Cabral, Liliana

Domingue, John

Tanasescu, Vlad

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsOCML ontologies to XML schema loweringConference or Workshop ItemHow to cite:Tanasescu, Vlad; Domingue, John and Cabral, Liliana (2004). OCML ontologies to XML schema lowering. In:First AKT Workshop on Semantic Web Services (AKT-SWS04), 8 Dec 2004, Milton Keynes, UK.For guidance on citations see FAQs.c© 2004 The AuthorsVersion: Version of RecordCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukOCML Ontologies to XML Schema Lowering ?,??Vlad Tanasescu1,2, John Domingue2, Liliana Cabral21 Swiss Federal Institute of Technology (EPFL), 1015 Lausanne, Switzerlandvlad.tanasescu@epfl.ch2 Knowledge Media Institute, The Open University, Walton Hall, Milton Keynes,MK7 6AA, UK{v.tanasescu, j.b.domingue, l.s.cabral}@open.ac.ukAbstract. Ontologies are explicit specifications of a conceptualizationintended for logical processing. They are used to express meaning and toapply it on otherwise less structured data. Nevertheless other models oforganization and structure of knowledge, like database models and XMLdocument definition standards, are widely used and valuable. In order toextend ontology usage to multiple layers of an application, or to integrateontologies with pre-existing software, the use of these various means tostructure data could be necessary. We are therfore using metadata inorder to adapt an ontology to these use cases and provide the example ofautomatically generating user interfaces for goal descriptions in IRS-III[1] by extracting an XML schema from an ontology.1 IntroductionThe semantic web vision foresees the advent of automatic machine operable ser-vices through the use of knowledge based technologies. An interesting class ofstandardized services are web services, which are widely accepted if not yet mas-sively used by the industry. Current web service implementations are, however,relatively inflexible, and not powerful enough to support automatic discovery,mediation and composition. Therefore ongoing research is investigating how se-mantic web technology can alleviate this. The IRS-III (Internet Reasoning Ser-vice) framework [1] supports the creation of semantic web services accordingto the WSMO ontology [2], and extends it. Users of IRS-III directly invoke webservices via goals i.e. IRS-III supports capability-driven service execution. A goalis described as a class in OCML (Operational Conceptual Modelling Language)[3] and related to one or more web services through mediators, which provide? This research was partially supported by the Advanced Knowledge Technologies(AKT) project. AKT is an Interdisciplinary Research Collaboration (IRC), whichis sponsored by the UK Engineering and Physical Sciences Research Council undergrant number GR/N15764/01. The AKT IRC comprises the Universities of Ab-erdeen, Edinburgh, Sheffield, Southampton and the Open University.?? This work was also partially supported by the DIP (Data, Information and ProcessIntegration with Semantic Web Services) project. DIP (FP6 - 507483) is an Inte-grated Project funded under the European Unions IST programme.the required flexibility by allowing to specify mapping mechanisms between goalinput roles, output roles, and matching web services.Ontologies, like the ones used in a goal description to define a capability, areintended for logical inference, and are often loosely related to applications basedon them; therefore one has to provide some glue to allow diverse aspects of anapplication to conform to an ontology; the problem of data consistency, of userinput for example, usually occurs and is hard to tackle in a generic way.Our motivating example is to provide a consistent interface to IRS-III goals.The input to these goals, i.e. the data one has to provide in order to expectachievement of the goal, is defined in an OCML ontology, and uses data typesprovided by it. For example an exchange rate goal will have three input roles:has-source-currency, has-target-currency and has-amount, which represents theamount which has to be converted from source to target currency.The difficulty is that OCML types are used to describe the roles, i.e. poundor euro are instances of a currency class. The enumeration of available currenciesis therefore well defined in the ontology and this constraint has to be reflectedin the actual web or Java goal access interfaces, task which is actually achievedby the developer, in a disconnected and hence highly error-prone way.2 Translating OCML ontologies to XML schemaThe relation between ontologies and XML schemas (XSD [5]) has already beendescribed (for example in [4]): ontology languages are a means to specify do-main theories while XML schemas provide integrity constraints for informationsources, but both provide vocabularies and structure for describing informationsources that are intended for exchange. However ontology modelling languageslike OCML do not offer rich collections of built-in data types for two main rea-sons:1. Providing clear semantics and reasoning support for a large collection ofcomplex data types is difficult.2. The precise representation of a data type is often superfluous in a knowledgemodeling context, i.e. a date may be an important aspect of a domain butvarious representations of it are not.The XML schema type system has been inspired by language independentdata types as well as actual query and object-oriented