Coordinating Activities in Collaborative Environments: A High Level Petri Nets Based Approach

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1 Coordinting Activities in Collortive Environments: A High Level Petri Nets Bsed Approch Alerto B. Rposo, Léo P. Mglhães nd Ivn L. M. Ricrte Stte University of Cmpins (UNICAMP) School of Electricl nd Computer Engineering (FEEC) Deprtment of Computer Engineering nd Industril Automtion (DCA) CP Cmpins, SP, Brzil {lerto, leopini, ricrte}@dc.fee.unicmp.r ABSTRACT The coordintion of interdependencies mong ctivities in collortive environments is very importnt nd difficult tsk. In this pper we present set of coordintion mechnisms for the specifiction nd control of interction mong collortive ctivities. To model these mechnisms, we use high level Petri nets, which hve proven to e n dequte pproch to evlute the ehvior of computer supported collortive system efore its implementtion. Keywords: Coordintion, Collortive Environments, Petri Nets, Computer Supported Coopertive Work, Multiuser Interction. 1. INTRODUCTION Some ctivities involving multiple individuls do not require forml plnning. Activities ssocited to the socil reltions re generlly well coordinted y the socil protocol, which is chrcterized y the sence of ny coordintion mechnism mong ctivities, trusting the prticipnts ilities to medite interctions. Exmples of computer supported ctivities of this kind re the chts nd videoconferences. On the other hnd, ctivities relted to coopertive work (not socil reltions) require sophisticted coordintion mechnisms to void tht prticipnts get involved in conflicting or repetitive tsks. This pper focuses on the coordintion of ctivities in computer supported collortive environments, defining set of interdependencies tht frequently occur mong collortive tsks nd presenting coordintion mechnisms for these dependencies. The ide is to seprte ctivities (tsks) from dependencies (controlled y the coordintion mechnisms), enling the use of different coordintion policies in the sme collortive environment, y chnging only the coordintion mechnisms. Moreover, the coordintion mechnisms cn e reused in other collortive environments. This pper is orgnized s follows. In the next section we introduce high level Petri nets, which is the tool we use to model the coordintion mechnisms. Then, in Section 3, we mke rief overview of works relted to coordintion in Computer Supported Coopertive Work. In Section 4 the dependencies nd coordintion mechnisms re presented, nd in Section 5 n exmple of use of the mechnisms is shown. The conclusions nd future work re discussed in Section HIGH LEVEL PETRI NETS Petri nets (PNs) re modeling tool pplicle to vriety of fields nd systems, specilly suitle for systems with concurrent events. Murt [11] presents very good introduction to the theme. Formlly, PN is defined s 5-tuple (P, T, F, w, M 0 ), where: P = {P 1,..., P m } is finite set of plces; T = {t 1,..., t n } is finite set of trnsitions; F (P T) (P T) is set of rcs; w: F {1, 2,...} is weight function; M 0 : P {0, 1, 2,...} is the initil mrking; with (P T) = nd (P T). In PN model, sttes re ssocited to plces nd tokens, nd events to trnsitions. A trnsition t is sid to e enled if ech input plce P i t is mrked with t lest w(p i, t) tokens, where w(p i, t) is the weight of the rc etween P i nd t. Once enled, trnsition will fire when its ssocited event occurs. Firing trnsition t, w(p i, t) tokens re removed from ech input plce P i nd w(t, P o ) tokens re dded to ech output plce P o t. Here, t nd t mens, respectively, the set of input nd output plces of trnsition t. A useful nottion for PNs is the grphicl nottion, where circles represent plces, rectngles represent trnsitions, dots represent tokens nd rrows represent the rcs, with weights ove. By definition, n unleled rc hs weight 1. In ddition to the sic PN model, severl extensions ppers in the literture. In this pper we use three of them: inhiitor rcs, nets with time nd high level nets.

