Research On Application Model Based Multi-Source Spatial Database

Usually, different spatial database systems (SDB) have significant differences in data structure, data schema and data format. This leads to a situation in which those application systems built on different SDB cannot share data and services between each other. Multi-source spatial database engine is brought forward to solve this problem.SDBE is a spatial database engine based on the Mediator-wrapper approach. It adopts common query language and common data model, and can hide the difference between different data sources from the client application. SDBE is a scalable system and easy to extend, its architecture is suitable for constructing distributed systems. But in the original SDBE system, the two kinds of application programming interface, the engine interface and the application interface, which have some limitations such as tight-coupling between data source and client application, inflexible in data view definition, etc. cannot meet the application requirement.Application model is a kind of description model based on objects, it uses the concept of object to define data views, and can describe complex data structures. The application model does not rely on any specific data sources, it provides client application with the description model of data sources using a uniform approach. The client transform the predefined data views, "feature class", in application model using the interface provided by system, and submit the query to retrieve data needed. Application model has following features: â‘ Object definition. Application model uses columns of feature tables in data sources as the basic object types, and combines those basic data types to construct complex objects. â‘¡Object nesting. When an object is being defined, existing feature classes can be used as object types to make more complex objects. â‘¢Object aggregation. When two feature tables are joined together, it may produce lots of redundant data. Using objects of application model, we can eliminate this redundancy by the object aggregation of one to many. The design of application model incorporates OOP methodology, and gives full consideration of extensibility. Objects creation of all classes in the application model class hierarchy use Factory approach, and the complex operation imposed on application model objects are implemented using Visitor approach. When needed, new derived classes can be introducedinto the class hierarchy of application model, and new derived Visitor classes can be used to define new complex operations.Application model is organized and managed through description script. The description script also provides data source access information management, user group management, and user management. The description script separates client application from data sources, and the users must be registered in SDBE system before they can access the application model. Those information such as user ID and password needed access data source is managed by the description script. Access authority to each application model is assign to user groups, and one user must have the identity of proper user groups to gain the access to specific application model.Because of the complexity of application model, a GUI modeling tool is necessary to manipulate application model efficiently. Through the modeling tool we developed, application model can be created, edited, and managed easily.