ATLAS, a platform for transparently developing distributed applications

A las : a pla o m o anspa en ly de eloping dis ibu ed applica ions M. Fai en and  A. Vinacua Depa men o So wa e, Ins i u e o Rob o ics and Indus ial In o ma ics, U.P.C. Diagonal 647, 8 ena plan a E08028 Ba celona, Spain +34 3 401 6739 m ai en,al a g @lsi.upc.es ABSTRACT We discuss he design and implemen a ion o a so - wa e de elopmen pla o m ha allows unsophis ica ed p og amme s o include ad anced ea u es o hei ap- plica ions wi h no o e y li le ex a in o ma ion and eo . These ea u es include he spli ing o he ap- plica ion in dis inc p o cesses ha may b e dis ibu ed o e a ne wo k, a p owe ul congu a ion and sc ip ing language, and se e al o ols including an inpu sys em o easily cons uc easonable in e aces. We a emp o desc ib e b o h he echniques used o achie e ans- pa ency o he p og amme and wha exac ly a use mus do o build new A las mo dules. KEYWORDS Dis ibu ed applica ions, So wa e de elopmen o ols, Ex e nal da a ep esen a ion 1 INTRODUCTION A las ep esen s an a emp o acili a e o p og am- me s in ou lab he inco p o a ion o hei applica ions o ce ain se ices a a minimum cos . In ac , A las helps hem build dis ibu ed applica ions (p oblems in ol ed in dis ibu ion we e o cused in [2]), p o ides hem wi h p owe ul congu a ion mechanisms and a mac o lan- guage and include a jou naling acili y, among o he hings. All hese asp ec s equi e a subs an ial amoun o amilia i y by he p og amme wi h he said ech- niques, and a ai amoun o p og amming o hem on each applica ion. In a esea ch lab like ou s a la ge amoun o so wa e is cons an ly b eing de elop ed o demons a e ideas, bu e en ually has o b e ew i en when ce ain unc ion- ali ies need o b e combined. I also emains in an un- nished s a us and is usually ha d o use o demon- s a e by anyb o dy excep he au ho , b ecause o he high cos o nishing up" he de ails. We hen eal- ized ha a de elopmen pla o m ha acili a ed he cons uc ion o his so wa e gi ing easy access o many sophis ica ed esou ces and es ablishing basic ules ha eased he join op e a ion o mo dules was aluable o us, since i enabled exp e imen al so wa e o g ow in a mo e ha monious way, and acili a ed e en he euse o old comp onen s. E ol ing om he ideas o iginally p esen ed in [9] we designed A las o his pu p ose. Since ou wo k is cen- e ed in Compu e G aphics applica ions we included ce ain mechanisms esp ecially designed o ha domain which we' e discussed in sp ecialized o ums [6]. How- e e by a he la ges p o ions o eo and o co de in ol ed e y gene al o ols usable by in e ac i e applica- ions in mos o he domains. The e o e A las can also b e iewed as a gene al p og amming o ol ha g ea ly simplies he cons uc ion o ai ly sophis ica ed appli- ca ions. In his pap e we shall p esen a iew o A las om his las p e sp ec i e, discussing he gene al asp ec s no p esen in o he publica ions ab ou i and showing how and in which ways i can enhance he so wa e de elop- men p o cess. In he nex sec ion we shall enume a e mo e conc e e ob jec i es and c i e ia used in he design o A las . Sec ion h ee will b iey p esen he solu- ion adop ed o he in e p o cess communica ion, and sec ion ou will desc ib e asp ec s o he design and im- plemen a ion o an A las applica ion om he p oin o iew o he use | he p og amme building he appli- ca ion. Finally we will u n b iey o conclusions and u u e wo k. 2 OBJECTIVES AND DESIGN CRITERIA The  s p io i y in he design o A las has b een o make i s inne wo kings as anspa en o he use as p ossible. To his end, some asp ec s may no ha e he in insically b es o mos p owe ul o mos exible so- lu ion, bu use s can build applica ions on A las wi h- ou almos any conce n ab ou i , ye ge ing subs an ial b ene s om i s p esence. In e ms o unc ionali y, A las includes hese ob jec- i es:  Low le el o pa al lelism . A las applica ions ea- u e se e al dis inc p o cesses unning concu en ly in he same o die en machines in a ne wo k; p o cesses encapsula e A las comp onen s o use mo dules. The use implemen s ou ines ha a e 1 accessible o o he p o cesses as emo e p o cedu es.  