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Figure 4 - Request transformation process
10.2.1 Request transformation
The WFS-G Best Practice only specifies an application schema for GML 3.1.1. Whereas the NGA gazetteer services was found to support GML 3.1.1, the USGS and NB
gazetteer services were found to support GML 3.2.1 and not GML 3.1.1. The impact of this difference was that requests received by the mediator could not be simply forwarded
to all three foundation gazetteer services without modification to specify the GML version supported by the target gazetteer service. The mediator was therefore configured
to check the GML version specified in a request and then depending on the target gazetteer service, the mediator either replaced or retained the GML version specified. The
following table explains how the request is transformed within the request transformation process presented earlier in this section:
Requirement NGA WFS-G
USGS WFS-G NB WFS-G
GML 3.1.1 Cascade
Transform to GML 3.2.1
Transform to GML 3.2.1
GML 3.2.1 Transform to
GML 3.1.1 Cascade
Cascade
10.2.2 XPath references
Another issue encountered was that WFS allows a feature property to contain its value as content encoded inline or to reference its value through a simple XLink. It was observed
that the USGS and NB services supported queries on properties in line, whereas the NGA service supported queries on properties both inline and by reference. This made it
necessary for the mediator to check the XPath expression used to name the locationType and then depending on the target gazetteer service specified, the mediator either replaced
or retained the XPath reference with an appropriate one.
Requirement NGA WFS-G
USGS WFS-G NB WFS-G
Cascade Cascade
Transform to locationType inline
Transform to locationType inline
locationType inline Cascade
Cascade Cascade
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10.2.3 PropertyIsSemanticallyRelatedTo Operator
In OWS-9, an operator was designed to support the use of semantic filters in WFS queries. The new operator, named PropertyIsSemanticallyRelatedTo allowed a client
application to filter features by property values that are semantically related to the search term.
Whereas in OWS-9 the location type hyperlinks were used to select relevant mappings, in Testbed-10 the actual place names were used.
The reason for the change of approach is that the WFS-G services used in the current testbed adopted a different URI scheme from the services used in the previous testbed.
The impact is that an update is required to the PropertyIsSemanticallyRelatedTo operator proposed in the previous testbed to allow it to accept a choice of hyperlinks or literal
values.
10.3 Implementation issues
