A XML-Based Composition Event Approach as an Integration and Cooperation Middleware

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1 A XML-Based Composton Event Approach as an Integraton and Cooperaton Mddleware Gang Xu, JanGang Ma, and Tao Huang Technology Center of Software Engneerng, Insttute of Software, Chnese Academy of Scences, Bejng , P.R. Chna {xugang, mjg, Abstract. Content-based Pub/Sub systems are ganng ncreasng popularty as an ntegraton and cooperaton mddleware supportng large-scale dstrbuted applcatons, especally for Grd. However, exstng content-based Pub/Sub systems manly focus on the one-to-one event matchng, not on the composte event matchng. The composte event matchng enables applcaton components to express more nterest n the occurrence of event composte patterns. In ths paper, we present a XML-based composte event model that conssts of the temporal logcal model and the event composte pattern. Based on the composte event model, a subscrpton language (EXML-QL) s ntroduced by extendng XML-QL to support the XML-based composte event computng. Fnally, n terms of the peculartes of the subscrpton language, a composte event matchng process s presented and analyzed. 1 Introducton As a loose couplng and scalable ntegraton mddleware, content-based Pub/Sub systems are ganng ncreasng popularty n the area of large-scale dstrbuted applcatons, such as Grd [1]. However, exstng content-based Pub/Sub systems only support the sngle event matchng and lose the ablty to express the more nterests of event composte patterns. For example, n Grd nformaton ntegraton, nformaton formatted as XML must be delvered tmely, whch rases the potental need that the users only receve relevant nformaton under certan event condtons, not the whole huge XML data. To solve ths problem, we propose a XML-based composte event model and ts subscrpton language. The composte event model descrbes the temporal relatonshp of composte events and the event forwardng patterns. At the same tme, ts subscrpton language supports the composte operatons on XML events. 2 Composte Event Model The composte event model of content-based Pub/Sub systems conssts of the event temporal logc models and the event composte patterns. The event temporal logc models descrbe the tme seuence relatonshp of events n dstrbuted envronment; the event composte patterns descrbe the event forwardng patterns. H. Zhuge and G.C. Fox (Eds.): GCC 2005, LNCS 3795, pp , Sprnger-Verlag Berln Hedelberg 2005

2 A XML-Based Composton Event Approach Temporal Logc Model In dstrbuted envronments, we use a two-part nterval tmestamp to llustrate events, rather than a sngle conventonal tmestamp due to network delays or rado transmsson lag. In terms of the characterstcs of nterval tmestamps, the temporal logc relaton defnton s defned as follows: f Defnton 1: t = [ t s, t ] denotes an nterval tmestamp of events, where t s denotes the tme at whch publshers release events; t f denotes the tme at whch Pub/Sub systems receve events. Temporal logc relaton s T Logc { <, p,, }, where < denotes the total order relaton, p denotes the partal order relaton. Temporal logc f s relatons meet the followng formal relatons: t < t t < t ; s s f f f s s f f f t p t ( t < t < t ) ( t < t ) ; t t ( t < t < t ) ( t < t ) ; s s f f t t [mn( t, t ), max( t, t )]. 2.2 Event Composte Patterns Event composte patterns llustrate dfferent event matchng and forwardng approaches. Accordng to the relaton of the matched content and the forwarded content and the relaton of the n-degree and the out-degree of Pub/Sub systems, we defne the event composte patterns as follows: Defnton 2: Event composte operator: Composte Operator = {,,, }, where = denotes forwardng pattern that s correspondent to the one-to-one event matchng, namely t resolves a sngle nput event matchng and forwards ths sngle event. denotes trggerng pattern that s correspondent to one of the many-to-one event matchng. For the trggerng pattern, Boolean evaluatng the temporal logc relatons of several nput events s the pre-condton. The events that be forwarded may be arbtrary nput events. denotes splttng pattern that s correspondent to the one-to-many event matchng pattern. Other than matchng the sngle nput event, the splttng pattern s reured to have the capablty of select the partcular part of the event s content, forwardng the selected contents to one or several subscrbers. denotes mergng pattern that s correspondent to the other of the many-to-one event matchng pattern. The mergng pattern needs to merge several events dstrbuted over the temporal seuence nto a sngle event, and further forward ths merged event. 3 Composte Event Subscrpton Language In Pub/Sub systems, nformaton provders publsh nformaton n the form of events, nformaton consumers subscrbe to events by subscrpton languages, and Pub/Sub systems delver the publshed events to all nterested subscrbers. Events and subscrpton languages are an nteractve entty. In other words, what form of events decdes the form of subscrpton languages; what form of subscrpton language also decdes the form of events.

