Page - 175 - in Intelligent Environments 2019 - Workshop Proceedings of the 15th International Conference on Intelligent Environments

back to the incomingmessage’s originator. This pinpoints interoperability at the site of the callee process. The observation coins the essence that interoperability on the con- solidation level ismore thancommunicationbecause it demandsadependencebetween message reception andmessage emission. This dependence is the result of some oper- ation beyond syntacticmessage parsing.As consequence, it enables us to discriminate betweenamessage-dependentbehavior andanarbitrarybehavior. 4.3. CausalModeling Our observation of themessage passing behavior of interoperable processes from sec- tion 4.2 establishes a cause-effect relation.We formulate this behavior as a structural causalmodel (SCM)whichdescribes in a functional formhow the components interact witheachother.Asaresult,wehaveclear rules foranalyzing interoperableprocesses.A structural causalmodeldefinedbyPearl[15] is anordered triple<U,V,F>with • U, a setof exogenousvariableswhicharenotdeterminedby themodel • V, a setof endogenousvariableswhicharedeterminedby themodel • E, set of structural equations toexpress thevaluesofvariables inV Wedefine theendogenousandexogenousvariables in table1for theSCMof interopera- bleprocesses.Foreachvariable in table1weneed todefine itsvalueusing thestructural Component EndogenousVariablesV ExogenousVariablesU Incomingmessage M UM Dispatcher P UP Outgoingmessage M′ UM′ Discriminator D UD Table1. Causalmodelvariabledefinition equations fM, fP, fM′, fD∈E. These equations relate the variableswith each other and conclude themodel. MessageM. TheincomingmessageMconsistsofanoperationspecificationandcorre- spondingarguments.Theoperationop is takenfromsetFM ofoperations.WespecifyM usingEBNF. M :=op, {Arg} op :=op1 |op2 | ... |opk , where Arg=UM are the arguments of the operation op unknown to the model and op∈FMwith |FM|=k. DispatcherP. The dispatcher parsesM andmaps the operation specifiedwithin to a local operation from the set FP which are accessible by the dispatcher on the callee process. If thedispatchercannotfind theoperationspecified in the incomingmessage, it returnsanunknownresult. P := fP(M,UP) := { op , ifop∈FP∩M UP , otherwise S. Kotstein and C. Decker /AnApproach for Measuring IoT Interoperability 175