This approach of the SWE Working Group (part of the 52°North Open Source initiative
(http://www.52n.org)) proposes a software framework named the OGC Web Service Access
Framework (OX-Framework) which is able to integrate all kinds of OGC Web Services and
thereafter to visualize and process the queried data. It is a generic solution
offering developers a customizable and extendable system of cooperating classes
supplying a reusable design. It can be used on the level of business logic in
different client AND server applications.

A demo client has been developed, which is build upon the proposed framework. This
client is a Java-Swing application which has the primary aim to visualize observed
sensor data provided by SensorObservationServices (SOS). You can see screenshots
& videos of the client here:
https://www.incubator52n.de/twiki/bin/view/Sensornet/OxFramework#Videos.
The videos show the client exploring gauge heights in North Rhine-Westphalia
(Germany) or checking out weather data (provided by a SOS) in South Africa.

As nowadays different types of geospatial data are available on different sources
through OGC Web Services the interest to integrate them is demanding. This is due to
the fact that to carry out reliable decisions different types of data from various
sources are required. Thus a generic framework is needed, which enables integration
of these Web Services. Though a number of approaches are available to support users
on a rather abstract level (most of the GI-software providers offer so called suites
or portal solutions), support for software developers is hard to find. Tillman &
Garnett (2006) support this integrative notion of OGC Web Services, but only by
focussing on client applications. As the integration of Web Services on service side
is also demanding especially in the context of sophisticated web processing and
service chaining (Kiehle et al. 2006), we propose an integrative approach for both
environments. Such an extended approach towards service applications would support
the development of sophisticated service chains and decrease the complexity of
OGC-based software development.
However until now only specific frameworks are available, either as pure client
solutions, such as uDig (http://udig.refractions.net), or as pure service solutions,
e.g. deegree (http://deegree.sourceforge.net/). A generic solution is still missing.
The proposed framework described below offers developers a customizable and
extendable system of cooperating classes supplying a reusable design which is
applicable for client and server applications. Looking from the perspective of the
Sensor Web Enablement (SWE) initiative (Sliwinski 2005), different sensors and other
support data are required to extract reliable information. This case was the driving
force for the SWE Working Group within the 52°North Open Source initiative
(http://www.52n.org; Kraak et al. 2005) to come up with an integrated framework named
the OGC Web Service Access Framework (OXFramework). It is the aim of the OX-Framework
to provide an integrative view to access all kinds of OGC Web Services and thereafter
to visualize and process the queried data. The variety of different services and data
encodings makes it necessary to build up a flexible architecture.

The OX-Framework supports flexibility by applying three concepts:
• Layer-Architecture
• Plugin-Concept
• Listener-Concept

The Layer-Architecture reduces the complexity of the OX-Framework by structuring it
into three layers: Service-Adapters, Core and Utilities. The Service-Adapters layer
contains realizations of adapters for specific OGC Web Services (e.g. SOS, WMS or
WCS). These adapters provide common facilities to the Core for service access in the
form of Service-Connectors, data visualization engines (Renderers) and feature
marshalling (Feature-Stores). The Service-Connector initializes the Common
Capabilities Model of the particular service type and is able to trigger its
operations. The Renderer converts the received data to a graphical representation.
The Feature-Store provides marshalling facilities for received feature data to the
Cores Feature Model. All the Service-Adapters communicate with each other through the
Core. This communication is enabled by common data models, which reflect the
integrative approach of the OX-Framework. In detail the Core incorporates a
three-folded data model: The Common Capabilities Model implements the OWS Common
Specification (Whiteside 2005) and introduces thereby the integrative view on service
access to the architecture. The Feature Model provides a basis for accessing,
visualizing and processing of vector data based on (Reynolds 2005; Kottman 1999). The
Context Model enables persistence and exchange functionality for client projects. It
maps a user session – so called “Context” - to an XML-encoding compliant to the Web
Map Context Documents specification (Humblet 2003). The Utilities provide
functionality for specific UI-frameworks (e.g. Struts or Swing). Those components
help the user of the OX-framework to build up a client application or a new service
using the framework for servicechaining. The Plugin-Concept enables the developer to
customize and extend the framework with the required Service-Adapters in a dynamic
way. Hence it is possible to build up client- and service-oriented applications with
the OX-Framework.

Additionally the Listener-Concept is an important feature of the OXframework because
it affords a high degree of extensibility and transparency which endows the developer
with the absolute control over the framework.

The OX-Framework is just evolving within 52°North’s SWE Working Group and provides a
valid basis for OGC-related software development as demonstrated by the concepts and
the implementation at 52°North. Now the framework has to show that it can stand the
test in coming sensor web projects. The further development will extend its
functionality in sense of web processing, coverage handling and additional
Service-Adapter realizations.


REFERENCES

Humblet, J.-P. (2003). Web Map Context Documents. OGC Implementation Specification.
OGC Document Number: 03-036r2.

Kiehle, C., K. Greve & C. Heier (2006). Standardized Geoprocessing - Taking Spatial
Data Infrastructures one Step Further. Proceedings AGILE 2006. 9th AGILE
International Conference on Geographic Information Science: 273–282. Visegrad, Hungary.

Kottman, C. (1999). The OpenGIS™ Abstract Specification Topic 5: Features (Version
4). OGC Abstract Specification. OGC Document Number: 99-105r2

Kraak, M.-J., A. Sliwinski & A. Wytzisk (2005). What happens at 52N? An Open source
approach to education and research. Joint ICA commission seminar 6.-8. July 2005,
part of the 22nd ICA conference, ICC 2005: Mapping approaches into a changing world,
2005, 16-20.

Reynolds, G. (2005). GO-1 Application Objects. OGC Implementation Specification. OGC
Document Number: 03-064r10.

Sliwinski, A., I. Simonis et al. (2005). Boosting the OGC Sensor Web Enablement
Initiative by Open Source Web Services – The Case of 52°North. AGIT 2005, July 6 - 8,
2005.

Tillman, S. & J. Garnett. (2006). OWS Integrated Client Architecture, Design, and
Experience. OGC Discussion Paper. OGC Document Number: 05-116.

Whiteside, A. (2005). OGC Web Services Common Specification. OGC Implementation
Specification. OGC Document Number: 05-008.