Lectures Software Engineering - Chapter 5: Software Requirements

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Software Requirements l Descriptions and specifications of a system ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 1
Objectives l To introduce the concepts of user and system requirements l To describe functional and non-functional requirements l To explain two techniques for describing system requirements l To explain how software requirements may be organised in a requirements document ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 2
Topics covered l Functional and non-functional requirements l User requirements l System requirements l The software requirements document ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 3
Requirements engineering l The process of establishing the services that the customer requires from a system and the constraints under which it operates and is developed l The requirements themselves are the descriptions of the system services and constraints that are generated during the requirements engineering process ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 4
What is a requirement? l It may range from a high-level abstract statement of a service or of a system constraint to a detailed mathematical functional specification l This is inevitable as requirements may serve a dual function • May be the basis for a bid for a contract - therefore must be open to interpretation • May be the basis for the contract itself - therefore must be defined in detail • Both these statements may be called requirements ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 5
Types of requirement l User requirements • Statements in natural language plus diagrams of the services the system provides and its operational constraints. Written for customers l System requirements • A structured document setting out detailed descriptions of the system services. Written as a contract between client and contractor l Software specification • A detailed software description which can serve as a basis for a design or implementation. Written for developers ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 7
Functional and non-functional requirements l 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. l Non-functional requirements • constraints on the services or functions offered by the system such as timing constraints, constraints on the development process, standards, etc. l Domain requirements • Requirements that come from the application domain of the system and that reflect characteristics of that domain ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 10
Functional requirements l Describe functionality or system services l Depend on the type of software, expected users and the type of system where the software is used l 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 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 11
Examples of functional requirements l The user shall be able to search either all of the initial set of databases or select a subset from it. l The system shall provide appropriate viewers for the user to read documents in the document store. l Every order shall be allocated a unique identifier (ORDER_ID) which the user shall be able to copy to the account’s permanent storage area. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 12
Requirements imprecision l Problems arise when requirements are not precisely stated l Ambiguous requirements may be interpreted in different ways by developers and users l Consider the term ‘appropriate viewers’ • User intention - special purpose viewer for each different document type • Developer interpretation - Provide a text viewer that shows the contents of the document ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 13
Requirements completeness and consistency l In principle requirements should be both complete and consistent l Complete • They should include descriptions of all facilities required l Consistent • There should be no conflicts or contradictions in the descriptions of the system facilities l In practice, it is impossible to produce a complete and consistent requirements document ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 14
Non-functional requirements l Define system properties and constraints e.g. reliability, response time and storage requirements. Constraints are I/O device capability, system representations, etc. l Process requirements may also be specified mandating a particular CASE system, programming language or development method l Non-functional requirements may be more critical than functional requirements. If these are not met, the system is useless ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 15
Non-functional classifications l Product requirements • Requirements which specify that the delivered product must behave in a particular way e.g. execution speed, reliability, etc. l Organisational requirements • Requirements which are a consequence of organisational policies and procedures e.g. process standards used, implementation requirements, etc. l External requirements • Requirements which arise from factors which are external to the system and its development process e.g. interoperability requirements, legislative requirements, etc. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 16
Non-functional requirements examples l Product requirement • 4.C.8 It shall be possible for all necessary communication between the APSE and the user to be expressed in the standard Ada character set l Organisational requirement • 9.3.2 The system development process and deliverable documents shall conform to the process and deliverables defined in XYZCo-SP-STAN-95 l External requirement • 7.6.5 The system shall not disclose any personal information about customers apart from their name and reference number to the operators of the system ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 18
Goals and requirements l Non-functional requirements may be very difficult to state precisely and imprecise requirements may be difficult to verify. l Goal • A general intention of the user such as ease of use l Verifiable non-functional requirement • A statement using some measure that can be objectively tested l Goals are helpful to developers as they convey the intentions of the system users ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 19
Examples l A system goal • The system should be easy to use by experienced controllers and should be organised in such a way that user errors are minimised. l 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. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 20
Requirements interaction l Conflicts between different non-functional requirements are common in complex systems l Spacecraft system • To minimise weight, the number of separate chips in the system should be minimised • To minimise power consumption, lower power chips should be used • However, using low power chips may mean that more chips have to be used. Which is the most critical requirement? ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 22
Domain requirements l Derived from the application domain and describe system characterisics and features that reflect the domain l May be new functional requirements, constraints on existing requirements or define specific computations l If domain requirements are not satisfied, the system may be unworkable ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 23
Library system domain requirements l There shall be a standard user interface to all databases which shall be based on the Z39.50 standard. l Because of copyright restrictions, some documents must be deleted immediately on arrival. Depending on the user’s requirements, these documents will either be printed locally on the system server for manually forwarding to the user or routed to a network printer. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 24