3 438 G. Xu, J. Ma, and T. Huang Wthout loss of generalty, we defne an XML-event as below: Defnton 3: an XML event s a tr-tuple: XML-event=(element, attrbute, structure), where elements and attrbutes have name, type and value, attrbutes belong to certan elements, structures restrct the structural relatons of elements, such as parent, sblng, ancestor. We further defne the temporal operators n the events as the followng: Defnton 4: Temporal operator: T-Operator { :, ;,,, }, where : denotes the concatenaton event that follows the partal order relaton and overlaps n tme, such as e 1 :e 2 ; ; denotes the seuence event that follows the complete order relaton and doesn t overlap n tme, such e 1 ;e 2 ; denotes the parallelzaton event that matches two events n parallel, and succeeds only f both are matched. Any order s allowed and events overlap n tme, such as e 1 e 2 ; denotes the tmng event that matches a event wthn a gve nterval, such as (e 1 e 2 )e 3 ; denotes the alternaton event that s satsfed f ether e 1 or e 2 s matched, such as e 1 e 2. In the envronment of the unrestrcted context, Pub/Sub systems may match several nput events and not all nput events may be meanngful to the event forwardng. For example, for the seuence event and the trggerng operator (e 1 ;e 2 e 3 ), the several events of e 3 may be matched after e 1 happened and before e2 happens. So, we need to further defne parameter contexts to desgnate whch event to be used. Defnton 5: Parameter-Context {Recent, Exclusve, Globe}, where Recent represents only the most recent nput event s used; Exclusve represents only the events that happen after the termnatng event s matched are used, Globe represents the events that happen after the ntatng event s matched are used. It s easy to prove that the synthess of temporal logc evaluatons and parameter contexts s the farness asserton of the composte event temporal logc. 3.1 Composte Event Subscrpton Language EXML-QL In the secton, we extend XML-QL to be a composte event subscrpton language EXML-QL (Event XML-QL). XML-QL tself s a declaratve relatonal complete uery language and s smple Fg. 1. An XML-QL example enough that t can be optmzed [2]. XML-QL s sutable for extractng data from exstng XML documents and constructng new XML documents. In the aspect of constructng new XML documents, XML-QL utlzes nested ueres to support the structure herarchy of XML data and

