Abdia Hamdani Ibn-Khaldoun University of Tiaret. hh abla@yahoo.fr Abdelkrim Abdelli LSI laboratory, USTHB university, Algiers. Abdelli@lsi-usthb.dz Abstract Dealing with synchronization in time constrained workflow is becoming a challenging issue. In this paper, we present a modelling approach based on Petri nets formalism for timed workflow systems with complex synchronization among tasks of different privileges (Mas- ter/Slave). To this aim, we consider the concept of rendezvous already introduced in the RTPN (Time Petri Nets with rendezvous), to define a subclass of RTPNs called Time Workflow-nets with Rendezvous (RTWF-nets). We discuss how this model can cover a large range of timed synchronization patterns in a very smart and compact framework. Keywords – Workflow, Real-time systems, Rendezvous, Petri nets, Patterns. 1 Introduction In the late of the 70’s, the research on workflow systems has started and a lot of approaches and powerful tools have been proposed and developed. Generally, workflows represent processes describ- ing how and when their elementary tasks should be accomplished, thus describing the control flow Copyright c  by the paper’s authors. Copying permitted for private and academic purposes. In:Proceedings of the 3rd Edition of the International Conference on Advanced Aspects of Software Engineer- ing (ICAASE18), Constantine, Algeria, 1,2-December-2018, published at http://ceur-ws.org of the workflow system. This may include dif- ferent mechanisms ( e.g., sequence, choice, paral- lelism and synchronizations), usually called work- flow patterns [Aal05]. For instance, synchroniza- tion in workflow system can be seen as a meeting point during the process where a set of tasks has to wait for others according to a given scheme (e.g the AND-join synchronizer pattern). Nowadays, the real challenge in workflow systems is to deal with situations where a variable number of tasks is processed under different synchronization and time constraints patterns. Indeed, adapting, replan- ning, and synchronizing workflows in response to an unexpected progress, delays, or technical con- ditions are necessary to maintain the safety of the systems. Furthermore, such requirements are be- coming critical aspects in many domains as for ex- ample healthcare workflows [Car09]. For example, in transplantation surgery activity, we require the concurrent presence of the organ to be implanted, blood for the patient and the patient, that must ar- rive within the same hour at the hospital to avoid their functional degradation. In this paper, we propose the use of RTPN (Time Petri Nets with Rendezvous) [Ham17], for the modelling and the analysis of complex workflow systems that include synchronization and time constraints among tasks. The RTPN introduces the paradigm of rendezvous to define various syn- chronization schemes under different time con- straint orchestrations. With its expressiveness power, RTPN provides a compact framework to represent complex workflow systems in elegant way, that could hardly be handled by other existing models. After presenting and discussing the work- flow patterns provided by RTPN, we define a sub- class of RTPN, called RTWFN (Time Workflow- nets with Rendezvous ), which is dedicated for the Page 76 Using the RTPN model for the modelling of complex Workflow systems