A Heuristic Approach to Detect Feature Interactions in Requirements

Transcription

1 A Huistic Appoach to Dtct Fatu Intactions in Rquimnts Maitta Hisl Janin Souquiès Fakultät fü Infomatik LORIA Univsité Nancy2 Univsität Magdbug B.P. 239 Bâtimnt LORIA D Magdbug, Gmany F Vandœuv-ls-Nancy, Fanc Fax: (49) Fax: (33) Abstact. W psnt a mthod to systmatically dtct fatu intactions in quimnts. Th quimnts a xpssd as constaints on systm vnt tacs. This mthod is pat of a boad appoach to quimnts licitation and fomal spcification. 1 Th Gnal Appoach Ou wok aims at poviding mthodological suppot fo analysts and spcifis of softwabasd systms. To this nd, w hav dvlopd an intgatd appoach to quimnts licitation and fomal spcification, which is sktchd in [3]. W do not invnt any nw languags, but giv guidanc how to pocd to (i) idntify and fomally xpss th quimnts concning th systm to b constuctd, and (ii) systmatically tansfom ths quimnts into a fomal spcification. Th diffnc btwn quimnts and a spcification is that quimnts f to th nti systm to b alizd, whas a spcification fs only to th pat of th systm to b implmntd by softwa. Ou mthod bgins with an xplicit quimnts licitation phas. Th sult of this fist phas is a st of quimnts. All systm fatus must b flctd in ths quimnts, which a xpssd fomally as constaints on squncs of vnts that can happn o opations that can b invokd in th contxt of th systm 1. Ths constaints not only fom th stating point fo th dvlopmnt of th fomal spcification. Thy also suppot th systmatic dtction of fatu intactions. W us agndas [2] to xpss ou mthods. An agnda is a list of stps to b pfomd whn caying out som task in th contxt of softwa ngining. Agndas contain infomal dsciptions of th stps. Ths may dpnd on ach oth. Usually, thy will hav to b patd to achiv th goal, bcaus lat stps will val os and omissions in ali stps. Th stps of an agnda may hav validation conditions associatd with thm that stat ncssay smantic conditions that th atifact must fulfill in od to sv its pupos poply. 2 Agnda fo Rquimnts Elicitation Ou mthod fo quimnts licitation is inspid by objct ointd mthods such as Fusion [1], and by th wok of Jackson and Zav [5]. It is pfomd in six stps. In th 1 A fatu can cospond to on o sval constaints.

2 following, w list th stps of th agnda w hav dvlopd fo quimnts licitation. Only th most impotant validation conditions a mntiond. 1. Intoduc th domain thoy. All ncssay notions must b intoducd. Ths can ith b ntitis, cosponding to nouns in a natual-languag dsciption, o lationships, cosponding to vbs in a natual-languag dsciption. 2. List all possibl vnts that can happn in connction with th systm, togth with thi paamts. 3. Classify th vnts as: (i) contolld by th nvionmnt and not shad with th softwa systm, (ii) contolld by th nvionmnt but obsvabl by th softwa systm, (iii) contolld by th softwa systm and obsvabl by th nvionmnt, and (iv) contolld by th softwa systm and not shad with th nvionmnt. Validation condition: th must not b any vnts contolld by th softwa systm and not shad with th nvionmnt. 4. List possibl systm opations that can b invokd by uss, togth with thi input and output paamts. Intoduc a lation btwn th input and output paamts. 5. Stat th facts, assumptions, and quimnts concning th systm in natual languag. It dos not suffic to just stat quimnts fo th systm. Oftn, facts and assumptions must b intoducd to mak th quimnts satisfiabl. Facts xpss things that always hold in th application domain, gadlss of th implmntation of th softwa systm. Oth quimnts cannot b nfocd bcaus.g., human uss might violat gulations. Ths conditions a xpssd as assumptions. 6. Fomaliz th facts, assumptions, and quimnts as constaints on th possibl tacs of systm vnts. W xpss quimnts, assumptions, and facts by fing to th cunt stat of th systm, vnts that happn, and th tim an vnt happns: S 1 1?!t1 S 2 2?!t2 : : : S n n?!tn S n+1 : : : Th systm is statd in stat S 1. Whn vnt 1 happns at t 1, thn th systm nts stat S 2, and so foth. On lmnt of a tac of th systm thus consists of ths th pats. Using constaints to talk about th bhavio of th systm has th advantags that, fist, it is possibl to xpss ngativ quimnts, i.., to qui that ctain things do not happn. Scond, it is possibl to giv scnaios, i.., xampl bhavios of th systm. Thid, giving constaints dos not fix th systm bhavio ntily. Constaints do not stict th spcification unncssaily. Any spcification that fulfills thm is pmittd. 3 Agnda to Incopoat Singl Constaints In Stp 6 of th agnda fo quimnts licitation, th constaints must b fomalizd on by on. Each nw constaint is addd to th st of constaints dfind so fa. But bfo th constaint is addd, its possibl intactions with oth constaints should b analyzd. Th following agnda givs guidlins how to incopoat a nw constaint into a st of alady

