▁end - users ▁to ▁derive ▁new ▁knowledge ▁from ▁devices , ▁sens ors , ▁databases , ▁analysis ▁components ▁and ▁computational ▁resources ▁that ▁res ide ▁across ▁the ▁Internet ▁or ▁grid . ▁ ▁Architecture ▁and ▁workflow ▁server ▁▁ ▁The ▁system ▁is ▁based ▁on ▁a ▁multi - t ier ▁architecture , ▁with ▁a ▁workflow ▁server ▁providing ▁a ▁number ▁of ▁supporting ▁functions ▁needed ▁for ▁workflow ▁author ing ▁and ▁execution , ▁such ▁as ▁integration ▁and ▁access ▁to ▁remote ▁computational ▁and ▁data ▁resources , ▁collaboration ▁tools , ▁visual is ers ▁and ▁publishing ▁mechan isms . ▁The ▁architecture ▁itself ▁evol ved ▁over ▁the ▁years ▁foc using ▁on ▁the ▁intern als ▁of ▁the ▁workflow ▁server ▁( G han em ▁et ▁al . ▁ 2 0 0 9 ) ▁to ▁support ▁extens ibility ▁over ▁multiple ▁application ▁domains ▁as ▁well ▁as ▁different ▁execution ▁environments . ▁ ▁Visual ▁workflow ▁author ing ▁▁ ▁Disc overy ▁Net ▁workflow s ▁are ▁represented ▁and ▁stored ▁using ▁D P ML ▁( Dis cover y ▁Process ▁Mark up ▁Language ), ▁an ▁XML - based ▁representation ▁language ▁for ▁workflow ▁graphs ▁supporting ▁both ▁a ▁data ▁flow ▁model ▁of ▁computation ▁( for ▁analyt ical ▁workflow s ) ▁and ▁a ▁control ▁flow ▁model ▁( for ▁orch estr ating ▁multiple ▁dis joint ▁workflow s ). ▁ ▁As ▁with ▁most ▁modern ▁workflow ▁systems , ▁the ▁system ▁supported ▁a ▁drag - and - drop ▁visual ▁interface ▁en abling ▁users ▁to ▁easily ▁construct ▁their ▁applications ▁by ▁connecting ▁nodes ▁together . ▁ ▁Within ▁D P ML , ▁each ▁node ▁in ▁a ▁workflow ▁graph ▁represents ▁an ▁executable ▁component ▁( e .