Requirements Engineering by dffhrtcv3

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									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]


                                                  Slide 1
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)




                                                                      Slide 2
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……...


                                                                         Slide 3
Requirements definition/spec. - example
  Requirements definition

    1. The software must provide a means of repr esenting and
    1. accessing external files created by other tools.


  Requirements specification

  1.1 The user should be provided with facilities to define the type of
  1.2 external files.
  1.2 Each external file type may have an associated tool which may be
  1.2 applied to the file.
  1.3 Each external file type may be represented as a specific icon on
  1.2 the user’s display.
  1.4 Facilities should be provided for the icon repr esenting an
  1.2 external file type to be defined by the user.
  1.5 When a user selects an icon repr esenting an external file, the
  1.2 effect of that selection is to apply the tool associated with the type of
  1.2 the external file to the file represented by the selected icon.


                                                                                  Slide 4
Requirements readers
                    Client managers
                    System end-users
    Requirements
                    Client engineers
     definition
                    Contractor managers
                    System architects


                    System end-users
    Requirements    Client engineers
    specification   System architects
                    Software developers


                    Client engineers (perhaps)
      Software
                    System architects
    specification
                    Software developers

                                                 Slide 5
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

                                                              Slide 6
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’.



                                                                   Slide 7
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


                                                       Slide 8
Non-functional requirement types

                                                         Non-functional
                                                          requir ements




                                Product                  Or ganizational                   External
                             requir ements                requir ements                  requirements




          Ef ficiency         Reliability        Portability        Interoperability        Ethical
         requir ements       requir ements     requirements          requirements        requirements




  Usability                               Delivery       Implementation         Standards         Legislative
requirements                            requirements      requir ements       requirements       requirements




Performance           Space                                                      Privacy            Safety
requirements       requir ements                                              requirements       requirements


                                                                                                   Slide 9
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
    •   9.3.2 The system development process and
        deliverable documents shall conform to the process
        and deliverables defined in XYZCo-SP-STAN-95
   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

                                                   Slide 10
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

                                                                     Slide 11
Requirements measures

Prope rty       Measure
Speed           Processed transactions/second
                User/Event response time
                Screen refresh t ime
Size            K Byt es
                Number of RAM chips
Ease of use     Training t ime
                Number of help frames
Reliability     Mean time t o failure
                Probabilit y of unavailability
                Rate of failure occurrence
                Availabilit y
Robustness      Time to restart after failure
                Percent age of events causing failure
                Probabilit y of dat a corruption on failure
Port abilit y   Percent age of t arget dependent statements
                Number of t arget syst ems
                                                              Slide 12
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...



                                                                     Slide 13
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


                                                                     Slide 14
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’.


                                          Slide 15
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.




                                           Slide 16
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



                                                                          Slide 17
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’.
                                           Slide 18
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)




                                                                     Slide 19
Editor example: structured presentation




    2.6 Grid facilities
    2.6.1    The editor shall provide a grid facility where a
             matrix of horizontal and ve rtical lines provide a
             background to the editor window. This grid shall be
             a p assive grid where the alignment of entities is the
             user's responsibility.
             Rationale: A grid helps the user to create a tidy
             diagram with well-spaced entities. Although an active
             grid, where entities 'snap-to' grid lines can be useful,
             the positioning is imprecise. T he user is the best person
             to decide where entities should be positioned.
    Specification: ECLIPSE /WS/ T ools/DE/FS Section 5.6



                                                                          Slide 20
Editor example: detailed user requirement


3.5.1 Adding nodes to a design
3.5.1.1       The editor shall provide a f acility for users to add nodes of a specified type to their
              design.
3.5.1.2       The sequence of actions to add a node should be as follows:
              1. The user should select the type of node to be added.
               2. The user should move the cursor to the approximate node position in the diagram and
               indicate that the node symbol should be added at that point.
               3. The user should then drag the node symbol to its final position.
                Rationale: T he user is the best person to decide where to position a node on the diagram.
                This approach gives the user direct control over node type selection and positioning.
Specification: ECLIPSE/WS/ T ools/DE/FS. Section 3.5.1




                                                                                                   Slide 21
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…)


                                                                     Slide 22
Alternatives to NL specification
    Notation         Description
    Structured       This approach depends on defining standard forms or
    natural          templates to express the requirements specification.
    language
    Design           This approach uses a language like a programming language
    description      but with more abstract features to specify the requirements
    languages        by defining an operational model of the system.
    Graphical        A graphical language, supplemented by text annotations is
    notations        used to define the functional requirements for the system.
                     An early example of such a graphical language was SADT
                     (Ross, 1977; Schoman and Ross, 1977). More recently, use-
                     case descriptions (Jacobsen, Christerson et al., 1993) have
                     been used. I discuss these in the following chapter.
    Mathematical      These are notations based on mathematical concepts such
    specifications   as finite-state machines or sets. These unambiguous
                     specifications reduce the arguments between customer and
                     contractor about system functionalit y. However, most
                     customers don’t understand formal specifications and are
                     reluctant to accept it as a system contract. I discuss formal
                     specification in Chapter 9.
                                                                                     Slide 23
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