In e p ocess communica ion . Since he applica ion is spli in se e al p o cesses, hese need o com- munica e o e he ne wo k and exchange da a b e- ween p ossibly die en a chi ec u es. This gi es ise o many die en p oblems ha need o b e ad- d essed [1].  S anda dized inpu model . A g ea deal o eo in an applica ion's de elopmen is sp en in i s use in e ace. A las p o ides a uni o m bu exible iew o inpu s ha allows many die en dialogue mo des in a uni o m way, somewha ela ed o Ab- s ac Da a Views [3].  Congu a ion and mac o language . A exible ye p owe ul way mus b e p o ided o a p og amme o desc ib e wha is in each o his mo dules, and how i should ela e o o he s, and also o dene he dialogues o he applica ion and i s b eha io in a simple way.  Jou naling mechanism . This is no ye imple- men ed and only men ioned he e in passing. I is pa ially designed and will b e a he hea o o he se ices oe ed by A las .  Faul ole ance . Since he applica ion is sp ead ou among se e al hos s, i b ecomes mo e exp osed o ansien o p e manen ailu es (o he communica- ions o o any o he hos s in ol ed). Faul ole - ance is anspa en ly p o ided based on he jou - naling mechanism and on hea b ea messages sen by all p o cesses so ha hei s a us can b e assessed.  Reusabili y . Each use mo dule is comple ely iso- la ed om o he s (in a sepa a e p o cess) excep h ough a well dened in e ace desc ib ed in A - las 's p og amming language. Thus new comp o- nen s can inco p o a e and use eliably old ones. No all hese asp ec s can b e discussed he e, as hey would ake an ino dina e amoun o space. The ollow- ing sec ions cen e a ound asp ec s o he p esen b e a e sion conce ning he housekeeping o p o cesses and communica ions, and how A las helps use s build ap- plica ions wi h hese ea u es easily. 3 IMPORTANT ISSUES IN THE A las COM- MUNICATIONS MECHANISM 3.1 P oblems Tha Need To Be Add essed Since A las  s p io i y is o oe he maximum ans- pa ency o he de elop e , he design o A las a chi ec- u e mus hide he in icacies o he in e p o cess com- munica ions om he p og amme . The communica ions mechanism has o add ess p ob- lems like how o s a a p o cess in an applica ion, how o manage he in e change o in o ma ion b e ween p o- cesses and also how o de ec ailu es in he applica- ion communica ions in o de o know when he aul - ole ance mechanism should b e ac i a ed. To s a a p o cess in a die en machine i is necessa y o ha e a p o cess lis ening o a connec ion in he cho- sen hos ( his equi es o ha e knowledge o low le el connec ions in he op e a ing sys em), and i would b e also desi able ha he p o cess o b e s a ed inhe i s he applica ion en i onmen . The in e change o in o ma ion b e ween p o cesses has also an added dicul y when hey a e unning in an he e ogeneous ne wo k b ecause da a can b e in e p e ed wi h die en meaning dep ending on he a chi ec u e whe e hey a e used. Nex subsec ion explains how hese p oblems ha e b een add essed in A las . 