3 xisting constaints. It is pfomd in six stps. W illustat th stps that a impotant fo fatu intaction dtction by th cas study of a lift systm [4]. In th following, w will us th tm lital to man pdicat o vnt symbols, o ngations of such symbols. An vnt symbol is supposd to man vnt must o may occu, whas : is supposd to man vnt dos not occu. If w f to pdicat symbols and thi ngations, w will us th tm pdicat lital. Evnt litals a dfind analogously. 1. Fomaliz th nw constaint as a fomula on systm tacs. W commnd to xpss if possibl constaints as implications, wh ith th pcondition of th implication fs to an ali stat o an ali point in tim than th postcondition, o both th p- and postcondition f to th sam stat (invaiants). 2. Giv a schmatic xpssion of th constaint. Schmatic xpssions hav th fom x 1 ^ x 2 ^ : : : ^ x n y 1 _ y 2 _ : : : _ y k wh th x i, y j a litals. Th symbol indicats that th pcondition fs to an ali stat than th postcondition. If th constaint is an invaiant of th systm stat, w us th implication symbol ) instad of th symbol. Tansfoming a constaint into its schmatic fom, w abstact fom quantifis and fom paamts of pdicat and vnt symbols. Exampl: A lift systm could hav th quimnt Whn th lift has stoppd, it will opn th doo. Th cosponding schmatic xpssion is stop opn. 3. Updat th tabls of smantic lations. Th dtction of constaint intactions cannot b basd on syntax alon. W also must tak into account th smantic lations btwn th diffnt symbols. A pdicat may imply anoth pdicat, an vnt may only b possibl if th systm stat fulfills a pdicat, and fo ach pdicat, w must know which vnts stablish and which vnts falsify it. W constuct th tabls of smantic lations: (a) Ncssay conditions fo vnts. If an vnt can only occu if pdicat lital pl is tu, thn this tabl has an nty pl. Exampl: Th doo can only opn whn it is closd: doo closd opn (b) Evnts stablishing pdicat litals. Fo ach pdicat lital pl, w nd to know th vnts that stablish it: pl Exampl: Th pdicat doo closd is stablishd by th vnt clos: clos doo closd (c) Rlations btwn pdicat litals. Fo ach pdicat symbol p, w dtmin th st of pdicat litals it ntails: p ) = fq : PLit j p ) qg th st of pdicat litals its ngation ntails: : p ) = fq : PLit j : p ) qg th st of pdicat litals that ntail it: ) p = fpl : : p ) : plg th st of pdicat litals that ntail its ngation: ) : p = fpl : p ) : plg Not that only two of th fou sts must b dtmind xplicitly. Exampl: doo closd implis : doo opn: doo closd ) = f: doo opng

4 p(c ) 1 1 p(c ) p(c ) 2 2 o p(c 2)... 2 stat i stat i+1 stat i stat i+k k > 1 Figu 1: Intaction candidats 4. Dtmin intaction candidats, basd on th list of schmatic quimnts (Stp 2) and th smantic lation tabls (Stp 3). Th dfinition of th intaction candidats is givn in Sction Dcid if th a intactions of th nw constaint with th dtmind candidats. It is up to th analysts and customs to dcid if th conjunction of th nw constaint with th candidats yilds an unwantd bhavio o not, and how dtctd intactions can b solvd. 6. If an intaction occus, tak on of th following actions: (i) coct a fact, (ii) lax a quimnt (usually by adding a nw p- o postcondition, as pconditions a usually conjunctions, and postconditions a usually disjunctions), o (iii) stngthn an assumption. Pfom an intaction analysis on thos litals that w changd o nwly intoducd into th changd constaint. 4 Dtmining Intaction Candidats In gnal, two constaints a intaction candidats fo on anoth if thy hav common pconditions, but incompatibl postconditions, as is illustatd in Figu 1. Th lft-hand sid of th figu shows th situation wh th incompatibility of postconditions manifsts itslf in th stat immdiatly following th stat that is fd to by th pcondition. Th ight-hand sid shows that th incompatibility may also occu at a lat stat. Ou mthod to dtmin intaction candidats consists of two pats: pcondition intaction analysis dtmins constaints with pconditions that a nith xclusiv no indpndnt of ach oth. This mans that th a situations wh both constaints might apply. Thi postconditions hav to b chckd fo incompatibility. Postcondition intaction analysis, on th oth hand, dtmins as candidats thos constaints with incompatibl postconditions. If in such a cas th pconditions do not xclud ach oth, an intaction may occu. 4.1 Pcondition Intaction Analysis To dcid if two constaints 2 x y and u w might intact on thi pcondition, w pfom th following asoning: if th two constaints hav common litals in thi pcondition (x \ u 6= ), thn thy a ctainly intaction candidats. 2 Undlind idntifis stand fo sts of litals.