Train protection system l The deceleration of the train shall be computed as: • Dtrain = Dcontrol + Dgradient 2 where Dgradient is 9.81ms * compensated gradient/alpha and where the values of 9.81ms2 /alpha are known for different types of train. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 25
Domain requirements problems l Understandability • Requirements are expressed in the language of the application domain • This is often not understood by software engineers developing the system l Implicitness • Domain specialists understand the area so well that they do not think of making the domain requirements explicit ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 26
User requirements l Should describe functional and non-functional requirements so that they are understandable by system users who don’t have detailed technical knowledge l User requirements are defined using natural language, tables and diagrams ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 27
Problems with natural language l Lack of clarity • Precision is difficult without making the document difficult to read l Requirements confusion • Functional and non-functional requirements tend to be mixed-up l Requirements amalgamation • Several different requirements may be expressed together ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 28
Database requirement 4.A.5 The database shall support the generation and control of configuration objects; that is, objects which are themselves groupings of other objects in the database. The configuration control facilities shall allow access to the objects in a version group by the use of an incomplete name. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 29
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. ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 30
Requirement problems l Database requirements includes both conceptual and detailed information • Describes the concept of configuration control facilities • Includes the detail that objects may be accessed using an incomplete name l Grid requirement mixes three different kinds of requirement • Conceptual functional requirement (the need for a grid) • Non-functional requirement (grid units) • Non-functional UI requirement (grid switching) ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 31
Guidelines for writing requirements l Invent a standard format and use it for all requirements l Use language in a consistent way. Use shall for mandatory requirements, should for desirable requirements l Use text highlighting to identify key parts of the requirement l Avoid the use of computer jargon ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 34
System requirements l More detailed specifications of user requirements l Serve as a basis for designing the system l May be used as part of the system contract l System requirements may be expressed using system models discussed in Chapter 7 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 35
Requirements and design l In principle, requirements should state what the system should do and the design should describe how it does this l In practice, requirements and design are inseparable • A system architecture may be designed to structure the requirements • The system may inter-operate with other systems that generate design requirements • The use of a specific design may be a domain requirement ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 36
Problems with NL specification l Ambiguity • The readers and writers of the requirement must interpret the same words in the same way. NL is naturally ambiguous so this is very difficult l Over-flexibility • The same thing may be said in a number of different ways in the specification l Lack of modularisation • NL structures are inadequate to structure system requirements ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 37
Structured language specifications l A limited form of natural language may be used to express requirements l This removes some of the problems resulting from ambiguity and flexibility and imposes a degree of uniformity on a specification l Often bast supported using a forms-based approach ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 39
Form-based specifications l Definition of the function or entity l Description of inputs and where they come from l Description of outputs and where they go to l Indication of other entities required l Pre and post conditions (if appropriate) l The side effects (if any) ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 40
PDL-based requirements definition l Requirements may be defined operationally using a language like a programming language but with more flexibility of expression l Most appropriate in two situations • Where an operation is specified as a sequence of actions and the order is important • When hardware and software interfaces have to be specified l Disadvantages are • The PDL may not be sufficiently expressive to define domain concepts • The specification will be taken as a design rather than a specification ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 42
PDL disadvantages l PDL may not be sufficiently expressive to express the system functionality in an understandable way l Notation is only understandable to people with programming language knowledge l The requirement may be taken as a design specification rather than a model to help understand the system ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 44
Interface specification l Most systems must operate with other systems and the operating interfaces must be specified as part of the requirements l Three types of interface may have to be defined • Procedural interfaces • Data structures that are exchanged • Data representations l Formal notations are an effective technique for interface specification ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 45
The requirements document l The requirements document is the official statement of what is required of the system developers l Should include both a definition and a specification of requirements l It is NOT a design document. As far as possible, it should set of WHAT the system should do rather than HOW it should do it ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 47
Users of a requirements document
Requirements document requirements l Specify external system behaviour l Specify implementation constraints l Easy to change l Serve as reference tool for maintenance l Record forethought about the life cycle of the system i.e. predict changes l Characterise responses to unexpected events ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 49
IEEE requirements standard l Introduction l General description l Specific requirements l Appendices l Index l This is a generic structure that must be instantiated for specific systems ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 50
Requirements document structure l Introduction l Glossary l User requirements definition l System architecture l System requirements specification l System models l System evolution l Appendices l Index ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 51
Key points l Requirements set out what the system should do and define constraints on its operation and implementation l Functional requirements set out services the system should provide l Non-functional requirements constrain the system being developed or the development process l User requirements are high-level statements of what the system should do ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 52
Key points l User requirements should be written in natural language, tables and diagrams l System requirements are intended to communicate the functions that the system should provide l System requirements may be written in structured natural language, a PDL or in a formal language l A software requirements document is an agreed statement of the system requirements ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 5 Slide 53