3.2 The App oach Used In A las The A las a chi ec u e is ep esen ed in gu e 1, whe e he o als deno e p o cesses and he a ows ep esen communica ions b e ween hem. I is a cen alized a - chi ec u e whe e he p o cess dis ac s as he mas e p o cess and is he cen e o each A las applica ion. This a chi ec u e allows an in elligen dis ibu ion o b e managed, i.e. he dis p o cess is able o decide he p o cesses dis ibu ion dinamically dep ending on he ap- i ude o each hos in he ne wo k o un each applica- ion p o cess. The e o e, his mas e p o cess is he mos c ucial one in his a chi ec u e and also in he communica ions mech- anism b ecause i is he communica ions cen e o each applica ion. I is also he one o ake ca e ab ou he s a us o each p o cess in execu ion a any ime. This is easy b ecause o he hea bea mechanism designed in A las . The hea bea mechanism makes e e y p o cess b eing ex- ecu ed in he applica ion send a sho message p e io di- cally o he mas e p o cess gi ing he equi ed in o ma- ion o con ol he global s a us o he execu ion. This mechanism is e y use ul o de ec i a p o cess o he communica ion wi h i ail, he e o e i will b ecome nec- essa y o he aul - ole ance mechanism which, al hough i is no a ailable ye , has b een almos comple ely de- signed. This mechanism will b e sho ly discussed in he u u e wo k sec ion b elow. Se e P ocess Design The A las se e p o cess is a simple bu e y imp o - an p o cess in he A las a chi ec u e. I s ole, in he 2 B-Rep Volume Command Subsys em Inpu Subsys em Sol e Cons ain s dis se e @hos Oc ee Machine Figu e 1: A sample execu ion o an A las applica ion. cu en e sion, is o accep connec ions om he mas- e p o cess dis and un he applica ion p o cess e- ques ed by i . In u u e e sions i will also implemen a handshake wi h dis o achie e a ce ain deg ee o load-balancing. As he se e daemon is no mally un a b o o ime wi h oo p e missions, i mus change i s p e missions o he use 's p e missions b e o e execu ing he use 's p o cess in o de o p o ec he sys em. Mo eo e , o do ha i mus b e su e o he iden ica ion o he use and also inhe i he use en i onmen o achie e a success- ul execu ion. This en i onmen in o ma ion is sen by he dis p o cess in he connec ion message. The use iden ica ion mechanism is based on a sp ecial 32 by es long use iden ie ha e e y A las use mus ha e gene a ed b e o e he execu es A las o he  s ime. Communica ions D i e s Design An A las applica ion p o cess execu ion is based on a emo e-p ocedu e-cal l -like pa adigm. A p o cess can b e hen conside ed as a se o ou ines o do he p o cess e- la ed wo k plus a communica ions d i e o manage he in e change o messages wi h he es o he applica ion (see sec ion 4). The communica ions d i e is he main p og am o he p o cess, and i s ole is o lis en eques s o messages om he mas e p o cess and o he connec ions added and dispa ch hem as needed. The mos ele an e- ques s o messages sen by he mas e p o cess can b e ela ed wi h a ou ine call, da a answe ing a eques , o an A las e en no ica ion (see sec ion 4.3). The main execu ion o an A las p o cess is hen he dispa ch ou ine o i s communica ions d i e . I en e s in a lo op lis ening in he g oup o channels ac i ed by his p o cess ( he de aul is only he connec ion wi h he mas e p o cess), and when i ecei es a message i dispa ches i and keep on lis ening o ano he one (see gu e 2). Apa om dispa ching messages he d i e can also dispa ch in e up ions (like signals). In ac he hea bea mechanism uses he SIGALRM in e up ion. D i e add ime add channel ou ine call ATLAS e en ea men answe da a dispa ch dispa ch dispa ch imeou handling imeou sub channel op ional Figu e 2: D i e ole scheme. The de aul ea men o an A las p o cess d i e is managing he messages comming om he mas e p o- cess and he in e up ion o SIGALRM. Bu he ac ual d i e is much mo e exible (as can b e seen in gu e 2). I also allows he p o cess o add channels o lis en o, add a ime ea men o emo e channels added b e- o e. This exibili y only equi es o ha e implemen ed he ea men o messages b eing ecei ed by hese new channels. The A las communica ions mechanism is implemen ed using so cke s, and i s implemen a ion akes p o o he w app e classes o so cke s oe ed by he public domain package ACE W app e s (see [7] and [8]). 