Lezione 4. Requirements [S2001, Cap. 5] [AC96, Cap. 1]

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1 Lezione 4. Requirements [S2001, Cap. 5] [AC96, Cap. 1] Functional, non-functional, domain requirements User requirements System and software requirements Requirements languages The requirements document Requirements analysis [AC96]

2 What is a requirement? It may range from a high-level abstract statement of a service or of a system constraint to a detailed mathematical functional specification… …since requirements may serve a dual function May be the basis for a bid for a contract - therefore must be open to interpretation (client ==> potential developers) May be the basis for the contract itself - therefore must be defined in detail (developer ==> potential client)

3 Requirements definition/specification
Requirements definition (user requirements) A statement in natural language plus diagrams of the services the system provides and its operational constraints. Based on information from Client, and written for him (or even by him) Requirements specification (system requirements) A structured document setting out detailed descriptions of the system services. Written as a contract between Client and Developer Software specification (software requirements) A detailed software description which can serve as a basis for a design or implementation. Written for technical developers (design team, programmers…). May be omitted……...

4 Requirements definition/spec. - example

5 Requirements readers

6 Functional, non-functional, domain requirements
Requirements engineering is the process of establishing the services that the Client requires from a system and the constraints under which it operates and is developed Requirements may be functional or non-functional Functional requirements describe system services or functions, often expressed in terms system reactions to inputs from the environment Non-functional requirements are constraints on the services offered by the system, and on the development process Domain requirements (funct./non funct.)come from the application domain of the system and reflect characteristics of that domain

7 Functional requirements - examples
‘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’. (*) User intention - special purpose viewer for each document type (*) Developer interpretation - Provide a text viewer that shows the contents of the document ‘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’.

8 Non-functional requirements
On: Reliability, response time, storage capacity, I/O device capability, data representation. On: CASE system, programming language or development method Non-functional requirements may be more critical than functional requirements. If these are not met, the system is useless

9 Non-functional requirement types

10 Non-functional requirements examples
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 Organisational 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

11 Verifiable non-functional reqs. Vs. goals
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. 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. … nevertheless, goals are helpful to developers as they convey the intentions of the Client

12 Requirements measures

13 Non functional requirements conflicts
... are common in complex systems Example: Spacecraft system Req.1 - System should fit into 4Mbytes of memory Req.2 - System should be written in ADA However, it may be impossible to compile an ADA program with the required functionality into 4Mbytes: drop one of the requirements...

14 Domain requirements Derived from the application domain; describe system features that reflect the domain May be new functional requirements, constraints on existing requirements or define specific computations Problems: Understandability. Requirements are expressed in the language of the application domain. This is often not understood by software engineers developing the system Implicitness. Domain specialists understand the area so well that they do not think of making the domain requirements explicit

15 Example: Library system domain requirements
‘There shall be a standard user interface to all databases which shall be based on the Z39.50 standard’ (a standard for this Library). ‘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’.

16 Example: train system domain requirement
The deceleration of the train shall be computed as: Dtrain = Dcontrol + Dgradient where Dgradient is 9.81ms2 * compensated gradient/alpha and where the values of 9.81ms2 /alpha are known for different types of train.

17 User requirements Should describe functional and non-functional requirements so that they are understandable by non-technical system-users. Externally visible behaviour User requirements are defined using natural language, tables and diagrams. Problems: Lack of clarity, ambiguity (‘Dogs must be carried’) Precision is difficult without making the document difficult to read Requirements confusion Functional and non-functional requirements tend to be mixed-up Requirements amalgamation Several different requirements may be expressed together

18 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’.

19 Problems in the Editor grid requirement
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)

20 Editor example: structured presentation

21 Editor example: detailed user requirement

22 System and software requirements
More detailed specifications of user requirements Serve as a basis for the Design in principle Reqs. and Design are separated (WHAT vs. HOW) in practice they are interdependent May be used as part of the system contract May be complemented with, or expressed by system models (Entity-Relation, Data-Flow, Petri nets, Communicating Finite State Machines, Statecharts, Basic LOTOS…)

23 Alternatives to NL specification

24 Structured natural language specifications
A limited form of natural language may be used to express requirements This removes some of the problems resulting from ambiguity and over-flexibility and imposes a degree of uniformity on a specification Often supported by a forms-based approach

25 Form-based req. spec. - Editor example

26 PDL (Program Descr. Language)-based requirements definition
Requirements may be defined operationally using a programming language (e.g. Java) enriched by constructs for further flexibility 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 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

27 PDL Example: Part of an ATM specification

28 System requirements: interface specification
Most systems must operate with other systems and the operating interfaces must be specified as part of the requirements Three types of interface may have to be defined Procedural interfaces Data structures that are exchanged Data representations Formal notations are an effective technique for interface specification

