In thii paper,weconsiderMOBY. a distributedarchitectureto support the developmentof expert databasesystemsin a rule based language.It combinesstandardindexing andhorizontaldatapartitioning techniqueswith a rule basedinterpreterto achievethe reasonableperformance.The majordifficulty in developingthis architectureis to maintaina high effectiveparallelismasthe numberof processorsincreases.Analytic resultssuggestthatwhendatais masonablywell balancedacrossa local areanetwork,MOBY hasa high effectiveparallelism. Simulationresultssupportthis claim by showingthattheeffectiveparallelismis proportionalto 40%of the numberof processors.A discussionof somecrucial issuesin our currentnetworkbasedimplementationis alsogiven.