3 The bigges p oblem o sol e in communica ions b e- ween A las p o cesses is he in e change o da a. Since hese p o cesses may b e unning in die en a chi ec- u es, da a mus b e sen h ough he ne wo k using a s anda d ep esen a ion in o de ha hey ha e he same meaning o he die en p o cesses. Fu he mo e, hese da a need o b e {a leas e y o en{ accessible o A las i sel , which includes a p og am- ming language o dene use -machine dialogs o o he - wise in e connec o die en mo dules (see sec ion 4). The p oblem o ac ually ans e ing he da a obus ly has long since b een sol ed. Indeed we jus ely on XDR [10] o ha pu p ose. To a ain hese da a sha ing wi h he maximum o anspa ency o he de elop e , A las includes a mechanism based on ou da a s uc- u es (called Va iables ) used o w ap use da a in each p o cess ( his mechanism is ho oughly explained in [5]). These s uc u es oe access me ho ds used by he in e - p e e o he command language, and also encapsula e me ho ds o enco de and deco de XDR s eams anspa - en ly. Using his mechanism and adding also he au- oma ic co de gene a ion explained in he nex sec ion, he de elop e needs no b e awa e o XDR and indeed almos no b e awa e o ou in e change me ho d a all. 4 DESIGNING AN APPLICATION. BENE- FITS TO THE DEVELOPER. 4.1 T anspa ency To The De elop e The mos imp o an aim ha A las wan s o achie e is a high le el o anspa ency o he de elop e . Almos all se ices ha A las oe s o applica ions de elop ed o e i a e echniques ha no mally equi e a lo o sp e- cialized p og amming. The emphasis in A las 's design is o elie he p og amme om his eo . Many acili ies oe ed by A las , like dis ibu ion o he communica ions mechanism, mus b e o ally man- aged by A las in o de o achie e he maximum ans- pa ency o he de elop e . The p og amme need no know ab ou hese mechanisms and can concen a e on he ob jec i es o his applica ion. Au oma ic Code Gene a ion The A las p o cess communica ions equi e qui e a bi o co de in each p o cess de o ed o handshaking wi h dis , gene a ing he hea bea messages a he ade- qua e a e, p epa ing he a gumen s o p o cess ou- ines o collec ing esul s and enco ding hem o b eing ansp o ed o e he ne wo k, and dispa ching calls o p o cess ou ines. To handle his, A las au oma ically gene a es co de s ubs ha he de elop e mus link wi h his p og am. These s ubs a e cons uc ed om he in- e ace decla a ion o he p o cess (like in gu e 3), which con ains he yp e deni ions used o a iables o b e exp o ed and he p o o yp e deni ions o he p o cess ex e n ou ines. The gene a ed co de also includes s ubs o au oma i- cally ans e he use 's da a in o A las Va iables and backwa ds, h ough b idge ypes used o isola e he de- elop e om he de ails o he A las Va iables (which an ad anced p og amme can use di ec ly i he wishes o). The b idge ypes a e used o build in e media e ob- jec s wi h he da a s uc u e o he p o cess ob jec s (as p e hei A las decla a ions) bu wi hou he me ho ds o he p o cess ob jec s (which emain unknown o A - las ). Each b idge ype has also me ho ds o ansla e o and om A las Va iables , making b o h ansla ions anspa en o he de elop e . The only bu den on he de elop e is hen o p o ide his classes wi h con e sion me ho ds o and om hese b idge ype ob jec s which is usually i ial (unless he de elop e choses o ha e a e y die en s uc u e o he A las da a ha he one used in e nally by his p og am). As an example we can see some ele an p o ions o his au oma ically gene a ed co de in app endix. 