5 p post q p p q => => q p => => p p facts ass. q. Figu 2: Candidats fo pcondition intaction But th common pcondition may also b hiddn. Fo xampl, if x contains th vnt, u contains th pdicat lital p, and is only possibl if p holds (p ), thn w also hav dtctd a common pcondition of th two constaints. Th common pcondition may also b dtctd via asoning on pdicats. If, fo xampl, x contains th pdicat lital p, u contains th pdicat lital q, and p ) q o vic vsa, thn th is a common pcondition. Figu 2 shows th gnal appoach to find intaction candidats of th pcondition fo a nw constaint among th facts, assumptions, and quimnts alady dfind. To fomally dfin th st C p (c 0 fa) of candidats of pcondition intaction of a nw constaint c 0 with spct to a st fa of constaints psnting facts, assumptions, and quimnts, w fist intoduc som auxiliay dfinitions: fo ach vnt, pdicat lital pl and constaint c, w dfin = fpl : PLit j pl g p pdicats(c) = (pcond(c) \ PLit) [ S 2pcond(c)\EVENT With ths pliminais, w can dfin C p (c 0 fa) = fc : fa j pcond(c) \ pcond(c 0 ) 6= g [ S x2p pdicats(c 0 )fc : fa j (( ) x [ x ) ) \ pcond(c) 6= ) _ (9 : pcond(c) \ EVENT y : ) x [ x ) y This dfinition can b xplaind as follows: all constaints c with a common lital in th pcondition a candidats. Fo vnts in th pcondition of c 0, all pdicats that a ncssay fo to occu a collctd. Togth with th pdicat litals containd in c 0 s pcondition, thy fom th st p pdicats(c 0 ). Fo ach x 2 p pdicats(c 0 ), th tansitiv closu with spct to implication is computd, wh both fowad (x ) ) and backwad chaining ( ) x) a pfomd. This is ncssay bcaus wak as wll as stong litals hav stats in common with x. Moov, this nsus that th dtmind candidats a indpndnt of th od in which th constaints a addd. Each constaint c whos pcondition contains an lmnt of th tansitiv closu of som x is a candidat. But also thos c that contain in thi pcondition an vnt that has a ncssay pcondition containd in th tansitiv closu of som x must b addd to th st of candidats. Not that on vnt litals : no chaining is pfomd, bcaus usually it is impossibl to inf anything fom th non-occunc of an vnt 3. Fom th dfinition of C p (c 0 fa), it follows that th st of candidats is indpndnt of th od in which th constaints a addd, and that th candidat function distibuts ov st union of th pconditions of constaints: 3 Othwis, w simply would st up a tabl with ntis pl sam way as postiv ons. )g : and tat ngativ vnt litals in th