2 An inhiitor rc connects plce P with trnsition t nd enles t only if P hs no tokens. In the grphicl nottion, inhiitor rcs re represented with circle on the edge. One wy to include the notion of time in PNs is to ssocite it with trnsitions firing. In this cse, tokens re removed from input plces of trnsition nd some time lter (firing time) re dded to the output plces. This kind of non-instntneous firing is clled firing with token reservtion. High level PNs include, mong other types, predicte/trnsition nets nd colored PNs [4]. The most importnt chrcteristic of high level PN is the distinction of tokens (clled colored tokens). The rcs hve lels defining vriles (or constnts) tht dictte how mny nd which kind of tokens will e removed from or dded to the plces. The sme vrile ppering in the incoming nd outgoing rcs of trnsition denotes the sme token type. A trnsition is enled if there is t lest one possiility of consistent sustitution of vriles into typed tokens. In the exmple of Figure 1, trnsition is enled ecuse there re two tokens of type in P1, eing possile to sustitute vrile <x> for type. After the firing of, P1 remins with token nd P2 receives token of type. P1 2<x> <x> P2 mechnism were developed. One of the most representtive system of tht genertion ws the Coordintor [13], which hs een developed sed on linguistic theories nd whose gol ws to help emil communiction. Since tht time, there hs een exmples of the use of PNs to coordinte ctivities in collortive environments [6], [9]. In the 90 s, systems hve een constructed with more flexile nd ccessile coordintion mechnisms. Inspired y coordintion lnguges [8], which proposed the seprtion of computtion from coordintion for multi-threded pplictions, some collortive systems seprtes the implementtion of coordintion components from their other prts. This llows more flexiility in the use of coordintion policies. COCA (Collortive Ojects Coordintion Architecture) [10] nd Trellis [7] re exmples of systems of this kind. The Trellis, in prticulr, uses vrition of PNs in server to specify interction protocols for group of collortive clients. Our work is more generic thn those presented ove. We define set of interdependencies mong tsks nd ssocited coordintion mechnisms (modeled y high level PNs) tht cn e used in workflows, multiuser interction, virtul environments, etc. The min gol is not to implement closed system, ut to provide set of mechnisms to enle the collortive system designer to preview nd test its ehvior efore implementing it, detecting possile prolems. Figure 1: Exmple of high level PN. Besides their modeling cpilities, PNs hve lso strong theoreticl support for nlysis nd numer of simultion techniques. There re three kinds of nlysis pplicle to PN model; verifiction, vlidtion nd performnce nlysis [2]. The verifiction nlysis is used to gurntee tht the net is correctly defined. It is verified whether the net hs dedlocks, whether there re ded trnsitions, whether it reches ny undesired stte, mong others. In the vlidtion nlysis, it is checked whether the model works s expected. Tests re mde vi itertive simultions of fictitious cses to ensure tht the model trets them correctly. Finlly, the performnce nlysis evlutes the cpcity of the system to chieve requisites such s verge witing time, throughput times, resource use, nd so on. 3. RELATED WORK In spite of their recognized importnce, coordintion mechnisms hve not een included in the first collortive systems. Only in the second hlf of the 80 s the first systems with some kind of coordintion 4. COORDINATION MODELS This work intends to provide mechnisms to mnge interdependencies mong collortive tsks nd gurntee tht these dependencies will not e violted. The ide is tht the designer of collortive environment e concerned only with the definition of tsks nd their interdependencies, nd not with the mngement of those dependencies. In the proposed schem, n environment is modeled in two distinct levels, workflow nd coordintion. In the workflow level, the tsks nd their interdependencies re defined. In the coordintion level, interdependent tsks re expnded nd the dequte coordintion mechnisms re inserted mong them. During the pssge from the workflow to the coordintion level, ech tsk which hs n interdependency with nother is expnded ccording to the model of Figure 2 [1]. The five plces ssocited to the expnded tsks represent the interfce with the resource mnger nd the gent tht executes the tsks. Plce indictes to the resource mnger tht the tsk needs resource. After the ssignment of the resource, the mnger puts token in plce ssigned_ to continue the tsk. Plces

