Software Requirements

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					Software Engineering

  6. Software Requirements

 To introduce the concepts of user and system
 To describe functional and non-functional requirements
 To explain how software requirements may be
 organised in a requirements document

Topics covered

 Functional and non-functional requirements
 User requirements
 System requirements
 Interface specification
 The software requirements document

Requirements engineering

 The process of establishing the services that the
 customer requires from a system and the constraints
 under which it operates and is developed.
 The requirements themselves are the descriptions of
 the system services and constraints that are generated
 during the requirements engineering process.

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.
 This is inevitable as requirements may serve a dual
  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.

Requirements abstraction (Davis)

 If a company wishes to let a contract for a large software
 development project, it must define its needs in a
 sufficiently abstract way that a solution is not pre-defined.
 The requirements must be written so that several
 contractors can bid for the contract, offering, perhaps,
 different ways of meeting the client organisation’s needs.
 Once a contract has been awarded, the contractor must
 write a system definition for the client in more detail so that
 the client understands and can validate what the software
 will do. Both of these documents may be called the
 requirements document for the system.
Types of requirement

 User requirements
  Statements in natural language plus diagrams of the services
   the system provides and its operational constraints.
  Written for customers.
 System requirements
  A structured document setting out detailed descriptions of the
   system’s functions, services and operational constraints.
  Defines what should be implemented so may be part of a
   contract between client and contractor.

Definitions and specifications

   Us er requ ir emen t d efinitio n

      1. The so ftw are m us t p rovid e a means o f representin g and
      1. acces sing e xtern al files cr ea ted b y o th er to ols .

                                 icatio n
   Sy stem requ ir emen ts sp ecif

     1.1 The us er sh ould b e pr ovid ed with facilities to defin e the ty pe o f
     1.2 external files .
     1.2 E ach e xtern al file typ e ma y h ave an ass ocia ted to ol w hich ma y b e
     1.2 applied to the file .
     1.3 E ach e xtern al file typ e ma y b e r epr esented as a sp ecific ico n on
     1.2 th e u ser’ s d is play
     1.4 Facilities s ho uld be pr o vid ed for the ico n r epresentin g an
     1.2 extern al file typ e to be def ed b y the us er .
     1.5 Wh en a us er selects an icon r epr esentin g an e xtern al file, th e
     1.2 effect o f that s election is to apply the to ol as so ciated with th e typ e of
     1.2 th e extern al f to th e file repres ented b y the s elected icon .

Requirements readers

6.1 Functional and non-functional

 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
 Non-functional requirements
  Constraints on the services or functions offered by the system such as
 Domain requirements
  Requirements that come from the application domain of the system and
   that reflect characteristics of that domain.

6.1.1 Functional requirements

 Describe functionality or system services.
 Depend on the type of software, expected users and
 the type of system where the software is used.
 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.

The LIBSYS system

 A library system that provides a single interface to a
 number of databases of articles in different libraries.
 Users can search for, download and print these articles
 for personal study.

Examples of functional 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
 (ORDER_ID) which the user shall be able to copy to
 the account’s permanent storage area.

Requirements imprecision

 Problems arise when requirements are not precisely
 Ambiguous requirements may be interpreted in
 different ways by developers and users.
 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

Requirements completeness and consistency

 In principle, requirements should be both complete and
  They should include descriptions of all facilities required.
  There should be no conflicts or contradictions in the
   descriptions of the system facilities.
 In practice, it is impossible to produce a complete and consistent
 requirements document.

6.1.2 Non-functional requirements

 These define system properties and constraints e.g.
 reliability, response time and storage requirements.
  Constraints are I/O device capability, system representations,
 Process requirements may also be specified
 mandating a particular 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.

Non-functional classifications

 Product requirements
  Requirements which specify that the delivered product must behave in a
   particular way e.g. execution speed, reliability, etc.
 Organisational requirements
  Requirements which are a consequence of organisational policies and
   procedures e.g. process standards used, implementation requirements,
 External requirements
  Requirements which arise from factors which are external to the system
   and its development process e.g. interoperability requirements, legislative
   requirements, etc.

