Petri net ontology

Abstract

The paper presents the Petri net ontology that enables sharing Petri nets on the Semantic Web. Previous work on formal methods for representing Petri nets mainly defines tool-specific descriptions or formats for model interchange. However, such efforts do not provide a suitable description for using Petri nets on the Semantic Web. This paper uses the Petri net UML model as a starting point for implementing the ontology. Resulting Petri net models are represented on the Semantic Web using XML-based ontology languages, RDF and OWL. We implemented a Petri net tool, P3, which can be used as a knowledge acquisition tool based on the Petri net ontology.

Introduction

The main idea of this paper is to propose a suitable way for Petri nets [35] to be used on the Semantic Web, i.e., to enable full semantic interoperability of Petri net models. Currently, Petri net interoperability is possible at the level of syntax for model sharing. It was first introduced in [3], where the authors said that it would be very useful if Petri net researchers could share their Petri net model descriptions. That way more software tools could be used for analyzing the same model. So far, all Petri net interchange attempts have been mainly tool-specific, but with very low (or without any) general acceptance. For example, DaNAMiCS tool (http://www.cs.uct.ac.za/Research/DNA/DaNAMiCS) uses regular text-based format, and Renew tool (http://www.renew.de) uses XML-based format. The Petri Net Markup Language (PNML) [4] is a recent Petri net community effort that tries to provide XML-based model sharing. PNML tends to be a part of the future ISO/IEC High-level Petri net standard [20]. A particularly important advantage of this approach is that XML documents can be easily transformed using eXtensible Stylesheet Language Transformations (XSLT) into other formats (that need not necessarily be XML-based). However, this initiative is also syntactically oriented, i.e., it introduces some constraints that enable validation of documents against their definition (e.g., validating whether an arc connects two nodes of different types, i.e., a place and a transition).

On the other hand, all approaches that formally define Petri nets are generally intended to be used as a basis for developing Petri net tools. Currently, these attempts are defined using Model Driven Architecture (MDA) standards [29] (e.g., Unified Modeling Language – UML [33], and Meta-Object Facility – MOF [32]).

A suitable way to represent Petri nets is needed in order to reuse them more effectively on the Semantic Web. It requires defining the Petri net ontology for semantic description of Petri net concepts and their relationships. The Petri net ontology enables describing a Petri net using Semantic Web languages (e.g., Resource Description Framework – RDF, and Web Ontology Language – OWL) [15], [40]. Petri nets described that way can be inserted into other, non-Petri net XML-based formats, such as Scalable Vector Graphics (SVG, the XML-based WWW Consortium (W3C) standard for 2D vector graphics [21]), which makes possible to reconstruct Petri net models using metadata and annotations according to the Petri net ontology.

We defined the Petri net ontology using experience from previous Petri net formal descriptions (metamodel, ontologies, and syntax). They indicate very useful directions for selecting key Petri net concepts and specifying their mutual relations. PNML is of primary importance here – it is closely related to the Petri net ontology. Actually, it is a medium (syntax) for semantics [36]. We additionally empowered the PNML usability by defining mappings to/from the Semantic Web languages (i.e., RDF and OWL).

The next section describes existing formal descriptions of Petri nets: metamodels, UML profiles, ontologies, and syntax. We concentrate on Petri net syntax because most work has been done in solving this problem (we specifically discuss the PNML). Section 3 enumerates advantages of the Petri net ontology, and gives guidelines for its conceptualisation. Section 4 outlines development of the Petri net ontology – its initial design and implementation using UML and Protégé [31] (i.e., RDF Schema (RDFS)-based implementation), whereas Section 5 extends the ontology using an OWL-based UML profile in order to support diversity of Petri net dialects. In Section 6, we present the tool we implemented to support the Petri net ontology, as well as an ontology-driven infrastructure for sharing Petri nets using PNML, XSLT, and RDF. This work is a part of the effort of the Good Old AI research group (http://goodoldai.org.yu) in developing its platform for building intelligent systems, called AIR.