Description Description Textual Description Textual
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6. Description Description A large part of RE is description Quite often we describe things by way of analytic models for n particular we must describe example the characteristics of the problem domain the behaviour of the solution system IK Bray 2004 Textual Description Data flow diagrams Entity relationship diagrams Decision tables Finite State Machines Class diagrams etc Where appropriate use them However all these models incorporate text and often we use text Natural Language Plain English alone and with good reason IK Bray 2004 Textual Description The bad news Jackson identifies 2 ways of defining terms Good documentation requires very good writing Designation Producing very good writing is difficult the informal definition of fundamental terms At the least it requires years of practice Formal Definition To be blunt some people will never get there The good news But all can improve and As we shall see we have some guidelines largely from KOVITZ99 IK Bray 2004 Designation The foundations for all other descriptions Takes the form of a recognition rule e g call button a press button situated in the foyer outside the lift that can be pressed in order to summon a lift Most dictionary definitions are designations always rely upon a common understanding ultimately te
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9. e g going the condition of some thing eg the path was rough going IK Bray 2004 Formal Definition The principles can be applied to NL but FD benefits from formal notations 3 are suggested predicate logic Other Ian s bit programming languages as in data declarations Extended Backus Naur Form E BNF Relatively few mechanisms are needed or known some are formalisations of designation mechanisms egcomponent parts boat boat name sail number boat class name helm name where all the elements on the right have been designated another is either or for example race class boat class race class name IK Bray 2004 Designation and Definition Notes neither designations nor formal definitions can be described as true or false we are simply stating what we mean by the terms they can however be poorly written and lack clarity and hence be open to misunderstanding not all useful information is amenable to formal definition Eg the helm is likely to be the owner IK Bray 2004 Designation Designations are inherently informal Rely upon some pre existing common understanding Stop when you are confident that there will be too little misunderstanding to cause significant problems Jackson also points out there is a big difference between Defining things that already exist typically in the PD and Defining things tha
10. ecking back with the elicitation sources So are you saying that idi Analysis checking that the PD description and requirements are correct Specification checking that the defined behaviour will meet the requirements And after RE checking that the design will produce the defined behaviour and ultimately that the system does behave as required These slides copyright by Ian K Bray Techniques e Simple checks e Review e Logical analysis e Prototypes and enactments e Functional test design e User manual development Simple checks Eg e On completion of the RD check through the elicitation notes and ensure that nothing 1s omitted e On completion of the specification check that all requirements have been reflected Review e Review of RE deliverables e Various types of review an example later Logical analysis Requires a formal specification VDM FSM Eg FSM can check e that all states are reachable and leavable e every transition has exactly one trigger e that every trigger event Is recognised e etc Prototypes and enactments e Very good for checking with users client etc Easter to relate to than specifications e Can use paper based or software based prototypes e Walk through various scenarios can use use cases Functional test design e System level functional tests must be designed sooner or later e Can should be derived from the specification e Desi
11. gning the tests can reveal errors in the specification prior to system design and build User manual development e Very similar considerations to FTD above T 9PIIS 8661 9III 1ouruog 1 pue e amp uojoN DCH sjuaurmnbersnongdiquy e SJOJJ BOTUYIOT SJOI JUOS sjuatuomboy Sspiepuejs oj SOUBUIJOJUO ouojsisuoo pu ss ou j duroo JO ju umoop SjUMIIMbaI e gau e pojuouro duit oq o uiojs s ayy Jo uonduosop o qe1doooe ue SI juoumoop sjuouroumboi DY PY sono e s oAno o qo uonepIi A I 9PUs 8661 IIA1oumuog 1 pue eAuojosy NO UOne pi e A sjuourodmboq t OPIS 8661 lltA49uttuoS 1 pue e amp uojoN DCH sprepum pes Sap ke Spe paar ty uorpepipe a E a pap oun yuca nha runne aus Sap E 1 E Strapp pops M ois sjndjno pue sindur uoneprye A 9PIIS 8661 IIA1oumuog 1 pue eAuojosy NO ogvjs STU 1 POJOMSUR oq o uons nb Ady oY sr JUSTI SJUQUISIMDII IY JOS IMIABH sjuouioJmbo pa913e pue p lenos ou uA 2371 Ju umo op sjuouromboad oy Jo 481p IEuU e YIM SYIOM UOMBPIRBA e FLIS STU 1 POJOMSUL oq 0 uons nb Ady oy sr sjusureumbog IYF I OS IMIABH SIOP OY9BIS urojs s oq DIOU PITS SE SJUTIIIMbII MBI YIM SYIOM SIS IEUN UOHBPILA pue sISA eu v 9 p IS 8661 lltA49uttuoS 1 pue e amp uojoN DCH Suorjoe pojeroosse ou MALY ABUL suro qoud auos suono 9AT29J1I00 TLIDADS JABY ABUT suro qoud euros suro qod sju ur rnb l 0 esuodsa1 ur suoroe pooJge JO SIT s
12. nd to be circular E g object any thing which can be seen touched or perceived by any of the senses thing any material object These and several other definitions are taken from Heineman English Dictionary 1993 IK Bray 2004 the more or less formal definition of derived terms in terms of designated terms And using defined terms we can produce 2 sorts of description Rough sketch an informal description generally work in progress Refutable description a description that is in principle testable IK Bray 2004 Designation When and what do we designate When precision is important pretty much always in RE We designate all fundamental specialist terms and concepts that cannot be formally defined In particular we must designate data inputs outputs stored data But other concepts may also require explanation states of the PD and SS problem domain jargon non standard use of terms K Bray 2004 Designation mechanisms equivalence means the same as gloaming twilight parsimonious mean classification and discrimination classic e g sea dog a sailor with many year s experience also see call button example above sum of parts or component list e g boat details the boat s class its sail number handicap and owner s name by example usually used to reinforce another poor definition