Non-functional requirement types

                                                                              Non-functiona l
                                                                               re quir e ments

                                            Product                           Organisa tiona l                              Externa l
                                        re quir e ments                       re quir e ments                           re quir e ments

                   Effic iency             Re lia bility            Porta bility            Inte r ope r a bility           Ethica l
                re quir e ments         re quir e ments          re quir e ments             re quir e ments            re quir e ments

      Usa bility                                           De live ry        Imple menta tion              Sta ndar ds                  Le gisla tive

    Pe rforma nc e               Spa ce                                                                           Priva cy                 Safety

Non-functional requirements examples

 Product requirement
  8.1 The user interface for LIBSYS shall be implemented as simple HTML
    without frames or Java applets.
 Organisational requirement
  9.3.2 The system development process and deliverable documents shall
 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.

Goals and requirements

 Non-functional requirements may be very difficult to state
 precisely and imprecise requirements may be difficult to verify.
  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 are helpful to developers as they convey the intentions of
 the system users.


 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
 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.

Requirements measures

Property      Measure
Speed         Processed transactions/second
              User/Event response time
              Screen refresh time
Size          M Bytes
              Number of ROM chips
Ease of use   Training time
              Number of help frames
Reliability   Mean time to failure
              Probability of unavailability
              Rate of failure occurrence
Robustness    Time to restart after failure
              Percentage of events causing failure
              Probability of data corruption on failure
Portability   Percentage of target dependent statements
              Number of target systems

Requirements interaction

 Conflicts between different non-functional requirements
 are common in complex systems.
 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?

6.1.3 Domain requirements

 Derived from the application domain and describe
 system characteristics and features that reflect the
 Domain requirements be new functional requirements,
 constraints on existing requirements or define specific
 If domain requirements are not satisfied, the system
 may be unworkable.

Library system domain requirements

 There shall be a standard user interface to all
 databases which shall be based on the Z39.50
 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.

Train protection system

 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.

Domain requirements problems

  Requirements are expressed in the language of the
   application domain;
  This is often not understood by software engineers
   developing the system.
  Domain specialists understand the area so well that they do
   not think of making the domain requirements explicit.

6.2 User requirements

 Should describe functional and non-functional
 requirements in such a way that they are
 understandable by system users who don’t have
 detailed technical knowledge.
 User requirements are defined using natural language,
 tables and diagrams as these can be understood by all

Problems with natural language

 Lack of clarity
  Precision is difficult without making the document difficult to
 Requirements confusion
  Functional and non-functional requirements tend to be
 Requirements amalgamation
  Several different requirements may be expressed together.

LIBSYS requirement

 4.5 LIBSYS shall provide a financial accounting
 system that maintains records of all payments made by
 users of the system. System managers may configure
 this system so that regular users may receive
 discounted rates.

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

Requirement problems

 Database requirements includes both conceptual and detailed
  Describes the concept of a financial accounting system that is to be
   included in LIBSYS;
  However, it also includes the detail that managers can configure this
   system - this is unnecessary at this level.
 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)

Structured presentation

2.6.1 Grid facilities
The editor shall provide a grid facility where a m atrix of horizontal and
vertical lines provide a background to the editor window. This grid shall be a
passive 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 u seful,
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
Source: Ray Wilson, Glasgow Office

Guidelines for writing requirements

 Invent a standard format and use it for all requirements.
 Use language in a consistent way.
  Use shall for mandatory requirements, should for desirable
 Use text highlighting to identify key parts of the
 Avoid the use of computer jargon.

6.3 System requirements

 More detailed specifications of system functions,
 services and constraints than user requirements.
 They are intended to be a basis for designing the
 They may be incorporated into the system contract.
 System requirements may be defined or illustrated
 using system models discussed in Chapter 8.

Requirements and design

 In principle, requirements should state what the system
 should do and the design should describe how it does
 In practice, requirements and design are inseparable
  A system architecture may be designed to structure the
  The system may inter-operate with other systems that
   generate design requirements;
  The use of a specific design may be a domain requirement.

Problems with NL specification

  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.
  The same thing may be said in a number of different ways in
   the specification.
 Lack of modularisation
  NL structures are inadequate to structure system

Alternatives to NL specification

Notation              Description
Structured natural    This approach depends on defining standard forms or templates to
language              express the requirements specification.
                      This approach uses a language like a programming language but with
Design description    more abstract features to specify the requirements by defining an
languages             operational model of the system. This approach is not now widely used
                      although it can be useful for interface specifications.

                      A graphical language, supplemented by text annotations is used to
                      define the functional requirements for the system. An early example of
Graphical notations
                      such a graphical language was SADT. Now, use-case descriptions and
                      sequence diagrams are commonly used .