4.2 Design P o cess O An A las Applica ion An A las applica ion is a se o p o cesses which can communica e b e ween hem h ough he A las com- munica ions mechanism. Each p o cess can b e seen as a mo dule oe ing some public me ho ds ha can b e used by he o he p o cesses. The e o e, i mus b e designed as a se o exp o ed ou ines ha may b e called by o he p o cesses. This should b e he gene al iew o a p o cess b elonging o an A las applica ion. F om he de elop e p oin o iew, a p o cess consis s o wo pa s: i s in e ace and i s implemen a ion. The P ocess In e ace The p o cess in e ace is a mo dule w i en in ATL lan- guage which denes he p o o yp e o he public ou ines o he p o cess and he needed yp es o hei pa ame e s and e u n esul s. The ATL language (desc ib ed in de ail in [4]) is an im- p e a i e and mo dula language designed o A las ap- plica ions. I allows o dene yp es, a iables, unc ions and p o cedu es ha can b e exp o ed ( isible o he o h- e s) o lo cal. I also accep s he mos common con ol s uc u es inside unc ions and p o cedu es (condi ion- als, lo ops, e c.) and ou ine calls b o h synch onous and asynch onous. Al hough only i s p o o yp es and yp es a e needed o a p o cess in e ace, he mo dule dening he p o cess in e - ace can also include unc ions o p o cedu es dened in ATL which desc ib e he in e ac ion wi h o he p o cesses 4 in he applica ion o wi h he use (e.g. asking o in- pu da a). ATL mo dules which a e no he in e ace o any p o cess bu dene he execu ion co o dina ion and in e ac ion b e ween p o cesses can also b e dened in he applica ion, and use s may dynamically add hei own. An example o an ATL mo dule wi h pa o he in e ace o a p o cess called olum can b e seen in gu e 3. Since i is an example i is no comple e, bu i shows he de - ini ion o a se o yp es exp o ed by he mo dule (some o hem a e needed as pa ame e s o ex e n ou ines), he p o o yp es o wo ex e n ou ines ( hese p o o yp es and he yp es o i s pa ame e s would o m he in e ace o he p o cess), and he desc ip ion o a p o cedu e (b e- ing also exp o ed o b e isible o o he mo dules) ha combines he execu ion o he p o cess ou ines, asks o an inpu da um ( h ough GETDATA ) and also calls o a p o cedu e o ano he mo dule ( se::Ou pu ). USE se; EXPORT #de ype poin STRUCT x -> eal; y -> eal; z -> eal; ENDSTRUCT EXPORT #de ype ace VECTOR [3] OF STRUCT p1 -> poin ; p2 -> poin ; id -> in ege ; ENDSTRUC T EXPORT #de ype simplex STRUCT name -> s ing; sides -> VECTOR [4] OF ace; ENDSTRUC T EXPORT #de ype scene VECTOR [100] OF simplex EXPORT #de ype p ope y in ege EXPORT scene o alsc; ... PROT EXTERN FUNCTION segmen a io n (scene sc, p ope y p) RETURNS scene; EXTERN PROCEDURE display_ sc ene (scene sc); ... ENDPROT ... EXPORT PROCEDURE Segmen S im pl ex () IS display_s ce ne (segmen a i on ( o a ls c, GETDATA(" In pu he p ope y alue"))) ; se::Ou pu ("Segmen a i on comple e d" ,"m ") ; ENDPROCED UR E Figu e 3: Po ion o he in e ace deni ion in A las o he olume mo deling p o cess ( olum"). The P ocess Implemen a ion Since gi ing he p o cess in e ace in he ATL mo d- ule allows A las o gene a e co de s ubs o implemen he communica ions d i e o his p o cess (see subsec- ion 4.1), om he de elop e p oin o iew he p o cess implemen a ion consis s o he se o C++ ou ines de- cla ed as ex e nals in he ATL mo dule plus he deni- ion o he C++ classes used