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Semantic Web services envision the automated discovery and selection of Web services. This can be realised by adding semantic information to advertised services and service requirements. The discovery and selection process finds matches between requirements and advertisements according to their semantic description. Based on the Web Ontology Language (OWL) an ontology for Web services (OWL-S) was introduced to standardise their semantic description. There are already some approaches available for matching of service requirements with service advertisements according to such an ontology. We propose an algorithm, which ranks the matching degree of service descriptions according to OWL-S. Different matching degrees are achieved based on the contravariance of the input and output types for requested and advertised services. Furthermore, additional elements of the service description, such as the service category, are either covered by reasoning processes or, such as quality of service constraints, by custom matching rules. Contrary to mechanisms that return only success or fail, ranked results provide criteria for the selection of a service among a large set of results. With such a discovery mechanism additional Web services can be found that might have normally been ignored.

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