                      These are notations based on mathematical concepts such as finite-
                      state machines or sets. These unambiguous specifications reduce the
                      arguments between customer and contractor about system functionality.
                      However, most customers don’t understand formal specifications and are
                      reluctant to accept it as a system contract.

6.3.1 Structured language specifications

 The freedom of the requirements writer is limited by a
 predefined template for requirements.
 All requirements are written in a standard way.
 The terminology used in the description may be limited.
 The advantage is that the most of the expressiveness
 of natural language is maintained but a degree of
 uniformity is imposed on the specification.

Form-based specifications

 Definition of the function or entity
 Description of inputs and where they come from
 Description of outputs and where they go to
 Indication of other entities required
 Pre and post conditions (if appropriate)
 The side effects (if any) of the function

Form-based node specification

Insulin Pump/Control Software/SRS/3.3.2
Function        Compute insulin dose: Safe sugar level
Description      Computes the dose of insulin to be delivered wh en the current measur ed sugar level is in
 the safe zone between 3 and 7 units.
Inputs Current sugar reading (r2), the previous two readings (r0 and r1)
Source Current sugar reading from sensor. Other readings from memory.
O utputsCompDose Š the dose in insulin to be delivered
Destination        Main control loop
Action: CompDose is zero if the sugar level is stable or falling or if the level is increasing but the rate of
 increase is decreasing. If the level is increasing and the rate of increase is increasing, then CompDose is
 computed by dividing the difference between the current sugar level and the previous level by 4 and
 rounding the result. If the result, is rounded to zero then CompDose is set to the minimum dose that can
 be delivered.
Requires          Two previous readings so that the rate of change of sugar level can be computed.
Pre-condition     The insulin reservoir contains at least the maximum allowed single dose of insulin..
Post-condition    r0 is replaced by r1 then r1 is replaced by r2
Side-effects       None

Tabular specification

 Used to supplement natural language.
 Particularly useful when you have to define a number
 of possible alternative courses of action.

Tabular specification

Condition                                   Action
Sugar level falling (r2 < r1)               CompDose = 0
Sugar level stable (r2 = r1)                CompDose = 0
Sugar level increasing and rate of          CompDose = 0
increase decreasing ((r2-r1)<(r1-r0))
Sugar level increasing and rate of          CompDose = round ((r2-r1)/4)
increase stable or increasing. ((r2-r1) ≥   If rounded result = 0 then
(r1-r0))                                    CompDose = MinimumDose

Graphical models

 Graphical models are most useful when you need to
 show how state changes or where you need to
 describe a sequence of actions.
 Different graphical models are explained in Chapter 8.

Sequence diagrams

 These show the sequence of events that take place
 during some user interaction with a system.
 You read them from top to bottom to see the order of
 the actions that take place.
 Cash withdrawal from an ATM
  Validate card
  Handle request
  Complete transaction

Sequence diagram of ATM withdrawal
                            ATM                     Database

                                    Card number

                                     Card OK
           PIN request
           Opt ion menu                                        Validate card

           invalid card

        Withdraw request          Balance request
          Amount request
                                                               Handle request
                                  Debit (amount)

                                  Debit response
        insufficient cash


            Card removed
             Cash                                               t ransact ion

         Cash removed

6.4 Interface specification

 Most systems must operate with other systems and the
 operating interfaces must be specified as part of the
 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.

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

6.5 The requirements document

 The requirements document is the official statement of
 what is required of the system developers.
 Should include both a definition of user requirements
 and a specification of the system requirements.
 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

Users of a requirements document

IEEE requirements standard

 Defines a generic structure for a requirements
 document that must be instantiated for each specific
  General description
  Specific requirements

Requirements document structure

 User requirements definition
 System architecture
 System requirements specification
 System models
 System evolution

Key points

 Requirements set out what the system should do and define
 constraints on its operation and implementation.
 Functional requirements set out services the system should
 Non-functional requirements constrain the system being
 developed or the development process.
 User requirements are high-level statements of what the system
 should do. User requirements should be written using natural
 language, tables and diagrams.

Key points

 System requirements are intended to communicate the
 functions that the system should provide.
 A software requirements document is an agreed
 statement of the system requirements.
 The IEEE standard is a useful starting point for defining
 more detailed specific requirements standards.


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