Requirements Reference: Chapters 5, 6, & 8

CMSC 345, Fall Objectives To introduce the concepts of user and system requirements To explain functional and non-functional requirements To present guidelines for writing system requirements To introduce the concept of use cases for describing functional requirements To discuss prototyping as a means for requirements elicitation

CMSC 345, Fall Requirements Engineering The process of establishing – the services that the required of the system, – and the constraints under which it operates and is developed The requirements themselves are the descriptions of the system services and constraints.

CMSC 345, Fall Who’s Who? Client – the person(s) paying for the development and will become the owner of the product Customer – the person who will buy the product off the shelf (mass marketing), or who has the final say as to whether the product is acceptable (in-house development). May be the same as the client Stakeholder – Anyone who should have some direct or indirect influence on the system requirements Reference: Mastering the Requirements Process, Robertson and Robertson

CMSC 345, Fall Types of Requirements User requirements –Should describe requirements so that they are understandable by those who do not have detailed technical knowledge. Written mainly for customers (end users) System requirements –A structured document setting out detailed descriptions of the system services and constraints. Written as a contract between client and contractor Software specification –A detailed software description that can serve as a basis for a more detailed design. Adds further detail to the systems requirements. Written for developers

CMSC 345, Fall Requirements Readers

CMSC 345, Fall Functional and Non-functional Requirements Functional requirements –Statements of services the system should provide, how the system should react to particular inputs, and how the system should behave in particular situations Non-functional requirements –constraints on the services or functions offered by the system such as timing constraints, constraints on the development process, standards, etc.

CMSC 345, Fall Functional Requirements Describe functionality or system services These services depend on –the type of software being developed –the expected users of the software Functional user requirements may be high- level statements of what the system should do, but functional system requirements should describe the system services in detail.

CMSC 345, Fall Examples of Functional Requirements (written as user requirements) The user shall be able to search either all of the initial set of databases or select a subset from it. The system shall provide appropriate viewers for the user to read documents in the document store. Every order shall be allocated a unique identifier which the user shall be able to copy to the account’s permanent storage area.

CMSC 345, Fall Requirements Imprecision Problems arise when requirements are not precisely stated Ambiguous requirements may be interpreted in different ways by developers and clients Consider the term ‘appropriate viewers’ –Client intention - special purpose viewer for each different document type –Developer interpretation - Provide a text viewer that shows the contents of the document

CMSC 345, Fall Requirements Completeness and Consistency In principle, requirements should be both complete and consistent Complete –They should include descriptions of all required functionality Consistent –There should be no conflicts or contradictions in the descriptions of the system functions In practice, it is impossible to produce a complete and consistent requirements document

CMSC 345, Fall Non-functional Requirements Define system properties and constraints Examples: –response time –storage requirements –process requirements (e.g., must use a particular CASE system, programming language, or development method) Non-functional requirements may be more critical than functional requirements. If one is not met, the system may be useless.

CMSC 345, Fall Non-functional Requirements Examples Product requirement –“It shall be possible for all necessary communication between the APSE and the user to be expressed in the standard Ada character set.” Organizational requirement –“The system development process and deliverable documents shall conform to the process and deliverables defined in XYZCo-SP-STAN-95” External requirement –“The system shall not disclose any personal information about customers apart from their name and reference number to the operators of the system”

CMSC 345, Fall Goals and Requirements Non-functional requirements may be very difficult to state precisely. Imprecise requirements are difficult to verify. Goal –A general intention of the user such as ease of use Verifiable non-functional requirement –A statement using some measure that can be objectively tested Goals may be helpful to developers as they convey the intentions of the system users. But all requirements, functional or non-functional, MUST be verifiable.

CMSC 345, Fall Example A system goal –“The system should be easy to use by experienced controllers and should be organized in such a way that user errors are minimized.” A verifiable non-functional requirement –“Experienced controllers shall be able to use all the system functions after a total of two hours training. After this training, the average number of errors made by experienced users shall not exceed two per day.”

CMSC 345, Fall Some Requirements Measures

CMSC 345, Fall Writing Requirements Requirements may be written using a natural language (common for user requirements) –Lack of clarity, possibly ambiguous –Requirements confusion Functional and non-functional requirements tend to be mixed-up –Requirements amalgamation Several different requirements may be expressed together Tables and diagrams may help

CMSC 345, Fall Example: Editor Grid Requirement 2.6 Grid facilities To assist in the positioning of entities on a diagram, the user may turn on a grid in either centimetres or inches, via an option on the control panel. Initially, the grid is off. The grid may be turned on and off at any time during an editing session and can be toggled between inches and centimetres at any time. A grid option will be provided on the reduce-to-fit view but the number of grid lines shown will be reduced to avoid filling the smaller diagram with grid lines.

CMSC 345, Fall Problems! Difficult to read Mixes three different requirements: –Functional requirement (the need for a grid) –Non-functional requirement (grid units) –Non-functional UI requirement (grid switching)

CMSC 345, Fall Alternatives to NL Specification

CMSC 345, Fall Some Guidelines for Writing Requirements Invent or find a standard format and use it for all requirements. Use language in a consistent way. Use “shall” or “will” for mandatory requirements, “should” for desirable requirements. Use text highlighting (e.g., italics) to identify key parts of the requirement. Do not use vague phrases (e.g., “around a month,” “have basic knowledge of”) Every requirement must be verifiable. Every requirement should be numbered for traceability.

CMSC 345, Fall Use Cases A way of describing a system’s functional requirements Describes the system’s behavior under various conditions as the system responds to a request from one of the stakeholders called the primary actor. The primary actor initiates some interaction with the system to accomplish some goal. The system responds, protecting the interests of the stakeholders. Different sequences of behaviors (scenarios) can unfold, depending on the request and the conditions surrounding the request. The use case gathers these scenarios together.

CMSC 345, Fall Use Case Format and Example (for this class) Use Case N Use Case: Context of Use: Scope: Priority: Level: Primary Actor:

CMSC 345, Fall Use Case N Use Case: Print letters Context of Use: User wants to print one or more of the same letter Scope: The Perfect Form Priority: 5 Level: User goal Primary Actor: Faculty member

CMSC 345, Fall Stakeholders and Interests: Precondition: Minimal Guarantees: Success Guarantees:

CMSC 345, Fall Stakeholders and Interests: Faculty member – wants to print letter(s) Precondition: User has opened a form letter Minimal Guarantees: No letters will print and a message stating why will be displayed Success Guarantees: All of the letters will be printed

CMSC 345, Fall Trigger: Main Success Scenario:

CMSC 345, Fall Trigger: Menu selection of “print” Main Success Scenario: 1) User selects “print” 2) A print dialog box appears 3) User changes prints settings 4) User presses “print” 5) The letters print 6) The print dialog box closes

CMSC 345, Fall Extensions: Related Information:

CMSC 345, Fall Extensions: 2a) User has not filled in the custom tags, the date tag, the sender tag, or the return address tag. 2a.1) The user will be informed of which tags to specify replacements for. 2a.2) The print dialog box will not open. Related Information: None