Services-based approach to support geographic model integration to simplify the cooperation of ... uniformly managed by geographic model services container.
Geographic Model Integration in Virtual Geographic Environments Hui Yang, Guonian Lu, Ying Hu, Yi Yang, Jia Wang Key Laboratory of Virtual Geographic Environment, MOE, Nanjing Normal University, China Abstract: To adapt well to modern distributed and heterogeneous network environment, Virtual Geographic Environments must be able to create, manage and optimize spatial analyses. Building up spatial analyses using pre-existing geographic models can lead to higher efficiency in solving complex geographic problems. Geographic model integration is concerned with the application of geographic models to distributed, cross-organizational geographic processes. We propose a Web Services-based approach, which meets spatial analyses requirements by integrating distributed and heterogeneous geographic model sources spread over the web. This paper suggests that geographic model integration should be wrapped by Web Services so that we can shield the heterogeneity of geographic model. The geographic model service container provides the management mechanism and execution engine for distributed geographic models, and supports the uniform management of the distributed models under open network environment. The loose coupling integration of geographic model resources will be easier to realize. The execution environment which is designed to support cooperation of the geographic model service containers under open network environment has been designed and will be introduced. The Web Services-based approach is presented to provide web services to locate, share, and publish model sources. This paper will emphasize on Web Services-based approach to support geographic model integration to simplify the cooperation of geographic scientists in VGE. Keywords: Geographic model, model integration, Web Services, Virtual Geographic Environments
1. INTRODUCTION In order to construct the collaborative environment for spatial analysis, Virtual Geographic Environments (VGE) may provide such a mechanism that many geo-analysis experts can put their geographic models on the Internet to solve the complex geographic problems. A complex geographic problem as the design of groundwater model will include multidisciplinary fields, such as geomorphology, geology, hydrology, and so on. Otherwise during the phase of geographic analysis, subjects such as physics, chemistry, dynamics, and hydraulics, must interact and negotiate in order to complete a complex geographic analysis. Looking at this situation, geographic problems are not only multidisciplinary in nature but also interdependent or interconnected to each other. Geographic model is an attempt to describe and simulate the real geographic world for the use of computer analysis and graphic display of spatial problems. However, traditional geographic modelling approaches are no longer appropriate for modern distributed, heterogeneous network environments, due to their closed architecture, and lack of interoperability, reusability, and flexibility. It has become a trend for giant research institutes especially in the geographic modelling field to have different model components to be built in different areas of the world. For instance, Geographic model A might be built in location A while the model B is designed in location B. All of these pre-existing geographic models are heterogeneous and distributed, so it is necessary for us to research geographic model integration of distributed network environment in VGE, so as to realize the collaborative spatial analysis. The ability to integrate geographic models efficiently and effectively on the Web is a critical step towards the development of the collaborative spatial analysis driven by the Virtual Geographic Environments. As geographic models are most likely autonomous and heterogeneous, building a model component with appropriate inter-service would be difficult. More importantly, the fast and dynamic integration of geographic models is an essential requirement for VGE to adapt their spatial analysis to the dynamic nature of the Web. In this paper, we propose a Web Services-based approach which provides Web Services to locate, share, publish, and integrate geographic models. The approach is so efficient to wrap geographic model that the autonomy and heterogeneity of geographic model can be shielded well. The geographic model service container provides the uniform management mechanism and execution engine for distributed geographic model; meanwhile, execution environment shows us the dynamic and transient relationships between the geographic model service containers when we integrate geographic model under the circumstance of open and distributed network. At last, the highly effective execution of integrated geographic model becomes true.
The remainder of this paper is organized as follows. Section 2 proposes the Web Services-based approach and discusses it from wrapping and integrated perspective. Section 3 gives a brief overview of related work. Finally, the conclusions are drawn and discussed in Section 4.
