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Metadata acquisition for web services

The promise of the emerging Semantic Web Services field is that machine understandable semantics augmenting web services will facilitate their discovery and integration. Several projects used semantic web service descriptions in very different application domains (bioinformatics grid, Problem Solving
Methods).

A common characteristic of these descriptions is that they rely on a generic description language, such as OWL-S, to specify the main elements of the service (e.g. inputs, outputs) and on a ontology containing knowledge in the domain of the service such as the type of offered functionality (e.g. TicketBooking, CarRental) or the types of service parameters (e.g. Ticket, Car).

The quality of the domain ontologies used influences the complexity of reasoning tasks that can be performed with the semantic descriptions. For many tasks (e.g. matchmaking) it is preferable that web services are described according to the same domain ontology. This implies that the domain ontology used should be generic enough to be used in many web service descriptions. Domain ontologies also formally depict the complex relationships that exist between the domain concepts. Such rich descriptions allow performing complex reasoning tasks such as flexible matchmaking.

We conclude that building quality (i.e. generic and rich) domain ontologies is at least as important as designing a generic web service description language such as OWL-S.


The acquisition of semantic web service descriptions is a time consuming and complex task whose automation is desirable, as signaled by many researchers in this field. Pioneer in this area is the work reported by Andreas Hess which aims to learn web service description from existing WSDL (WSDL stands for Web Service Description Language and is the industry standard for syntactic web service descriptions) files using machine learning techniques. They classify these WSDL files in manually built task hierarchies. Complementary, we address the problem of building such hierarchies, i.e. domain ontologies of web service functionalities (e.g. TicketBooking). This task is a real challenge since in many domains only a few web services are available. These are not sufficient for building generic and rich
ontologies.

Our approach to the problem of building quality domain ontologies is motivated by the observation that, since web services are simply exposures of existing software to web-accessibility, there is a large overlap (often one-to-one correspondence) between the functionality offered by a web service and that of the underlying implementation. Therefore we propose to build domain ontologies by analysing application programming interfaces(APIs).

We investigate two research questions:
- Is it possible and useful to build a domain ontology from software APIs?.
- Can we (semi-)automatically derive (part of) a domain ontology from APIs?

We verified our hypothesis in two different domains. First, we worked in the domain of RDF based ontology stores. Tools for storing ontologies are of major importance for any semantic web application. While there are many tools offering ontology storage (a major ontology tool survey reported on the existence of 14 such tools), only very few are available as web services (two, according to the same survey).

Therefore, in this domain it is problematic to build a good domain ontology by analysing only the available web services.

Nevertheless, good domain ontology is clearly a must since we expect that many of these tools will become web services soon. We attempted to build a domain ontology by analysing the APIs of three tools (Sesame, Jena , KAON RDF API).

The second domain was that of bioinformatics services.

We have experimented with extracting domain ontologies from the descriptions of a set of bioinformatics services employed by the myGRID project.

In both cases the results were very encouraging, as we were able to extract a significant amount of ontological knowledge. Therefore we believe that our method will be a crucial innovation in the field of Semantic Web Services by supporting domain engineers in their task of building adequate high quality domain ontologies, therefore boosting a web of semantically described services.

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Vrije Universiteit Amsterdam
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1081HV Amsterdam
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