Web Services Composition and Geographic Information - Geographical Information Systems: Trends and Technologies - page 121

<PARTNERLINKS> element contains the <PARTNERLINK> children, a
concrete reference to WSDL services (named Partner Services) that will
take part in the execution of the business process. In particular, for each
<PARTNERLINK> element, the partnerLinkType attribute identifi es the
portType of the referring service. The <VARIABLES> element defi nes the
data variables used during the process workfl ow and their defi nition is
provided in terms of WSDL Message Types, XML Schema Types or XML
Schema Elements. The <FAULTHANDLERS> element is used to defi ne
what to perform in case of fault (for example, during the invocation of a
service). Finally, the <SEQUENCE> element lets process designers organize
a series of building blocks (named activities) executed in a sequential and
predefi ned order. Besides the sequential order, the whole process logic can
be structured in several other ways, such as conditional branching (<if>
element), iterations, repetitions (<while> or <repeatUntil> element) and
parallel processing (<fl ow> element).
Composing OGC services
Due to its strong reliance on WSDL, it is quite diffi cult to use WS-BPEL for
the composition of OGC services. As a matter of fact, there is no need for
OGC services to be equipped also with a WSDL document. Moreover, WS-
BPEL lacks support for the direct management of non-XML data, e.g., the
binary data, such as the image fi les returned by a WMS. For these reasons
researchers have investigated other modalities to manage orchestration of
geographic services.
A valid example of an OGC services orchestration is provided in
Stollberg and Zipf (2007). After describing diffi culties of using WS-BPEL
for the orchestration of OGC services, in this paper the authors propose
the use of the OGC Web Processing Service (WPS) standard introduced in
Schut (2007) as a feasible solution.
The WPS standard provides for a standardized interface that aims at
supporting publication and discovery of geospatial processes. According
to the standard (Schut 2007), a geospatial process includes 'any algorithm,
calculation (e.g., polygon intersection) or model that operates on spatially
referenced data'. In particular, the purpose of the WPS interface is to
standardize the way processes and their input/output are described, how
a client can invoke the execution of a process and how the output should be
treated. Data that can be used during the elaboration can vary from image
data formats to data exchange standards such as GML.