Web Services Composition and Geographic Information - Geographical Information Systems: Trends and Technologies - page 130

exclusively with the W3C compliant wrapper. Based on such a solution,
a W3C service could be able to handle even binary data. For this task the
authors propose to use the standard MTOM or, in case the Web service
environment does not support this standard, the use of Base64 encoding
(Ioup et al. 2008).
The second problem the authors discuss concerns the mapping
of functionalities. In this case, some issues arise due to the fi xed set of
functionalities that OGC services have and that are described in the
Capabilities document. To solve this problem, in Ioup et al. (2008) two main
methods are discussed. In the fi rst method the WSDL document simply
lists all the available operations of an OGC service, thus the same WSDL
specifi cation can be used for all the OGC services of the same type. However,
a drawback exists, namely a direct mapping would lose information since
the exact dataset of each OGC service can be retrieved only by parsing the
document returned by the GetCapabilities function. For this reason the
authors propose an alternative solution which represents the second method
that they discuss in the paper. It consists in a direct mapping between the
OGC service dataset and the W3C service functionalities, i.e., the mapping
does not occur at individual functionality level. In particular, a wrapper
can expose directly the OGC service data layer. This task can be performed
in two different ways: 1) all data layers available in a single OGC function
are mapped into a single and atomic W3C service; 2) each data layer is
mapped into a different W3C service; in this case function names will be
the same for each Web service, but every service will return a single and
different data layer. However, from a Metadata Management point of
view, in the W3C services realm there is no standard for the management
of spatial metadata while almost every OGC service requires them, thus
removing them would mean to lose a lot of their usefulness. Then, it needs
to provide a way to offer such metadata in a WSDL document and in Ioup et
al. (2008) the authors suggest to consider that, although WSDL documents
do not contain metadata (except for those contained in functions provided
by the service) such documents are, to some extent, extensible. Therefore,
the proposed method includes metadata in the extensible part of a WSDL
document. In particular, they suggest to include them in the <SERVICE>
element of the document and to use XML Schema to encode the limits of the
input parameters. Some constraints have nevertheless to be satisfi ed, namely
the whole set of available metadata of an OGC service should be supported
and they must not interfere with the proper use of the WSDL document. A
further benefi t of this solution concerns the possibility of preserving the two
steps process of getting the capabilities and then executing an operation.
The diffi culties to integrate effi ciently Web services and OGC services
are analyzed also in Amirian et al. (2010). By using as a case study the
development of a software system for the management of the Urban Services