2. WEB SERVICES-BASED GEOGRAPHIC MODEL INTEGRATION There is no unified concept for model integration at present, and the intuitive comprehension is to construct a composite model combined with many models in order to finish certain task. A Geographic model has a specific task to perform and may depend on other geographic models during integration. For example, a groundwater model that is interested in hydrology could focus on this aspect while quotes other aspects such as precipitation and evaporation. The integration of two or more geographic models generates a new model providing both the original individual behavioural logic and a new collaborative behaviour for carrying out a new integration task. Geographic model integration could be static or dynamic. The former means models interact with each other in a pre-negotiated manner, while the latter means geographic models discover each other and negotiate on the fly. In this section we present a Web Services-based approach that allows the encapsulation of pre-existing geographic models using Web Services as geographic model services. These model services will be uniformly managed by geographic model services container. This kind of container offers the execution engine for distributed geographic model. We also give a Geographic Model Execution Environment (GMEE), which couples all distributed geographic model services over the network space through the uniform management mechanism offered by the container, and helps realize the highly effective execution of the integrated model by execution engine.
2.1 Geographic Model Wrapping For solution of the heterogeneity of pre-existing geographic model we draw on approaches from the study of database integration for multidatabase [1]. The solution requires individual, independent geographic model components to be encapsulated by a wrapper which can interact with each other. It means that existing geographic models are able to cooperate although the cooperation was not designed in advance. Web services are self-contained and modular applications that can be described, published, located, and invoked over the Web. They use open standards and common infrastructure for their description, discovery and invocation, (e.g., XML over HTTP, WSDL, SOAP, and UDDI). The standard specifications and general introductions to these concepts have been provided in some papers. Certainly for Web Services the importance of XML is paramount; all key Web Services technologies are based on it [2]. SOAP is the XML-based information which can be used for exchanging structured and typed information between peers in a decentralized, distributed environment [3]. WSDL defines an XML grammar for describing network services as collections of communication endpoints capable of exchanging messages [4]. UDDI is a “meta services” for locating Web Services by enabling robust queries against rich metadata [5]. Two of the great advantages of the Web Services architecture are its openness and its modularity [6]. Overall, Web services present many new potential opportunities to significantly reduce the complexity and costs of spatial analyses for developing and maintaining geographic models. Fig. 1 illustrates a general model for Web services which named Service Oriented Architecture (SOA). Register Service
Locate Service
UDDI Registry Return WSDL
Service Broker
Invoke Service by SOAP Call Return Result
Service Provider
Service Requesters
Figure 1. Service Oriented Architecture for Service Providers, Requesters and Broker
The service oriented architecture describes three key roles. The services provider implements the Web Service and publishes its description to one or more repositories for potential users to locate. The service requesters are searching for a Web Service that it needs to bind with. Finally, the service broker manages a repository and allows the service requestor to find an adequate service. In actual usage scenarios, multiple service requesters, providers and brokers can interact with one another. Web services perform encapsulated geographic models ranging from simple buffer operation to complex geographic process interactions. Effectively, the Web Services-based approach offers a mechanism which can wrap the pre-existing geographic model as the Web Services at which wrapper linking is performed. The wrapper translates queries on the geographic model services to direct queries on the constituent component in its native style. One unique feature of Web services is that they can be mixed and matched to construct a new and complete geographic process due to their standardized service interfaces and common communication protocols. They also allow dynamic integration with reduced human interaction through the embedded capability of services discovery and binding. Existing geographic models can, therefore, be integrated and extended to participate as part of a new composite geographic model services. Web services are standards-based and suited to build common infrastructure to reduce the barriers of geographic model integration. The simplicity of the overall Web services model helps accelerate its deployment. The dynamic nature of Web services also opens up new opportunities for both geographic and IT support.