programming idioms likeSQL or JAVA [5]. OCML is a frame-based, i.e. object-centered, knowledge repre-sentation system, which also provides a relational view. These systems thereforepresent similarities that we can exploit and a quite intuitive mapping appearsbetween the two (see table 1).Unfortunately the XSD type system does not support multiple inheritance,and even simple derived types can be only created either by restriction or by ex-tension but not both at the same time. However this kind of modelling is mostlyuseful for knowledge representation and have usually little role to play wheninteracting with other software architectures. Moreover we are only interestedTable 1. OCML to XSD data typesOCML XSDclass complexTypeslot elementstring, float, ... simpleTypeby the constraints allowed by an XSD definition, not by the reuse of the derivedXSD types since we are able to generate them on the fly. Therefore we chose totreat these cases by defining a new type for every OCML class, inherited or not,containing all the slots (local and inherited) as elements. Also there is actuallyno planned support in our approach for classes defined in OCML intensionallyby logical expressions.Still the result of this simple mapping is not very helpful from an applicativepoint of view since we do not have any validation constraints yet, i.e. we donot know, because it is not specified in the ontology, that for our purposes theslot/element called has-amount must be smaller than a given value (otherwisethe program may crash, or the database will not be able to store the value, etc).3 Constraint metadata applied to ontologiesConstraint information could simply be added to the ontology, however the ap-plicative environment (Java program, web interface, database) may change whilethe ontology is already shared in multiple contexts, therefore it is more appro-priate to only attach this information to the ontology, without any modification,hereby allowing for multiple views or aspects of it depending of the applica-tive context. Without doing so constraints on a goal could be superfluous oreven conflicting depending on the context, e.g. the maximum value of a numbercould be greater for a Java application than for a Web interface, while entirelymeaningless for the knowledge domain.During the translation process we therefore use a metadata system to storethe constraints we chose to add to the ontology XSD representation for ourspecific context dependent purpose. Our metadata system is composed of:1. A naming convention mimicking the has-a tree-like structure of the ontologyand allowing to request information about a particular element of it (class,slot or relation). For example:’(transform classes EXCHANGE-RATE-GOAL HAS-AMOUNT)2. A repository which can be anything from a DBMS to simple associationlists, able to store the information regarding a node according to the namingstructure, for example the required length of a string, as well as other relevantinformation like OCML basic types mappings to XSD:’((transform ((classes ((EXCHANGE-RATE-GOAL((HAS-AMOUNT((maxInclusive ((1000000)))))))))(types ((float (("xs:decimal"))))))))3. An access interface to the metadata repository that has to provide neces-sary access functions as well as commodity ones required by the generationprogram.By using the metadata convenience (through the repository called mddb) wecan now easily map an OCML ontology to a constrained XSD schema:Algorithm OCML2XSD(ontology,mddb)1. for c ∈ classes(ontology)2. do Create element complexType with attribute name = name(c)3. Create sequence element4. for s ∈ slots(c)5. if s is a type referenced as basic in mddb6. then create element simpleType7. add base attribute with value specified in mddb8. if s has attached constraints in mddb9. then add the constraints as restriction elements10. else create element with type attribute set to class(s)It is straightforward to restrict this algorithm to the slots representing in-put roles of an IRS-III goal. Then, by applying an XSL transformation to thegenerated schema, we obtain a web interface with embedded validation.4 ConclusionWe demonstrated the use of metadata applied to ontologies in order to adaptthem to practical application contexts, and used this technique to generate XMLschema from an OCML goal description in order to obtain a consistent validatingweb interface. Metadata can be used to produce many applicative views of anontology, like SQL Data Definition Language statements for persisting ontologyinstances, or JavaBeans template code to ease ontology driven enterprise appli-cation building. In further work we aim to generalise this process by providinga language for building generative mappings.References1. J. Domingue, L. Cabral, F. Hakimpour, D. Sell and E. Motta (2004). IRS-III: APlatform and Infrastructure for Creating WSMO-based Semantic Web Services.(WIW 2004)2. Web Service Modeling Ontology Standard, http://www.wsmo.org/2004/d2/3. E. Motta, An Overview of the OCML Modelling Language (KEML ’98)4. M. Klein, D. Fensel, F. van Harmelen, and I. Horrocks, ‘The Relation betweenOntologies and XML Schemas’ (2001)5. XML Schema - W3C Recommendations 28 October 2004,http://www.w3.org/TR/xmlschema-0/ and http://www.w3.org/TR/xmlschema-2/

OCML ontologies to XML schema lowering

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OCML ontologies to XML schema lowering

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