structural and behavioral issues. Consequences: Addresses what happens when the pattern is applied and what trade-offs exist during its application. Design: Discusses how the analysis pattern can be achieved through the use of known design patterns. Known uses: Examples of uses within actual systems. Related patterns: On e or more analysis patterns that are related to the named pattern because (1) it is commonly used with the named pattern; (2) it is structurally similar to the named pattern (3) it is a variation of the named pattern.

Negotiating Requirements Identify the key stakeholders –These are the people who will be involved in the negotiation Determine each of the stakeholders “win conditions” –Win conditions are not always obvious Negotiate –Work toward a set of requirements that lead to “win-win” 38

Validating Requirements - I Is each requirement consistent with the overall objective for the system/product? Have all requirements been specified at the proper level of abstraction? That is, do some requirements provide a level of technical detail that is inappropriate at this stage? Is the requirement really necessary or does it represent an add- on feature that may not be essential to the objective of the system? Is each requirement bounded and unambiguous? Does each requirement have attribution? That is, is a source (generally, a specific individual) noted for each requirement? Do any requirements conflict with other requirements? 39

Validating Requirements - II Is each requirement achievable in the technical environment that will house the system or product? Is each requirement testable, once implemented? Does the requirements model properly reflect the information, function and behavior of the system to be built. Has the requirements model been “partitioned” in a way that exposes progressively more detailed information about the system. Have requirements patterns been used to simplify the requirements model. Have all patterns been properly validated? Are all patterns consistent with customer requirements? 40