by hei pa ame e s. This implemen a ion can also include wha e e he de elop e wan s as a p i a e pa o he p o cess. This pa won' b e isible ou side he p o cess. Figu e 4 shows how an A las execu able p o cess is gen- e a ed om i s sou ce les. The les depic ed on he le mos column a e hose ha he de elop e mus imple- men . The au oma ically gene a ed co de is di ided in h ee les: he a l p ocess.hh le denes he C++ p o- o yp es o he p o cess ou ines decla ed as ex e n ou ines in he ATL mo dule; he a l p ocess.H le has he b idge ypes implemen a ion o ha yp es used by he ex e n ou ines o he p o cess; nally he a l p ocess.C le implemen s he main co de o he communica ions d i e and also auxila y ou ines which con e A las Va iables o he p o cess C++ classes and backwa ds in o de o b e able o call he co e- sp onding p o cess ou ine wi h he co ec pa ame e s and esul a iables. 4.3 O he Bene s To The De elop e The e a e also o he b ene s p o ided o he de elop e ha mayb e a e in e es ing o b e men ioned:  A un ime, he A las ke nel keeps some s uc- u es con aining in o ma ion ab ou he s a us o he applica ion (wha p o cesses a e in execu ion, i he e is some eques wai ing o an inpu da a, e c). To gi e he opp o uni y ha an applica ion p o cess b e in o med ab ou changes in his in e nal s uc u es, A las oe s he A las e en s mecha- nism ha allows he p o cess o ask o a subsc ip- ion o a pa icula A las e en (ADD PROCESS when a new p o cess s a s in he applica ion o ADD INPUT when an inpu is p o duced by he end use , o example). Whene e an A las e en is p o duced he dis p o cess sends he co esp ond- ing e en message o e e y p o cess subsc ib ed o ha e en , and he d i e o he p o cess, when i ecei es his e en message, calls he ou ine a - ached o his e en a subsc ip ion ime.  In o de o add ess he s anda dized inpu model p esen ed as an A las ob jec i e in sec ion 2, A - las also oe s a gene ic inpu handling p o cess. I p o ides a window in which all he ex ual in e - ac ions o ccu (issuing commands o en e ing nu- me ical da a), bu can also b e ins uc ed o cap- u e e en s om o he windows (owned by he es o he p o cesses in he applica ion) and conside hem inpu da a o b e channeled o hose p o cesses. The inpu sys em is also ex ensible. In ac i is also an in e ace b e ween A las and an ex ended Tcl/Tk [11] engine, so sc ip s in Tcl can b e sen o i o ins an ia e new in e ace comp onen s. Using his A las comp onen he de elop e can p epa e he use in e ace o his applica ion al- mos i ially, and dedica e mos o his ime o he p op e sub jec o his applica ion. 5 (p o o ypes) ATLAS p ocessgene a ed ilesde elope iles a l_p ocess.hh a l_p ocess.H a l_p ocess.C (b idge ypes) (d i e main) p ocess.a l in e ace p ocess.h (C++ classes) includes includes p ocess.C p i a e pa + ou ines ex e n implemen a ion p ocess Figu e 4: C ea ing an A las p o cess.  The i ual machine ha in e p e s ATL co de al- lows b o h synch onous and asynch onous calls o ex e nal ou ines. Some p o cesses may he e o e ac as la ge ba ch p o cesses ha a e execu ed con- cu en ly wi h he applica ion. To endez ous wi h hese asynch onous calls, A las uses a simple de- ice: he i ual machine ags all ou pu pa am- e e s o e u n alues o an asynch onous call as di y", and any a emp o use one o hem as an - alue eezes he execu ing h ead. Thus asyn- ch onous calls may b e issued and o he p o ions may p op e ly awai hei comple ion in a anspa - en manne . 