Web Geoprocessing Workflows

Geoprocessing

In the absence of a general definition, the term geoprocessing can be seen as a specialization of the term processing in a spatial context. In other words, geoprocessing is the processing of spatially related data. In classical desktop GIS applications such as GRASS or ArcGIS, geoprocessing represents the core GIS analysis functionality and thus is one of the key concepts. In contrast to monolithic desktop GIS, distributed GIS are based on loosely coupled services organized in SDIs. According to the ISO 19119 specification, there are four different types of geoprocessing services:

• Spatial processing, such as coordinate transformation services, feature manipulation services, or route determination services
• Thematic processing, such as thematic classification services, geographic information extraction services, or image processing services
• Temporal processing, such as temporal reference system transformation services, subsetting services, or temporal proximity analysis services
• Metadata processing, such as statistical calculation services and geographic annotation services

From the work of the Open Geospatial Consortium, the Web Processing Service specification evolved as a standardized but generic service approach to perform geoprocesses in an SDI on distributed data. These processes can be as simple as the sum of two numbers (e.g., population) or as complex as a global climate model. The data required by the service can be delivered across a network or made available on a server. Image data formats or data exchange standards such as geography markup language or KML can be used to encode the data. The Geoprocessing Web Services form the building blocks for Web-based geoprocessing workflows.

Web-Based Workflows

The term workflow was defined by ISO 19119 as “the automation of a business process, in whole or part, during which documents, information or tasks are passed from one participant to another for action, according to a set of procedural rules.”

This concept of a workflow has been extensively researched in the information technology (IT) and business fields for several decades. With the advent of the service-oriented architecture paradigm and the increasing data-processing power of network computing, Web-service-based workflows are currently a prominent workflow type. By integrating several Web services from different partners into a single workflow, it becomes possible to form virtual enterprises. These concepts of Web-based workflows rely heavily on service chains to exchange information between workflow participants. There are three different architectural patterns defined for service chains according to ISO 19119:

• User transparent chaining
• Translucent chaining
• Opaque chaining

Especially the opaque chaining pattern is applied by the “mainstream” IT workflow management systems, which mostly use the idea of Web service orchestration (WSO). This concept is defined as a description of how composed Web services interact on the message level. The description includes the business logic and execution order. Based on the predefined message exchange protocol, the interaction is controlled by a central orchestration engine that enables the Web services to be held loosely coupled. This common central orchestration approach is widely supported by several major IT WSO frameworks. The frameworks allow the visual composition of workflows instead of programming and hence enable flexible and rapid workflow modeling and execution. The Business Process Execution Language is mainly used by these frameworks since it is the de facto standard.

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