3 strt_tsks nd finish_tsks indictes, respectively, the eginning nd the end of the tsk. Finlly, relese_ indictes to the resource mnger tht the tsk hs finished nd the resource is ville gin. Figure 2: An expnded tsk in the coordintion level. The min gol of our work is to construct the coordintion level from the workflow level, ecuse once the interdependencies re defined, the expnsion of tsks ccording to the model of Figure 2 nd the insertion of coordintion mechnisms cn e utomted. The interdependencies presented elow re divided into two clsses: temporl nd resource mngement. Temporl Dependencies Temporl dependencies estlish the execution order of tsks. Coordintion mechnisms ssocited to this kind of dependency hve strt_tsks s input plce nd finish_tsks s output plce (Figure 2). The proposed mechnisms re sed on temporl reltions defined in clssic pper of temporl logic [3]. In this pper, set of primitive nd mutully exclusive reltions etween time intervls is defined. We dpted these reltions for the definition of temporl dependencies etween tsks in collortive environments, dding couple of new reltions nd few vritions of those originlly proposed. The following temporl dependencies re defined. Tsk input_ plces t P1 t P2 ti P3 tf P4 tc ssigned_ strt_tsks finish_tsks relese_ output_ plces tsk 1 equls tsk 2: oth tsks must e executed simultneously. tsk 1 strts tsk 2: Allen s originl definition estlishes tht oth tsks must strt together nd tsk 1 must finish efore tsk 2 (we clled this reltion strtsa). We lso relxed the second prt of the definition, creting vrition of the reltion in which it does not mtter which tsk finishes efore (strtsb). tsk 1 finishes tsk 2: the originl definition estlishes tht oth tsks must finish together, ut tsk 1 hs to strt fter tsk2 (finishesa). As in the previous cse, we creted new dependency relxing the restriction on which tsk must strt efore (finishesb). tsk 2 fter tsk 1: tsk 2 my only e executed fter the execution of tsk 1. Two vritions re possile for this reltion. In the first one (ftera) ech execution of tsk 1 enles single execution of tsk 2. In the second vrition, severl executions of tsk 2 re enled fter single execution of tsk 1. tsk 1 efore tsk 2: from the temporl logic point of view, this dependency cn e seen s the opposite of the previous one, ut it genertes totlly different coordintion mechnism. Essentilly, the difference is ecuse in this cse, the restriction occurs in the execution of tsk 1, which my not e executed nymore if tsk 2 hs lredy een executed. Here, tsk 2 does not wit for the execution of tsk 1, which ws the cse for tsk 2 fter tsk 1. tsk 1 meets tsk 2: tsk 2 strts immeditely fter the end of tsk 1. tsk 1 overlps tsk 2: the originl definition estlishes tht tsk 2 must strt efore the end of tsk 1, which must finish efore tsk 2 (overlpsa). Relxing the restriction tht tsk 1 must finish first, we defined vrition of this reltion (overlpsb). tsk 2 during tsk 1: two vritions re possile. In the first one (duringa), tsk 2 cn e executed only once during the execution of tsk 1. In the second one (duringb), tsk 2 cn e executed severl times. Since the gol of the coordintion mechnisms is to del with reltions tht sometimes elongs to complex procedures, it is interesting to dd mechnisms to void frequent dedlocks. One of such mechnisms is the use of s. We defined two kinds of s. In the first one (A), n lterntive tsk is defined if the originl one does not strt fter certin witing time. The other kind of (B) returns the tokens to the input plces of the tsk fter witing time, enling the following of lterntive pths tht do not execute the locked tsk. Resource Mngement Dependencies Coordintion mechnisms for resource mngement re complementry to those presented in the previous section nd cn e used in prllel to them. This kind of coordintion mechnism dels with the distriution of mong the tsks, nd hve nd relese_ s input plces nd ssigned_ s output plce (Figure 2). We define three sic mechnisms for resource mngement: Shring: limited numer of needs to e shred mong severl tsks. Simultneity: resource is ville only if certin numer of tsks requests it simultneously. Voltility: indictes whether, fter the use, the resource is ville gin. We hve lso defined composite mechnisms from the sic ones discussed ove. For exmple, shring M + voltility N indictes tht up to M tsks my shre the resource, which cn e used N times only. Differently from temporl dependencies, resource mngement dependencies re not inry reltions. It is

