Plain WS- Web Processing Service
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Copyright © 2009 Open Geospatial Consortium, Inc.Copyright © 2009 Open Geospatial Consortium, Inc.
Figure 13. Basic WPS interaction
Therefore, the interface is held very generic and is mainly based on three operations as shown in figure 13. Since a WPS should be able to be integrated in to the OpenGIS
framework, it has to offer the GetCapabilites operation generally described by the OWS Common specification. Besides the OWS Common metadata see Section 2.3.4, and
basic service metadata, all processes offered by the WPS are briefly described in the capabilities document. The GetCapabilities operation can only be invoked via HTTP-
GET and the request should be Key-Value-Pair KVP encoded. A minimal request can be found in OGC, 2007
A requestor first requests the service metadata via the GetCapabilities operation. Detailed information is obtained by calling the DescribeProcess operation for an operation listed in
the service metadata. Finally, the execute operation is invoked with all required input data identified by the DesribeProcess response. Especially, the flexibility of the WPS
specification can be used for the workflow wrapping approach: All required workflow input data can be mapped to WPS input data.
Since any kind of calculation or algorithm can be exposed as a WPS process, the same principle can be applied for workflows.
Copyright © 2009 Open Geospatial Consortium, Inc.Copyright © 2009 Open Geospatial Consortium, Inc.
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Figure 14. Workflow Engine Wrapping Architecture
As depicted in figure 14, the client sees the Workflow engine as a simple WPS process and is not necessarily aware of the underlying Workflow engine. The OGC Discussion
Paper 08-123 discusses a transactional extension for the WPS, which applies a generic approach and a concrete profile for BPEL. This profile allows clients to dynamically
deploy and undeploy workflows as WPS processes. In the case of OWS-6, a static-non transactional approach is applied which wires a WPS process directly to a workflow in a
BPEL engine.
This architecture has the advantage, that different Workflow engines can be exposed as a WPS processes without changing the workflow appearance to the client. The client
always can invoke a WPS process independent of the underlying workflow engine or workflow script language. Besides, by encapsulating a workflow engine with a WPS, the
advantages of a WPS can be used, such as integration into the OGC service stack, asynchronous execution and basic message structure interoperability combined with the
flexibility of input data description.