29 PDL interface description
Il costrutto ‘Interface’ di Java è molto adatto alla specifica… di interfacce

30 Requirements document structure
Introduction Describe need for the system and how it fits with business objectives Glossary Define technical terms used Functional requirements definition (user reqs.) Describe the services to be provided Non-functional requirements definition (user reqs.) Define constraints on the system and the development process System Architecture helps structuring requirements around subsystems

31 System and software requirements specification System models
Detailed specification of functional requirements System models Define models showing system components and relationships System evolution Define fundamental assumptions on which the system is based and anticipated changes Appendices System hardware platform description Database requirements (as an ER model perhaps) May include USER MANUAL and TEST PLAN Index

32 Analisi dei requisiti [AC96, fig 1.1]

33 La fase di Analisi Sottofase I (linguaggio naturale +...)
Studia e definisce il problema da risolvere Stretta interazione con il committente Sottofase I (linguaggio naturale +...) 1. studio di fattibilità 2. comprensione del dominio (==> glossario) 3. stesura (raccolta e definizione) dei requisiti 4. ispezione dei requisiti Sottofase II (linguaggio formale) 5. specifica formale dei requisiti ==> modello astratto del sistema …’analisi’

34 1. Studio di fattibilità Valutazione di costi, benefici e rischi
Disponibilità di librerie SW? HW adatto alle prestazioni attese? Uso di tecnologie non consolidate? Valore di mercato al tempo di consegna? Output n scenari di sviluppo, con relativi tempi e costi Società specializzate nel puro studio di fattibilità

35 2. Comprensione del dominio
Comprensione dei concetti e termini usati dal Committente per parlare del sistema e del suo contesto. Lo Sviluppatore acquisisce la competenza del Committente, non viceversa ==> migliore interazione Input: documenti dal Committente e altri reperiti autonomam. Es. strutture organizzative/commerciali, caratteristiche di impianti, leggi fisiche Output: Glossario Insieme chiuso e sintetico di definizioni che rifletta la complessità del dominio. Può includere descrizioni di algoritmi, procedure d’ufficio, ...

36 Stesura del Glossario

37 3. Stesura dei requisiti Ha valore contrattuale… (stesura e ispezione)

38 Il documento dei requisiti
Ha valore contrattuale… .. ma è soggetto a cambiamenti ‘tardivi’. E’ scritto in linguaggio naturale Ogni requisito cattura un aspetto o vincolo, completo e indipendente, del sistema Requisiti obbligatori, desiderabili, opzionali (==> contratto) Non dovrebbe contenere: inconsistenze (req. ==><== req.) ambiguità (req. ?!) imprecisioni terminologiche (req. ==><== glossario) ridondanze (req ==> req.) dettagli tecnici e rif. alla soluzione-implementazione

39 Dovrebbe essere completo
Elenca tutte e sole le esigenze del Committente Usa tutti e soli i termini del Glossario Dovrebbe essere ben strutturato bilanciando la granularità dei requisiti minimizzando riferimenti in avanti. Lemmario: elenco dei termini usati nei requisiti, ciascuno con lista di puntatori ai requisiti che lo usano. Facilita la ricerca di inconsistenze o ridondanze in requisiti semanticamente vicini

40 4. Ispezione dei requisiti
Boehm: “Trovare e riparare un difetto nel software consegnato costa 100 volte meno che farlo durante l’analisi dei requisiti”. Fagan: “la maggior parte degli errori si manifesta dopo la consegna del sistema, ma ha origine durante l’analisi dei requisiti”.

41 È economica e rivela il 60% degli errori [Boehm] ESEMPIO
Lettura strutturata È economica e rivela il 60% degli errori [Boehm] ESEMPIO Analisi dei requisiti di CTC (Centralised Traffic Controller) delle ferrovie nordamericane 10 gruppi di analisti in parallelo Dei 92 difetti del documento dei requisiti 77 vengono trovati durante l’ispezione dei requisiti 15 nelle fasi successive Ogni gruppo trova mediamente ‘solo’ 25 difetti. L’ispezione parallela e ridondante paga.

42 5. Specifica formale (dei requisiti…) [AC96]
Descrizione tecnica del comportamento di un sistema che risponde ai requisiti enfasi sull’osservatore esterno: sistema come black box ==> Modello astratto del sistema primo passo dalla caratterizzazione verso la soluzione del problema

43 Specifica simultanea dei requisiti (problema) e di un modello astratto del sistema (soluzione)
User Requirements Formal specification P3 P1 R1 ... P2 Modello formale astratto del sistema, dal comportamento osservabile desiderato R2 ... R3 ... In linguaggio naturale ??? In linguaggio formale eseguibile, analizzabile