Flavours of XChange,a Rule-Based Reactive Language for the(Semantic)WebJames Bailey1,Fran¸c ois Bry2,Michael Eckert2,and Paula-Lavinia P˘a trˆa njan2 1The University of Melbourne,Australia,http://www.cs.mu.oz.au/~jbailey/ 2University of Munich,Germany,http://pms.ifi.lmu.de/Abstract.This article introduces XChange,a rule-based reactive lan-guage for the Web.Stressing application scenarios,itfirst argues thathigh-level reactive languages are needed for both Web and Semantic Webapplications.Then,it discusses technologies and paradigms relevant tohigh-level reactive languages for the(Semantic)Web.Finally,it presentsthe Event-Condition-Action rules of XChange.1IntroductionA common perception of the Web is that of a distributed repository of hyper-media documents,with clients(in general browsers)that download documents, and servers that store and update documents.Although reflecting a widespread use of the Web,this perception is not completely accurate.In fact,many Web applications rely on the updating of server data in response to the requests of clients,or client data in response to the requests of servers.Indeed,there are many kinds of reactions for both servers and clients,to events or messages ex-changed on the Web.The Web has an infrastructure for updates and reactivity: the protocol HTTP.This articlefirst argues that complementing HTTP with high-level languages for updates and reactivity is needed for both standard Web and Semantic Web applications.It then introduces XChange,a novel high-level language for Event-Condition-Action rules for updates and reactivity on the (Semantic)Web.2MotivationUpdates on the Web.Many Web applications build upon servers that update data according to client requests or actions.This is the case for e-Commerce systems that receive,process and buy orders,e-Learning systems that select and deliver teaching materials depending on students’test performances,and of communication platforms such as wikis,where several users modify the same documents.Conversely,some Web applications also build upon clients that up-date data according to server requests.This is the case with so-called cookies, i.e.descriptions on a client of the states of a connection to a server,or when a client keeps,after a connection to a server,data collected during the connection, e.g.for a railways or airline electronic ticket.
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