Web Service Composition Based On Object-oriented Petri Nets

With the development of Internet, Service-Oriented Computing (SOC) has become one of the most popular content among software domain. As web service standards and technologies becoming mature, web service has been made as significant computing resources on the World Wide Web, and has promoted the application and development of the service-oriented computing greatly. The study of SOC involves many research areas, and web service composition is one of the most important fields.So far, around the research topic of web service composition, many domestic and foreign researchers at academic institutions have carried out extensive and in-depth research in their own areas of research. Petri net is a formal method which is suitable for descripting and analysising the concurrent, asynchronous and distributed software systems. It has both a strict mathematical definition and an intuitive graphical representation, therefore, it has been widely applied in the field of web service composition. However, as classical Petri net can't express the semantics of web services accurately, besides the number of web services is increasing and the requirements from service requestors are complex and diverse, web service composition based on classical Petri net shows some difficulties, such as closed and the state space explosion issues. That is why it promotes the development and research of advanced Petri nets.Object-Oriented Petri Net combines the advantages of both Petri net and object-oriented. Petri net has rich and rigorous analytical methods and presentation skills, while object-oriented has features of abstraction, encapsulation and modular, etc. So, OOPN can describe concurrent, distributed system models and the interactions during modeling simpler and more intuitive. In this paper, we'll make OOPN as a theoretical basis. Based on OOPN, we'll formally define and graphically represent a single web service, then design a model WSC-OOPN to study web service composition. There are many service composition operators over WSC-OOPN model, we've formally defined and graphically represented the operators. Then, verify OOPN's three properties which are safeness, reachability and liveness by Coverability Tree, Incidence Matrix and State Equation, Transitive Matrix respectively. Finally, we define the First Priority and Second Priority, by which we dispose a priority algorithm based on WSC-OOPN model to implement web service composition. In the text, the steps and pseudo codes have been given, and a simple example is used to verify the feasibility of proposed algorithm. The priority algorithm is suitable tor the situation that when the methods requested by user are only partly contained in several web services, we can calculate the priorities of message sequence which is automatic generated from the requested methods, and then in accordance with the priority algorithm processes to choose the optimal web services sequence to composite.