4 possile, for instnce, tht more thn two tsks shre resource. Moreover, ech of the ove mechnisms requires prmeter indicting the numer of to e shred, the numer of tsks tht must request resource simultneously, or the numer of times resource cn e used (voltility). A Lnguge for the Definition of Interdependencies In order to define the interdependencies mong tsks, we hve creted lnguge tht descries, one t ech line, ll the dependencies of collortive environment. The interdependencies re defined ccording to the following syntx <dependency nme> [prmeters] <tsk1 nme> <tsk2 nme> [ <tskn nme> ] [s], where prmeters re needed for resource mngement dependencies nd the list of s is optionl. Modeling Coordintion Mechnisms using High Level Petri Nets Initilly, we hve modeled coordintion mechnisms for ll dependencies discussed ove using ordinry PNs [12]. However, typicl prolem in the use of ordinry PNs is the stte explosion, which cn occur in our context when the numer of interdependencies increses. High level PNs reduce this prolem ecuse they generte simpler models, with less plces nd trnsitions. Therefore, we remodeled them using high level PNs. To illustrte one of the coordintion mechnisms, Figure 3 shows the mechnism for simultneity 2 ( resource is ville only if two tsks request it simultneously). In the figure, the rcs with expression <x>+<y> ensure tht two different tsks (tokens of different colors) which re requesting the (tokens r) re going to receive them if they re ville in plce Pn. Tsk 1 i1 t_1 P1_1 t_1 P2_1 ti_1 P3_1 tf_1 P4_1 tc_1 O1 <x> + <y> <x> + <y> ssigned_ <x> + <y> relesed_ Pn t2 i2 O2 t_2 P1_2 t_2 P2_2 ti_2 P3_2 tf_2 P4_2 tc_2 Tsk 2 Figure 3: Coordintion mechnism for simultneity EXAMPLE To illustrte the use of the coordintion mechnisms, we present n exmple descriing typicl sitution in multiuser interction. In our hypotheticl environment, two users interct y mens of whiteord. The workflow level of this environment is shown in Figure 4. As cn e seen in the model, ech user cn enter the whiteord, then write on it severl times efore leving it. The following interdependencies ppers in the model. The whiteord is ville only if oth uses request it simultneously (simultneity 2); only one user my write on the whiteord t ech time (shring resource, e.g., pen); nd when user A leves the whiteord, user B must follow him/her (meets). Using the lnguge for the definition of interdependencies, the following file is written for the exmple ove: sim 2 entera enterb time_outb time_outb div 1 writea writeb time_outb time_outb meets levea leveb The model of the coordintion level for this exmple is shown in Figure 5. At first glnce, this model my seem complicte, ut it is highly modulr nd esily uilt from the model of Figure 4, y expnding interdependent tsks (open rectngles in Figure 4) nd inserting the predefined coordintion mechnisms.

5 User A entera w ritea levea ia oa Simultneity 2 Mutul exclusion Meets ib ob enterb w riteb leveb User B Figure4: Workflow level of the exmple. User A entera w ritea levea ia oa <x>+<y> <x>+<y> ssigned_ relese_ <x>+<y> t2 Pn r r <x> <x> r ssigned_ <x> relese_ strt_ tsks + tska R finish_ tsks R tskb Simultneity 2 Pn t2 Mutul Exclusion Meets ib ob enterb writeb leveb User B Figure 5: Coordintion level of the exmple. It is necessry to oserve tht interdependent tsks of Figure 5 re not expnded exctly ccording to the model of Figure 2. The reson is tht in the cse of temporl dependencies, only plces strt_tsks nd finish_tsks re used, nd in the cse of resource mngement dependencies, only, ssigned_ nd relese_ re used. Therefore, we used simplified versions of the model.