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14. riting guidelines the Naming Rules As far as is sensible adopt the terminology of the problem domain Never use the same term for different things As far as is sensible always use the same term for the same thing Define all special terms see DD later Don t invent unnecessary terms including acronyms IK Bray 2004 Unnatural Language Structured English tends to be short precise sentences formatted in a similar way to program code For example 2 1 The user is prompted to enter their old PIN 2 2 The user enters their old PIN 2 3 If the old PIN is incorrect 24 The message incorrect PIN is displayed for 4 seconds 2 5 The system returns to the main menu display 2 6 Ifthe old PIN is correct 2 7 The user is prompted to enter the new PIN etc A similar style is often found in use cases IK Bray 2004 Unnatural Language Extended Backus Naur Form E BNF provides a well defined grammar for precise and concise definitions So far I have concentrated upon semantics meaning BNF helps there but particularly useful for defining syntax Syntactic definitions are built upon terminals aka literals or primitives IK Bray 2004 Writing guidelines Examples IK Bray 2004 Unnatural Language Pseudo code even more codey For example 2 1 output message enter old PIN 2 2 input user pin 2 3 if user pin stored pin then 24 output mes
15. rminology of the problem domain By all means clarify the meaning of existing terms but do not invent new terminology including acronyms just for the sake of it 3 Never use the same term for different things If this already happens in the PD you must invent new terms or number them E g in e YRR PD class is used for both boat class and race class As far as is sensible always use the same term for the same thing If duplicates already exist show the equivalence in the ata dictionary IK Bray 2004 EBNF Example boat boat name boat class name sail number helm boat name alphanumeric 25 boat class name alphanumeric sail number digit helm alphanumeric J25 alphanumeric a SZ TSA SZ digit 18 ee digit UM sg Exercise Which of the following are valid values of boat Trapeze TS 240 23 Ian K Bray D eau solo 3575 Ian K Bray Bori Free spirit K24570 Fred Basset IK Bray 2004 The Data Dictionary Definitions of all relevant data items syntax and semantics A convenient place to also include designations and definitions of Problem domain jargon Problem sub domains sub systems he etc K Bray 2004 Validation e Checks that the right product is being built e Helps ensure delivery of what the client wants e highly pertinent to RE Need to be performed at every stage during the process Elicitation ch
16. rovide navigation clues Reinforcement not repetition A trade off avoid redundancy in general why but use overviews examples etc to aid understanding IK Bray 2004 Rough sketches Ordinary English descriptions A useful starting point Hopefully develop into refutable descriptions but quite often don t K Bray 2004 Writing guidelines Natural language Write for the reader assume the reader is intelligent and co operative Constantly ask Is there an easier to understand alternative expression Overloading with too much information at once What is most important less important to them Too abstract without illustration Too disconnected without underlying principle Any reasonable misinterpretations possible Any benefit to the reader in this part What is the feel formal stuffy rambling etc Is the document boring IK Bray 2004 Writing guidelines Avoid Decoy text Metatext text that describes the text eg This document is the requirements document for the XYZ lift controller system which should be clear from the title Generalities Eg All requirements should be testable We already know that s a good idea but is it feasible And what if it s not Inclusions Don t copy other documents reference them Duckspeak Meaningless padding eg The lift request validation function will validate lift requests IK Bray 2004 W
17. sage incorrect PIN 2 5 wait 4 seconds 2 6 goto 2 1 2 7 else 2 8 output message enter the new PIN etc IK Bray 2004 Unnatural Language Extended Backus Naur Form E BNF A terminal is shown in double quotes eg A Non terminals are defined using a defining expression or rule composed of terminals logical symbols defined non terminals Eg my name Ian ie non logical terminal terminal symbol K Bray 2004 Meaning is defined as is followed by and exclusive or a is repeated zero or more times nfa a is repeated at least n times a ais repeated up to m times na ais repeated between n and m times inclusive a Zero or one occurrences of a optionality indicates a contiguous range as in a z O delimits a group or ai delimits a comment 3 terminates a definition I K Bray 2004 EBNF EBNF comments can be used for semantic definitions designations For example sail number n digit the unique identifier of a boat within its boat class Generally this is the only kind of double definition syntactic semantic that should be used IK Bray 2004 The naming rules version 2 Designate or wherever possible formally define all terms that might give rise to misunderstanding 2 As far as is sensible adopt the te
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19. t as yet do not typically in the SS You must be even more careful with the latter Designations should be kept to the minimum wherever possible use formal definitions instead IK Bray 2004 Formal Definition More complex are formal definitions of relationships quick advert for VDM Suppose that if a boat enters a series then it is automatically entered into every race in that series We may designate a race entry and a series race boat b is entered in series s series entry b s race r is part of series s series race r s We could also designate a race entry thus boat b is entered in race r race entry b r But a formal definition is better V b r s e series entry b s series race r s gt race entry b r This can be read as for any boat race and series if the boat is entered in the series and the race is part of the series the boat will be entered in the race IK Bray 2004 Description Designations and definitions are a means to an end they provide the building blocks to build useful descriptions Things to describe the problem domain the requirements the effects that are required to be produced within the PD the behaviour of the envisioned solution system This may be generalised to descriptions of systems in terms of their components or sub domains the relationships between sub domains in terms of the shared phenomena often data flows
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