Requirements Reference: Software Engineering, by Ian Sommerville, 6th edition, Chapters 5, 6, & 8

Objectives To introduce and contrast 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 CMSC 345, Spring 2003

Requirements Engineering The process of establishing the services that are required of the system, and the constraints under which it operates and is developed The requirements themselves are the descriptions of both of these items. CMSC 345, Spring 2003

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, Spring 2003

Types of Requirements User requirements System 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 design specification An abstract description of the software design that can serve as a basis for a more detailed design. Bridges the gap between requirements and design. Written for developers CMSC 345, Spring 2003

Requirements Readers CMSC 345, Spring 2003

Functional and Non-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, Spring 2003

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, Spring 2003

Functional Requirement Examples The user shall be able to add or delete problems to/from the problem collection. The user shall be able to preview an examination on the monitor. A student shall be able to take an examination on-line. The system will automatically grade an examination upon completion by the student. CMSC 345, Spring 2003

Requirements Imprecision Problems arise when requirements are not precisely stated. Ambiguous requirements may be interpreted in different ways by developers and clients. Requirements should also be verifiable. Examples: The user shall be able to modify the problem collection. The user shall be able to use an existing exam. CMSC 345, Spring 2003

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, Spring 2003

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, Spring 2003

Non-functional Requirements Examples Product requirements - An examination shall accommodate true/false, multiple choice, and short answer questions. An exam question and the space for its answer must not be divided between two printed pages. - The system must run under Red Hat Linux, Version 6.2. - The system must be written in C++, compilable using Microsoft Visual C++, Version 6.0. CMSC 345, Spring 2003

More Non-functional Requirements User interface requirements The user interface shall be text-based. The user interface shall be menu-driven. The UMBC logo shall always be displayed in the upper right-hand corner of the screen. CMSC 345, Spring 2003

More Non-functional Requirements Organizational requirements The system development process and deliverable documents shall conform to the process and deliverables defined in XYZCo-SP-STAN-95. External requirements 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, Spring 2003

NFRs Must Be Verifiable, Too! Non-verifiable The system should be easy to use by experienced controllers and should be organized in such a way that user errors are minimized. Verifiable 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, Spring 2003

Some Requirements Measures CMSC 345, Spring 2003

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, Spring 2003

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, Spring 2003

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, Spring 2003

Alternatives to NL Specification CMSC 345, Spring 2003

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, Spring 2003

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 all 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, Spring 2003