5 CONCLUSIONS AND FUTURE WORK We ha e p esen ed a so wa e pla o m designed o al- low de elop e s o inco p o a e ad anced ea u es in hei de elopmen wi h he leas hassle. I is p esen ly b eing used wi hin ou lab o p o se e al packages de elop ed he e, and also o build new ones. I 's design a o s he cons uc ion o eusable mo dules ha can ela i ely easily b e combined wi h each o he . This seems e y desi able, esp ecially in an en i onmen like ou s, whe e la ge p o ions o co de a e cons an ly b eing gene a ed by s uden s which la e depa . The use s may jus as easily add p o cesses o implemen a new applica ion o o ex end A las i sel . P esen ly, o ins ance, a menu-handling mo dule is b eing con- s uc ed, ha will hen b e a ailable o all o he A las applica ions o dene hei own, e y exible, menus. We a e cu en ly p o ing he cu en e sion (0.2) o A las o die en pla o ms, cu en ly including Suns unde b o h Sola is 1.x and 2.x, and HPUX, bu so on o include also SGI's IRIX 6.x and Windows NT. We a e also comple ing he jou naling mechanism, which will no only allow eplays o sessions, bu will supp o he aul - ole ance wi hin A las , and will p o ide un- limi ed (alb ei exp ensi e) undo's and edo's h ough a commi " blo cking ins uc ion wi hin he jou nal, and supp o o in e se unc ions. Among he nea u u e p o jec s, we plan o add some supp o o CSCW by he simple de ice o cloning he applica ion o he die en use s collab o a ing, and es- ablishing sp ecial connec ions b e ween he co esp ond- ing dis p o cesses, only one o which ac s as mas- e . This, al hough limi ed, would u n essen ially e e y A las applica ion in o an CSCW-capable applica ion, wi h no o ex emely li le eo by he de elop e s, as p e A las 's equi emen s. REFERENCES [1] G. R. And ews. Pa adigms o P o cess In e ac ion in Dis ibu ed P og ams. ACM Compu ing Su - eys , 23(1), Ma ch 1991. [2] R. S. Chin and S. T. Chanson. Dis ibu ed Ob jec - Based P og amming Sys ems. ACM Compu ing Su eys , 23(1), Ma ch 1991. [3] D. D. Cowan and C. J. Lucena. Abs ac Da a Views: An In e ace Sp ecica ion Concep o En- hance Design o Reuse. IEEE T ansac ions on So wa e Enginee ing , 21(3), Ma ch 1995. [4] M. Fai en and A. Vinacua. ATLAS. Sis ema de Co- mandes: Manual ecnic (in Ca alan). Repo LSI- 95-11-T , 1995. h p://www.lsi.up c.es/~ m ai en. [5] M. Fai en and A. Vinacua. In e p o cess da a ans- e in A las , a pla o m o dis ibu ed applica- ions. 1997. Submi ed o he OPENARCH'98 con- e ence. [6] M. Fai en and A. Vinacua. A las , a pla o m o dis ibu ed g aphics applica ions. 1997. To app ea in he p o ceedings o Eu og aphics Wo kshop on P og amming Pa adigms in G aphics. [7] D. C. Schmid . The ADAPTIVE communica ion en i onmen : Ob jec -o ien ed ne wo k p og am- ming comp onen s o de eloping clien /se e ap- 6 plica ions. In 12 h Sun Use s G oup Con e ence , 1994. [8] D. C. Schmid . Reac o : An ob jec b eha io al pa - e n o concu en e en demul iplexing and e en handle dispa ching. In P oceedings o he 1s Pa - e n Languages o P og ams Con e ence , Augus 1994. [9] A. So o, S. Vila, and A. Vinacua. A To olki o cons uc ing command d i en g aphics p og ams. Compu e & G aphics , 16(4):375{382, 1992. [10] R. S ini asan. R c 1832: Xd : Ex e nal da a ep- esen a ion s anda d, Augus 1995. [11] B. B. Welch. P ac ical P og amming in Tcl and Tk . P en ice Hall PTR. Upp e Saddle Ri e , New Je sey 07458, 1995. APPENDIX: AN EXAMPLE OF THE AUTOMATI- CALLY GENERATED CODE Using he p o ion o he olum p o cess in e ace shown in gu e 3 he A las co de gene a o makes au o- ma ically he les a l olum.hh , a l olum.H and a l olum.C which a e pa ially depic ed in gu es 5 h ough 7. #i nde __ATL_ o lu mh h_ _ #de ine __ATL_ o lu mh h_ _ #i nde NOHEADER #include " olum.h" #endi #include "a l_ olu m. H" scene segmen a i on (s ce ne, p op e y) ; oid display_ sce ne (s ce ne) ; #endi Figu