6 p post p q p p => q => p facts ass. q. Figu 3: Candidats fo postcondition intaction 8 c c 1 c 2 : Constaint cs : Constaint c 2 2 C p (c 1 cs [ fc 2 g), c 1 2 C p (c 2 cs [ fc 1 g) ^ pcond(c) = pcond(c 1 ) [ pcond(c 2 ) ) C p (c cs) = C p (c 1 cs) [ C p (c 2 cs) Th latt implis that, whn a constaint is changd by adding a nw lital to its pcondition, th intaction analysis has to b pfomd only on this nw lital. 4.2 Postcondition Intaction Analysis To dtmin th candidats fo postcondition intaction, w pocd similaly. To find conflicting postconditions, w pfom fowad chaining on th postconditions of th nw constaint, ngat th sulting litals, and chck if on of th ngatd litals follows fom th postcondition of anoth constaint. This constaint is thn idntifid as an intaction candidat. To pfom fowad chaining on vnts, th infomation containd in th tabl of vnts stablishing pdicat litals ( p) is usd. Again, on ngativ vnt litals, no chaining is pfomd. Figu 3 givs an ovviw of th pocdu. W nd th auxiliay dfinitions = fpl : PLit j plg post pdicats(c) = (postcond(c) \ PLit) [ S 2postcond(c)\EVENT ls 1 opposit ls 2, 9 x : ls 1 : x 2 ls 2 wh ls 1 ls 2 a sts of litals and : : l = l. Now, w can dfin C post (c 0 fa) = fc : fa j postcond(c) opposit postcond(c 0 )g [ fc : fa j 9 x : post pdicats(c) y : post pdicats(c 0 ) x ) opposit y ) g This dfinition is symmtic, too, and C post distibuts ov st union of postconditions of constaints. 4.3 Empiical Rsults W hav usd this automatic pocdu to dtmin intaction candidats fo a lift systm [4]. It tund out that compad to a complt analysis about 70% of th analysis ffot could b savd. Howv, th pocdu did not find th ight intaction candidats whn th constaints mad statmnts about systm stats that a not conscutiv (as shown on th lft-hand sid of Figu 1), but wh indfinitly many intmdiat stats a possibl (as shown on th ight-hand sid of Figu 1). Th ason was that ou smantic tabls (s Sction 3) did not contain nough infomation to dtct such intactions. Th ncssay infomation, howv, can b addd systmatically.

7 Constaints can b assignd a distanc, which chaactizs th numb of intmdiat stats that a possibl btwn th p- and post stats latd by th constaint. Fo ach constaint with a distanc gat than on, additional infomation is ndd. It can b xpssd as scnaios that show on th on hand how to pocd on stp fom th bginning stat (to pfom pcondition intaction analysis) and on th oth hand on stp that lads to th final stat (to pfom postcondition intaction analysis). Whn such scnaios a addd to th sts of constaints, ou pocdu finds as candidats all constaints wh an intaction actually occus. Mo cas studis must b pfomd to find out if this nhancmnt suffics to find all pactically occuing fatu intactions and if th pcntag of analyss savd is stabl fo diffnt application domains. 5 Discussion Th appoach fo th dtction of fatu intactions w hav psntd is tuly huistic. This mans, w cannot guaant that all intactions that might occu a found by ou automatic pocdu. Ou aim is to povid a simpl pocdu that woks wll in pactical cass and that may b applid whn a complt intaction analysis is infasibl. Th vitu of ou appoach lis in th fact that intactions on th quimnts lvl can b dtctd vy aly, bfo th fomal spcification is st up, and with lativly littl ffot. Evn though dtmining th intaction candidats is tdious if pfomd by hand, th pocdus to dtmin th sts C p and C post as dfind in Sction 4 a vy asy to implmnt. Thom poving tchniqus a unncssay. Th numb of intaction candidats that a yildd by ou pocdu and that must b inspctd is much lss than if a complt analysis w pfomd. Th smantic infomation collctd in th tabls of ncssay conditions fo vnts, vnts stablishing pdicat litals, and lations btwn pdicat litals not only contibuts to a btt undstanding of th quimnts, but also gatly facilitats th pocss of stting up and validating a fomal spcification fo th softwa systm to b built. Ou appoach to dtct fatu intactions is indpndnt of th od in which th fatus a addd. W do not attmpt to solv fatu intactions automatically. Such dcisions influnc th ovall bhavio of th systm and hnc should b takn by th systm dsigns o customs. Rfncs [1] D. Colman, P. Anold, St. Bodoff, Ch. Dollin, H. Gilchist, F. Hays, and P. Jmas. Objct- Ointd Dvlopmnt: Th Fusion Mthod. Pntic Hall, [2] M. Hisl. Agndas a concpt to guid softwa dvlopmnt activits. In R. N. Hospool, dito, Poc. Systms Implmntation 2000, pags 19 32, London, Chapman & Hall. [3] M. Hisl and J. Souquiès. Mthodological suppot fo quimnts licitation and fomal spcification. In Poc. 9th IWSSD, pags IEEE Comput Socity, [4] M. Hisl and J. Souquiès. Dtcting fatu intactions a huistic appoach. In G. Saak and C. Tük, ditos, Poc. 1st FIREwoks Wokshop, Ppint 10/98, pags 30 48, Fakultät fü Infomatik, Univ. Magdbug. Availabl via db/voffntlichungn/98/saatu98.html [5] M. Jackson and P. Zav. Diving Spcifications fom quimnts : an Exampl. In Poc. ICSE 95, pags ACM Pss, 1995.