2.2 Geographic Model Services Container For the purpose of the uniform management of encapsulated geographic model services and the execution of geographic model services, this paper presents the concept of geographic model services container, which provides the management mechanism and execution engine for the distributed geographic models. The geographic model services containers, which are distributed on the different host computers, have contact with each other organically through SOA framework. They become a whole with using the management mechanism and the execution engine of distributed geographic models. Geographic Model Services Container Interface and Interaction Protocol
Execution Engine of Distributed Geographic Model Model Analysis, Precompiled and Execution Control Long-distance Model Transfer
Local Model Transfer
Model Data Accessing and Transmission Control Model Life Cycle Manage
Execution Engine of Local Geographic Model Model Execution Model Data Accessing and Transmission Control Model Life Cycle Manage Model Execution Monitor
Model Execution Monitor
Distributed Geographic Model Resource Management Geographic Concept & the Filtration, Match and Choice of Geographic Model Geographic Model Deployment
Basic Geographic Model
Geographic Conceptual Model Geographic Computational Model
Model Metadata Management
Geographic Model Metadata Repository
Model Qos Character Management
Model Concept Management
Geographic Model Qos Character Repository
Figure 2. Structure of Geographic Model Services Container
Geographic Concept and Icon Repository Restraint Rule Repository
The geographic model services container is composed of three layers as illustrated in figure 2: geographic model resource management, geographic model execution management, and geographic model services container interface and interaction protocol. In addition, the geographic model services container contains the standard, open interface and interactive protocol itself. The interface ensures that there is interoperability between the geographic model services, while the interactive protocol promises the interconnection and communication between the geographic model services container. The container manages geographic models, responds to the user’s transfer request, and fulfils the interaction and transmission of information in the data layer. Such structure provides the benefit of the united management of geographic models in the open network environment and allows for the distributed integration of geographic models. As a result, the loose coupling of geographic model resources could be realized easily.
2.3 Execution Environment The management modules of geographic model resources among the different geographic model services containers cooperate with each other. The application by means of any interface of the services container may get the whole view of all geographic model resources which belong to the entire frame and may obtain the access method for any geographic model resources. Distributed management of geographic model resources also may become true while using UDDI (Universal Description, Discovery, and Integration) for reference. The execution of geographic computational model is accomplished collaboratively by the distributed geographic model execution engine among the different services containers. Each execution engine can result in the execution of one geographic computational model and parse the script model; it also can invoke local or long-distance geographic model resources with the support of the management modules. Geographic Model Services Container
Relative Resource of Geographic Model
Distributed Execution Engine
Distributed Resource Management
Geographic Model Services Container Interface and Interaction Protocol
Geographic Conceptual Model Modelling Tool
Geographic Model Services Container
Open Internet Environment
Geographic Computational Model Mapping Tool
Geographic Model Services Container
Figure 3. Distributed Geographic Model Execution Environment
Figure 3 briefly illustrates the distributed geographic model execution environment. This environment allows the geographic model services container for connection and communication with each other in the open network environment. The execution environment offers two main tools: geographic conceptual model modelling tool and geographic computational model mapping tool. On the basis of describing the complex semantics of geographic problems to be solved, the geographic conceptual model modelling tool expresses a set of specific geographic concept collection and the relationship between them in the visualized way. It provides the tool that may construct a geographic conceptual model, using a set of matching rules between geographic models and spatial data and the associated rules among a lot of geographic concepts. The figure 4 is the sketch map of geographic conceptual modelling.
Conceptual Icon Geographic Concept Concept/Relation Restraint Engine
Concept Description
Model/Data Matching Engine Model Matching Rule Graphics Interaction Engine Data Matching Rule
Concept Association Rule
Modelling Environment of Geographic Conceptual Model Geographic Model Repository Geographic Conceptual Icon Repository & Restraint Rule Repository
Figure 4. Sketch Map of Geographic Conceptual Modelling
As shown in figure 5, with the conceptual associated relationship among the geographic conceptual models, the geographic computational model mapping tool automatically scans and matches in the geographic model repository, so as to find the geographic conceptual models and spatial data which are suitable to solve the geographic problems. Therefore, the mapping between the geographic conceptual models and geographic computational models as well as the automatically selection of geographic models become true. These two tools may help the geographic model services container manage and long-distance invoke the geographic model services.