e 5: The au oma ically gene a ed a l olum.hh le. Figu e 5 shows he co de gene a ed o dene he C++ p o o yp es o he wo ex e n ou ines decla ed in he in e ace. This le also includes he olum.h le im- plemen ed by he de elop e b ecause he p o o yp es use p o cess yp es only known by he de elop e co de. In gu e 6 we can see wo o he  e b idge ype deni- ions co esp onding o he exp o ed yp es dened in he in e ace: he mos simple one is he one co esp onding o a yp e deni ion which dep ends on ano he yp e de- ned b e o e, and he o he one is he one co esp onding o he mos complex one whose con en s dep end on an- o he yp e dened b e o e and i s me ho ds show how his b idge ype is made om an A las Va iable and back- wa ds. These wo me ho ds make p ossible he au oma ic ansla ion b e ween he b idge ype and he co esp ond- ing Va iable , isola ing he de elop e om his A las ex e nal ep esen a ion. The las one, gu e 7, shows a p o ion o he main co de o he d i e . This co de includes an auxilia y ou ine o each one dened as an ex e nal ou ine in he in e ace (in he gu e only he one o segmen a ion ou ine is shown), and he main ou ine o he communica ions d i e . The auxilia y ou ine is he one o ansla e he pa ame e yp es om Va iables o he p o cess yp es ( h ough b idge ypes ) in o de o call he p o cess ou- ine in he co ec way, and also o ansla e back he esul o he p o cess ou ine o ha e a Va iable o go h ough he ne wo k. The main ou ine o he d i e only ha e o make some ini ializa ions o i and en e in he dispa ching lo op. 7 ... namespace olum { ypede a l_py a mi d a l_simpl ex; } namespace olum { s uc a l_scene { a l_simp le x con [10 0]; ope a o Va iable () { // au oma i c ansla io n o a Va iable Type (" olum: :s cen e" , "V[100]S( nam e s ing,ba se V[3]S(p1 S(x eal,y eal,z eal), p2 S(x eal,y eal,z eal), iden in ege ) , sides V[3]V[3] S( p1 S(x eal,y eal,z eal), p2 S(x eal,y eal,z eal), iden in ege ) )" ); Va iable ( ,"") ; .build_ ee () ; o (in i1=0;i1 <10 0; i1 ++) { *((*( .T e e( ))) .a cc ede (i 1) ) = *(((Va i ab le )c on [i 1] ).T e e( )) ; } e u n ( ); } a l_scen e( ) {} a l_scen e( Va ia bl e & ) { // cons uc o om a Va iable i ( .T ee() == NU LL) a l_exi (- 1) ; // In alid a iable o (in i1=0;i1 <10 0; i1 ++) { Va iable 2("S(nam e s ing,ba se V[3]S(p1 S(x eal,y eal,z eal), p2 S(x eal,y eal,z eal), iden in ege ) , sides V[3]V[3] S(p 1 S(x eal,y eal,z eal), p2 S(x eal,y eal,z eal), iden in ege ) )" ," "); 2.build_ ee (); *( 2.T ee () )= *(( *( . T e e( )) ). acc ed e( i1) ); a l_simpl ex paux( 2 ); con [i1]= p au x; } } }; } ... Figu e 6: Po ion o he au oma ically gene a ed a l olum.H le. Comunic_D is dis ib( CAN AL _C OMU NI C_ DI STR ); S ing namep og am ; D i e d i (dis i b) ; ... oid aux_segm en a io n( S in g codi, DLLis <Va ia bl e *> ¶me e s ) { Pix p=pa ame e s. i s () ; a l_scen e p p0(*(pa am e e s(p )) ); scene pa 0(p p0 ); pa ame e s .ne x (p ); p ope y p p1(((n od ein *)pa ame e s(p )- >T ee () )- >G e al ue ()) ; pa ame e s .ne x (p ); --> scene es=segme n a ion (p a 0,p p 1) ; a l_scen e es p; es p= e s. con e s io n_ o_ b idg e_ y pe (); Va iable * =new Va iable( es p ); Re u nVa lu e * =new Re u nVal ue (c od i, ) ; dis ib. se nd( ); } ... oid main(in a gc,cha **a g ) { namep og a m=a g [ 0] ; ini_ o_c al ls( ); // some inicializ a i on s o he d i e d i .se _n ame _p o g am( na me p o g am ); ini_p oc es s() ; // inicial iza i on s o he p ocess i sel d i .Dis pa ch () ; // loop close(CA NA L_C OM UN IC _DI ST R) ; exi (0); } Figu e 7: Po ion o he au oma ically gene a ed a l olum.C le. The a ow has b een added p oin ing o he p oin whe e use co de is ac ually in oked. 8