4 A XML-Based Composton Event Approach 439 use Skolem functons to control how the result s grouped. A XML-QL example s gven n Fg. 1. In the example, varable names are preceded by $ to dstngush them from strng lterals. The WHERE clause contans the content needed to be matched and extracted. The CONSTRUCT clause constructs a XML document whose root element s a <person> element. Among the CONSTRUCT clause, PersonID s a Skolem functon that can produce a new tag for each new ($fn, $ln). Though XML-QL has many advantages n constructng XML documents, t can t satsfy the need of composte event subscrpton languages as follows: In Pub/Sub systems, detectng events s based on the content of events, so t s mpossble to name events n advance. In XML-QL, f omttng IN clauses, the XML matchng sectons for dfferent events can t be dstngushed. XML-QL doesn t contan the tme concept, so t doesn t support the temporal logc evaluaton. XML-QL doesn t support the event composte patterns. Based on the exstng functon of XML-QL, we further defne EXML-QL by addng the tme operators and the composte operators, usng the block-varable substtutng the IN clauses n XML-QL to support the composte event subscrpton. EXML-QL s defned as follows: Defnton 6: EXML-QL s a extended XML-QL language for the composte event subscrpton: EXML-QL={#Block-varable} {T-Operator} {Composte- Operator} {XML-QL}, where # s the prefx of block varables, T-Operator s the tme operators, Composte-Operator s the composte operators. So, we change the grammar of XML-QL as the followng. EXML-QL::=(Functon Query ComposteCondton)<EOF>; ComposteCondton::=Block-varable T-Operator EP; Condton::=Pattern BndngAs* # Block-varable Predcate. Fg. 2 s a smple EXML-QL example. Ths example contans two block varables #P and #T representng the <person> element and the <orderpayer> element from the dfferent events. In the end of the WHERE clause, we add the temporal operators and utlze the block varables referencng to events to buld the temporal logc relatons of dscrete events. Ths temporal logc relaton n the example s a seuence event, namely the event #P weekly follows the event #T. The CONSTRUCT clause uses the content of the event #P to construct a new output Fg. 2. An example of EXML-QL XML event.

5 440 G. Xu, J. Ma, and T. Huang 4 Composte Events Matchng 4.1 Composte Events Matchng Based on EXML-QL, matchng composte events has the followng characterstcs: Decomposng the WHERE clause nto a set of block varables and mplementng the XML event matchng. Based on the temporal logc bult from the block varables, the temporal logc relatons are evaluated on the matched events. Transformng the XML events that satsfy the temporal logc relaton nto the namng XML document, furthermore transformng EXML-QL nto XML-QL and usng XML-QL nterpreters to construct new output XML events. In the process of matchng XML events, we convert block varables to Fnte State Machnes (FSM). The events that drve the executon of the matchng are generated by the XML parser. In the executon model, a subscrpton s consdered to match a XML event when the fnal state of ts FSM s reached. We construct a man element ndex table that ndexes all the subscrptons. The man elements are the root elements n block varables. The matchng process s based on the Push mechansm, namely XML event are used to match subscrptons. For each parsng event, we mplemented three functons to deal wth the element start tag, the element end tag and the element nternal data. Frstly, the man element ndex table s checked to match the root element n block varables; secondly, other element nodes are performed three checks: the name check, the level check and the predcate check. The name check makes sure that the parsng element name euals the name of the node. The level check makes sure that the level of the element appeared n the XML event matches the level of element n EXML-QL. The predcate check makes sure that the values of the element and attrbute satsfy the evaluaton of the matchng predcates. For the WHERE clauses that may contan the temporal logc, we construct event graphs to evaluate the composte event temporal logc. An event graph s a rooted, drected acyclc graph whose each node has an out-degree edge and two n-degrees edges and s labeled by a tme operator. The termnal output of the whole event graph s true or false representng whether the XML event s trggered or not, or whether a new XML event s constructed or not. The non-termnal outputs of event graph are the unon calculaton of the tme of the two nput events. Besdes, each node s appended a parameter context to restrct the selecton of nput events. In the process of constructng new XML events, currently we only utlze the exstng XML-QL nterpreter and do the followng change: transformng EXML-QL nto XML-QL; Namng the XML events that satsfy the content matchng and the temporal logc evaluaton and nput these named XML events nto XML-QL nterpreter; XML-QL nterpreter constructs new output XML events. 4.2 Performance Analyss Fg.3 s the result of calculatng the matchng rate of the seuence event. In the matchng rate experment, we restrct the EXML-QL only contans one seuence temporal