Geographic Concept
Model Style & Data Style Choose Introduction Model
Concept Description Data ? Model Matching Rule
Data Model
Data Matching Rule
Concept Association Rule
Geographic Conceptual Model
Concept Association Relation
Geographic Geographic Database Computational Model Repository
Model
Model Running Script
Data
Data
Model Matched With Data Model Model Integration & Data Configure Choose Introduction
Model Associated With Data
Figure 5. Sketch Map of Geographic Computational Model Mapping
3. RELATED WORK In an overview of approaches to model integration, J. E. Kottemann et al. (1992)[7] identify two aspects of an integration specification: variable correspondence-the connections of inputs and outputs between models, and synchronization- the sequencing of solver execution. R. G. Ramirez et al. [8] address similar but simpler problems in their design for DAMS, aimed to support model integration by modellers with a high degree of understanding of the components. The SWAMP system (D. J. Abel et al. 1994) [9] approached the model integration problem with architecture, comprising a graphical user interface, a database, a model driver, and the HSPF solver. In other related work, C. H. Sham et al. (1995) [10] have developed an application that combines a legacy ground-water model, a custom spatio-temporal nitrogen loading model, and a GIS to study water quality. In this case the model integration is loose-data is passed manually between components via translation package. The GOODES (D. Djokic et al. 1996) [11] approach to model integration in the hydrological domain focuses on the problem of data transfer between models. Its file translation utilities translate data formats of heterogeneous models to a common framework. The common framework is not just a syntactic framework but also a semantic one- a universal global schema for hydrology. Thereafter application components are easily linked because they respect the same language, both syntactically and semantically. The work of D. A. Bettett (1997) [12] also supports the development of spatial systems incorporating multiple users, demonstrated in a hydrological domain. The approach defines a customizable objectoriented type system and some programmer’s tools to support the process of building models. The process of integrating models is an intrinsic part of the model building process, that is, the models are built to the common type system which plays the role of a common schema. K. Taylor et al. (1999) [13] proposes the FMISDS integration approach that offers a problem-oriented method for designing the integration system. It distinguishes syntactic and semantic issues in model integration, enabling a declarative description of the models and their conceptual relationships. The integration effort is supported by a reusable toolkit that interprets the declarative descriptions. The EU HarmonIT project is the most sophisticated approach for model integration. Great effort is spent on the development of concepts called Open Modelling Interfaces and Environment (OpenMI, 2002) [14], which allow the combination of various models. In OpenMI existing models are integrated into model components, which are connected by well-defined interface based on a pull driven structure. GeoVISTA Studio[15], which is developed by the Pennsylvania State University, is a Java-based, visual programming environment that allows for the rapid, programming free development of complex data exploration and knowledge construction applications to support geographic analysis. The European Network for Earth System Modelling (ENES) organised the PRISM project[16], funded in 2001, is to share the development, maintenance and support of a comprehensive Earth System Modelling software environment. It is key to facilitate assembling, running, archiving and postprocessing of ESM based on state-of-the-art component models developed in the different climate research centres in Europe and elsewhere.
4. CONCULSION Geographic model integration is a hot research of Geographic Information Systems (GIS), and helpful to enhance the VGE’s ability to solve complex geographic problem. Web Services-based approach is the base of model integration and management. In conclusion, the reader’s attention is drawn to the three key parts in Web Services-based approach to geographic integration. The first is the delineation of the syntactic wrapping technologies which encapsulate the heterogeneous geographic models as Web Services. The second is the geographic model services container, which provides uniform management mechanism and execution engine for distributed geographic model services. The third is the execution environment, which allows the geographic model services container for connection and communication with each other in the open network environment, and helps user solve geographic problem collaboratively via model integration. The architecture designed by Web Services-base approach utilizes Web Services to provide encapsulation in integrating heterogeneity models to solve the spatial analysis problems. The geographic model services, managed by the geographic model services container, are helpful to the uniform management and execution control for the distributed and local geographic model services. Finally, through the Service oriented architecture (SOA), they can register, locate and invoke geographic model services containers in execution environment. This paper introduces the Web Services-based approach to share and integrate model sources by model users from multiple domains via Web in a new perspective view.
We can safely conclude that the work on Web Services-based geographic model integration is not finished yet and that many challenging tasks lie ahead. Some questions should be better studied in future work: parsing pre-existing geographic computational models with the methods of automatically code analysis and artificial assistant analysis, disassembling the basic geographic model as different kinds of granularities, and putting it to the geographic model repository, algorithms library and method repository. As far as this is concerned, it is well possible that development of Web Services-based geographic model integration is ahead of schedule.
5. ACKNOWLEDGEMENTS This research has been supported by Major Program of National Natural Science Foundation of China, National Natural Science Foundation of China (No. 40671147), and Doctoral Fund of Ministry of Education of China (No.20060319004). The authors would like to express special thanks to Professor Guonian Lu who has carefully reviewed the manuscript and provided many valuable comments and suggestions that help improve the quality of the paper. We also thank Yongjun Wang and all other VGE group members for their fruitful discussions.
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