312 The Semantic Web The Network of
312 The Semantic Web The Network of Meanings in the Network of Documents 5. Mediation: This is a necessary functionality aimed at solving semantic mismatches, which can happen at the data level, protocol level and/or at the process level. 6. Execution: This is the invocation of services following programmatic conventions. This supports Monitoring (control over the execution process), Compensation (providing transactional support and mitigating unwanted effects), Replacement (facilitating the substitution of services by equivalent ones) and Auditing (verification that the execution took place as expected). Semantic Web Service Ontologies were discussed in detail in the previous sections. However for the case of ontologies for the semantic web services the following two stand out: OWL-S: Semantic Markup for Web Services 2004: OWL-S is an OWL ontology to describe (semantic) web services. It does not aim at replacing existing web services standards but at providing a semantic layer over existing Web Services standards such as Web Services Description Language. It relies on WSDL for the invocation of web services and on Universal Description, Discovery and Integration for web services discovery. From the OWL-S perspective, a Semantic Web Service is provided by a Resource, presented by a Service Profile, described by a Service Model and supported by a Service Grounding. The Service Profile represents what the Service provides and capabilities of the Service; the Service Model describes how a service works and which are internal processes (Input; Preconditions; Outputs and Results) of the service. Processes can be of the types Atomic, Simple, or Composite. Finally, the Service Grounding builds upon WSDL descriptions to define the message structure and physical binding layer and maps an XML schema to OWL concepts. Web Service Modeling Ontology (WSMO): Feier and Domingue (2005) is a conceptual model for Semantic Web Services. It is an ontology for core elements for the services and consists of a formal description language (Web Services Modeling Language) and an execution environment (Haller et al. 2005). WSMO defines the modeling elements for Semantic Web services based on the conceptual grounding specified in the four main components of the Web Service Modeling Framework (Fensel and Bussler 2003): Ontologies provide the formal semantics to the information used by all other components. Goals specify objectives that a client might have when consulting a Web service. Web services represent the functional (and behavioral) aspects, which must be semantically described in order to allow semi-automatic use. Mediators used as connectors to provide interoperability facilities among the other elements. Web Services have an aspect of capability, which defines their functionality in terms of pre-and post-conditions, assumptions and effects. A Web Service defines one and only one capability. The interface of a Web Service provides further information on how the functionality of the Web Service is achieved. It contains: A Choreography, which describes the communication pattern, that allows to one to consume the functionality of the Web service, named choreography. In other words Choreography is how to interact with a service to consume its functionality. An Orchestration, which describes how the overall functionality of the Web service is achieved by means of cooperation of different, Web service providers, named
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