6 A XML-Based Composton Event Approach 441 operator and construct a new output event n the CONSTRUCT clause. The number of subscrpton s 500; the whole number of Block varable s We have the 500 nput XML events that are dvded nto ten groups of XML events, where only four groups meet the subscrptons. Each group of XML event has the same 50 XML event. The 500 nput XML events are nputted n an average tme nterval way. Fg. 3 shows the event matchng rate depends on the event nput seuence and the matchng rate ncreased when the number of the accumulated events ncreased. In calculatng the matchng tme of the seuence event, we restrct the EXML-QL only contans one seuence temporal operator, does not contan the CONSTRUCT clause and uses the event composte pattern operator to pont out the event that s need to be forwarded. The number of block varables n each subscrpton s two; the number of subscrptons s from 500 to 10,000. Fg. 4 shows the result of the experment that denotes the matchng tme s mostly proportonal to the number of nput events. Fg. 3. Results of matchng rates Fg. 4. Results of matchng tme 5 Related Works Currently, XML-based Pub/Sub systems are manly XFlter [3], YFlter [4], XTre [5] and WebFlter [6]. They adopt XPath as subscrpton languages to performng effcent flterng of XML documents for large-scale nformaton dssemnaton systems. XPath has more expressve capablty for user nterests than XML-QL, but XPath has not ablty to construct new XML document. Compared to EXML-QL, XML-QL can t support composte event operatons. The researches of composte events manly focus on actve databases. Snoop was to desgn an expressve composte event specfcaton language wth powerful temporal support [7]. Peter dscussed composte event detecton n dstrbuted envronments [8]. Compared to ths paper, t emphaszes partcularly on the topologcal structure analyss of composte event detecton n dstrbuted envronments and don t make reference to composte patterns such as splttng and mergng. 6 Conclusons We present the study of XML-based composte event matchng approach, an mportant problem n next-generaton, scalable content-based Pub/Sub systems. We ntroduce the temporal logcal model to descrbe the temporal relatonshp of events n

7 442 G. Xu, J. Ma, and T. Huang dstrbuted envronments and the event composte pattern to descrbe the dfferent forwardng and trggerng patterns. Based on composte event model, a descrpton language (EXML-QL) s presented by extendng XML-QL. Fnally, accordng to the peculartes of the subscrpton language, the process of matchng composte events s presented and further analyzed. Acknowledgements. Ths work s partally supported by the Chnese Natonal 973 Key Research Program (2002CB312002, 2002CB312005), the Chnese Natonal 863 Hgh-Tech Program (2001AA113010, 2001AA414020, 2001AA414330). References 1. Zhuge H.: Chna's E-scence knowledge grd envronment. IEEE Intellgent Systems 1 (2004) Deutsch A., Fernndez M., Florescu D.: XML-QL: A uery language for XML Altnel M., Frankln M. J.: Effcent flterng of XML document for selectve dssemnaton of nformaton. In: Proceedngs of 26th Internatonal Conference on Very Large Data Bases, Caro, Egypt (2000) Yanle Dao, Peter Fsher, Mchael J. Frankln, Raymond T.: Yflter: effcent and scalable flterng of XML documents. In: Proceedngs of the 18th Internatonal Conference on Data Engneerng, San Jose, CA (2002) Chan C. Y., Felber P., Garofalaks M., Rastog R.: Effcent flterng of XML documents wth XPath expressons. The VLDB Journal 4 (2002) Perera J., Fabret F., Llrbat F., Jacobsen H. A., and Shasha D.: WebFlter: A hgh throughput XML-based Publsh and Subscrbe system. In: Proceedngs of 27th Internatonal Conference on Very Large Data Bases, Roma, Italy (2003) Chakravarthy S., Mshra D.: Snoop: An expressve event specfcaton language for actve databases. Techncal Report UF-CIS-TR , Department of Computer and Informaton Scence, Unversty of Florda, Florda (1993) 8. Peter R. Petzuch, Bran Shand, Jean Bacon: Composte event detecton as a generc mddleware extenson. IEEE Network 1 (2004) 44-55