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data into XML messages that will be sent over the network (according to se-
rialization details obtained from the WSDL binding), and also specify how
received XML messages are interpreted as semantic data.
A behavioral grounding must handle the multiplicity of operations of a
Web service interface in WSDL. WSDL itself does not specify any ordering
or dependencies between operations; only the semantic description within a
choreography interface tells the client what particular operations can be in-
voked at a specific point in the interaction.
9.5.1 Location of Grounding
We have described what kind of information must be specified in the ground-
ing; however, we have not yet elaborated on the place of that information.
There are three options for placing grounding information: (1) placing the
grounding in the semantic description document, (2) embedding the ground-
ing within WSDL documents, and (3) externalizing the grounding to a third
document.
Placing the grounding within the semantic description document makes
it directly accessible from the semantic data, which follows the order of the
tasks in the Semantic Web service framework, i.e. discovery uses the semantic
data and then relies on some sort of grounding mechanism in order to invoke
the discovered service. On the other hand, placing the grounding directly
in the WSDL documents (option 2) can enable the discovery of semantic
descriptions in WSDL repositories such as UDDI [10]. This approach is taken
by WSDL-S [1], a specification of a small set of simple WSDL extensions
that can be used with any Semantic Web service modeling framework. An
externalized grounding (outside both WSDL and the semantic descriptions)
does not provide easy access to the grounding information from either the
semantic or the syntactic side, but it may introduce more flexibility for reuse.
However, at the time of publication, externalized grounding is not supported
by any specification.
The options listed above are not exclusive, as grounding information can
be put redundantly in both the semantic description document and the WSDL
document. This could be done by using the native grounding mechanism in
WSMO to point to WSDL and at the same time annotate the WSDL docu-
ment with WSDL-S elements pointing back to WSMO.
9.5.2 Data Grounding
Web services generally communicate with their clients using XML messages
described in XML Schema. At the semantic level, however, Web service in-
puts and outputs are described using ontologies. A semantic client then needs
grounding information that describes how the semantic data should be writ-
ten in XML and then sent to the service. Additionally, the semantic client
needs grounding information that describes how XML data coming back from
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