6 The coordintion level model of the environment cn e simulted nd nlyzed with ny tool tht supports high level PNs (see [5] for list of PN simultion tools). Verifiction nd vlidtion nlysis indicted tht our exmples hs eight finl sttes. Seven of them cn e eliminted y the correct use of s (i.e., with dequte witing times). The eightieth finl stte is the correct one (tokens in oa nd ob). Performnce nlysis could e relized, for instnce, to mesure verge witing times of n user, given the rtes with which the other requests the. Finlly, it is necessry to reinforce tht the exmple represents just hypotheticl sitution. We did not stress ll detils for the modeled scenrio. Its min gol ws to show how the coordintion mechnisms cn e used in prcticl sitution. 6. CONCLUSION The coordintion of interdependent ctivities in collortive environments is prolem tht should e ddressed to ensure the effectiveness of the coopertion. The seprtion etween ctivities nd dependencies, nd the utiliztion of reusle coordintion mechnisms re steps towrds this gol. Petri nets, due to their support for modeling, simultion nd nlysis, hve proven to e powerful tool for verifying the correctness nd vlidting the effectiveness of collortive environments efore their ctul implementtion [2], [12]. Furthermore, the hierrchicl description of PNs showed n pproprite wy to define the coordintion structure in different strction levels (workflow nd coordintion levels). In prticulr, high level PNs lso reduces the prolem of stte explosion. The set of interdependencies presented in this pper does not clim to e complete. It would e very difficult to estlish frmework of ll possile interdependencies. For tht reson, we opted for n extensile pproch. When new kind of interdependency rises, corresponding coordintion mechnism cn e modeled nd esily inserted etween corresponding tsks. One of the next steps of this work is to utomte the pssge from the workflow to the coordintion level of models in high level PN simultion tool. We hve lredy done this for ordinry PN models [12]. Due to their generlity, the presented coordintion mechnisms re dequte to wide rnge of collortive systems, from interorgniztionl workflows to virtul environments. Presently, we re implementing the mechnisms to e used in the development of collortive virtul environments. The coordintion of ctivities will fcilitte the use of this kind of environment for the reliztion of tsks tht cnnot e controlled y the socil protocol. 7. REFERENCES [1] W. M. P. vn der Alst. Modelling nd nlysing workflow using Petri-net sed pproch. Proc. 2 nd Workshop on Computer-Supported Coopertive Work, Petri nets nd relted formlisms, pp [2] W. M. P. vn der Alst. The Appliction of Petri Nets to Workflow Mngement. The Journl of Circuits, Systems nd Computers, 8(1): [3] J. F. Allen. Towrds Generl Theory of Action nd Time. Artificil Intelligence, 23: [4] W. Bruer, W. Reisig nd G. Rozenerg (Eds.). Petri Nets: Centrl Models nd Their Properties Advnces in Petri Nets Lecture Notes in Computer Science, 254. Springer-Verlg, [5] CPN Group, Univ. of Arhus, Denmrk. The World of Petri Nets Tools on the We < [6] F. De Cindio, G. De Michelis nd C. Simone. The Communiction Disciplines of CHAOS. In Concurrency nd Nets, pp Springer- Verlg, [7] R. Furut nd P. D. Stotts. Interpreted Collortion Protocols nd their use in Groupwre Prototyping. Proc. of the Conf. on Computer Supported Coopertive Work (CSCW 94), pp [8] D. Gelernter nd N. Crriero. Coordintion Lnguges nd their Significnce. Communictions of the ACM, 35(2): Ferury [9] A. W. Holt. Coordintion Technology nd Petri Nets. In G. Rozenerg (Ed.). Advnces in Petri Nets, pp Lecture Notes in Computer Science, 222. Springer-Verlg, [10] D. Li nd R. Muntz. COCA: Collortive Ojects Coordintion Architecture. Proc. of the Conf. on Computer Supported Coopertive Work (CSCW 98), pp [11] T. Murt. Petri Nets: properties, nlysis nd pplictions. Proc. of the IEEE, 77(4): [12] A. B. Rposo, L. P. Mglhães nd I. L. M. Ricrte. Petri Nets Bsed Coordintion Mechnisms for Multi-Workflow Environments. To e pulished in the Int. J. of Computer Systems Science & Engineering, Septemer [13] T. Winogrd nd F. Flores. Understnding Computers nd Cognition: A New Foundtion for Design. Alex, Acknowledgements: The first uthor is sponsored y FAPESP (Foundtion for Reserch Support of the Stte of São Pulo), process numer 96/ We lso would like to thnk DCA FEEC Unicmp for the support grnted to this reserch.

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