                                                  Slide 24
Form-based req. spec. - Editor example
ECLIPSE/Workst at ion/Tools/DE/FS/3.5.1

Fu nction         Add node
Des cripti on    Adds a no de t o an exist ing design. The user select s t he t ype of node, and it s position.
                                              t
When added t o the design, the node becomes he current select ion. T he user cho oses t he no de po sit ion by
mov ing the cursor t o the area where the node is added.
Inputs Node type, Node position, Design identifier.
Source            Node type and Node po sition are inp ut by the user, Design identifier from the database.
O utputs          Design identifier.
Des ti nati on    The design dat abase. T he design is committ ed t o the dat abase on complet ion of t he
op erat ion.
Requires          Design graph root ed at input design identifier.
Pre-condition               The design is open and displayed on the user's screen.
Post-condi ti on            The design is unchanged apart from theaddition of a no de of the specified type
at the given posit ion.
Si de-effe cts    Non e
Definition : ECLIPSE/Workstation/Tools/DE/RD/3.5.1

                                                                                                         Slide 25
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

                                                                        Slide 26
PDL Example: Part of an ATM specification


    class ATM {
            // declarations here
            public static void main (String args[]) throws InvalidCard {
                      try {
                                thisCard.read () ; // may throw InvalidCard exception
                                pin = KeyPad.readPin () ; attempts = 1 ;
                                while ( !thisCard.pin.equals (pin) & attempts < 4 )
                                           {           pin = KeyPad.readPin () ; attempts = attempts + 1 ;
                                           }
                                           if (!thisC ard.pin.equals (pin))
                                                       throw new InvalidCard ("Bad PIN");
                                thisBalance = thisCard.getBalance () ;
                                do { Screen.prompt (" Please se lect a service ") ;
                                           service = Screen.touchKey () ;
                                           switch (service) {
                                                       case Services.withdrawalWithReceipt:
                                                                          receiptRequired = true ;




                                                                                                      Slide 27
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


                                                     Slide 28
PDL interface description


 interface PrintServer {

 // defines an abstract printer server
 // requires:          interface Printer, interface PrintDoc
 // provides: initialize, print, displayPrintQueue, cancelPrintJob, switchPrinter

           void initialize ( Printer p ) ;
           void print ( Printer p, PrintDoc d ) ;
           void displayPrintQueue ( Printer p ) ;
           void cancelPrintJob (Printer p, PrintDoc d) ;
           void switchPrinter (Printer p1, Printer p2, PrintDoc d) ;
 } //PrintServer




 Il costrutto ‘Interface’ di Java è molto adatto alla specifica…
 di interfacce
                                                                                    Slide 29
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

                                                                       Slide 30
   System and software requirements specification
    •   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


                                                                          Slide 31
Analisi dei requisiti   [AC96, fig 1.1]




                                          Slide 32
La fase di Analisi

       •   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   …’analisi’
       •   4. ispezione dei requisiti
      Sottofase II (linguaggio formale)
       •   5. specifica formale dei requisiti ==> modello astratto del
           sistema


                                                                         Slide 33
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



                                                                    Slide 34
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, ...




                                                                             Slide 35
Stesura del Glossario




                        Slide 36
3. Stesura dei requisiti
      •    Ha valore contrattuale…




      (stesura e ispezione)




                                     Slide 37
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



                                                                          Slide 38
•   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



                                                                        Slide 39
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”.




                                                                   Slide 40
Lettura strutturata

       È economica e rivela il 60% degli errori [Boehm]
       ESEMPIO
        •   Analisi dei requisiti di CTC (Centralised Traffic Controller)
            delle ferrovie nordamericane - 1990
        •   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.

                                                                       Slide 41
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




                                                                        Slide 42
Specifica simultanea dei requisiti (problema)
e di un modello astratto del sistema (soluzione)

 User Requirements                           Formal specification

                                                P1        P3
       R1 ...                                        P2
                                             Modello formale
       R2 ...                                astratto del sistema,
                                             dal comportamento
       R3 ...                                osservabile
                                             desiderato

 In linguaggio naturale                    In linguaggio formale
                             ???           eseguibile, analizzabile




                                                                      Slide 43

								
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