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Scalable and Interoperable Tourism Information System based on the WebGIS
3. XML-based Data Integration
The integration of geospatial data from heterogeneous data sources is a challenging problem when trying to utilize existing TIS on the web, because of heterogeneity of sources and lack of source metadata. Another new challenge occurs in the integration of GIS and TIS. The reason is that GISs were originally developed independently by software vendors, who tailored their applications for their specific user needs, using locally created terminologies and approaches (Goodchild 1999). In the TATIS, the information integration of geospatial data which are stored in GIS database and tourism data in TIS are achieved by using a two-part middleware between the information sources and the tourists’ application (Figure 2). The first part is called a mediator. Its function is to accept tourist request, breaks up the request into small fragments according to the capabilities of the sources and delegates the request-fragments to the appropriate sources. When the sources process the requests and return the results, the mediator integrates the results and sends the combined information back to the user. In the process, the spatial mediator browses the schema information exported by GIS wrapper as an XML DTD, and dynamically evaluates the minimal combination of sources that satisfy the query attributes and predicates. The aspatial mediator integrates and transforms data from several sources using a declarative specification. For example, it is used to convert aspatal data between different models by translating data from a relational database into XML format and integrating data from different sources into a common view. The key technology in the mediator consists in query rewriting.  Figure 2 The spatial mediator and XML-wrapped sources of geospatial data and GIS and TIS services The second part of the middleware is called a wrapper. The task of the wrapper is to translate a request from the mediator’s language to that of the information source and transform the results provided by the information source back to the mediator’s language. Acting as a proxy of an information source, the wrapper communicates with an information source in SVG DOM, and communicates with the mediator in XML and SVG. In this way, “wrapping” each information source into the translation software makes the protocol diversity of particular sources manageable. 4. Strategies for transferring data between XML documents and relational databases and middleware As design examples of our work, we discuss the strategies for transferring data between XML documents and relational databases according to two mapping rules of a table-based mapping and an object-based mapping. The table-based mapping views an XML document as a serialized table or set of tables. The following code is the example of structure of the document.
<Tables>
<Table>
<row>
<column_1>......</columN-1>
<column_2>......</columN-2>
..........................................
<column_n-1>......</columN-n-1>
<column_n>......</columN-n>
</row>
<row>
.........
</row>
</table>
.........
</tables>
From the above text we can see the table-based mapping has the advantage of simplicity. Simplicity makes it useful as a model for writing data transfer tools, especial using XML to transfer data from the relational databases. Furthermore, the code to transfer data between an XML document and a relational database is independent of whether SAX or DOM is used. The reason is that the table-based mapping allows the document to be processed in single pass in document order. The table-based mapping is commonly used by middleware to transfer data between XML documents and relational databases. In TATIS we store some data in object-relational model. For this case, we adopt an object-relational mapping to transferring the data. Contract to the table-based mapping, the object-relational mapping views an XML document a tree of data-specific object and then uses an object mapping to map these objects to the database. In this model, elements types with attributes, element content, or mixed content are generally modeled as classes. Element types are modeled as scalar properties. The model is then mapped to relational databases using traditional object-relational mapping technology or SQL objective view. That is, classes are mapped to table, scalar properties are mapped to columns, and object-valued properties are mapped to primary key/foreign key pairs. For example, the following figure shows the structure of the XML document (Figure 3).  Figure 3 Structure of XML document using object-relational model |