XML:XML Applications

XML Applications

A central design goal for XML is to make it easier to communicate information, on the Web and between applications, by allowing the semantics of the data to be described with the data itself. Thus, while the large amount of XML data and its use in business applications will undoubtably require and benefit from database technologies, XML is foremost a means of communication. Two applications of XML for communication — exchange of data, and mediation of Web information resources — illustrate how XML achieves its goal of supporting data exchange and demonstrate how database technology and interaction are key in supporting exchange-based applications.

Exchange of Data

Standards are being developed for XML representation of data for a variety of specialized applications ranging from business applications such as banking and shipping to scientific applications such as chemistry and molecular biology. Some examples:

• The chemical industry needs information about chemicals, such as their molecular structure, and a variety of important properties such as boiling and melting points, calorific values, solubility in various solvents, and so on. ChemML is a standard for representing such information.

• In shipping, carriers of goods and customs and tax officials need shipment records containing detailed information about the goods being shipped, from whom and to where they were sent, to whom and to where they are being shipped, the monetary value of the goods, and so on.

• An online marketplace in which business can buy and sell goods (a so-called business-to-business B2B market) requires information such as product catalogs, including detailed product descriptions and price information, product inventories, offers to buy, and quotes for a proposed sale.

Using normalized relational schemas to model such complex data requirements results in a large number of relations, which is often hard for users to manage. The relations often have large numbers of attributes; explicit representation of attribute/- element names along with values in XML helps avoid confusion between attributes. Nested element representations help reduce the number of relations that must be represented, as well as the number of joins required to get required information, at the possible cost of redundancy. For instance, in our bank example, listing customers with account elements nested within account elements, as in Figure 10.3, results in a format that is more natural for some applications, in particular for humans to read, than is the normalized representation in Figure 10.1.

When XML is used to exchange data between business applications, the data most often originate in relational databases. Data in relational databases must be published, that is, converted to XML form, for export to other applications. Incoming data must be shredded, that is, converted back from XML to normalized relation form and stored in a relational database. While application code can perform the publishing and shredding operations, the operations are so common that the conversions should be done automatically, without writing application code, where possible. Database vendors are therefore working to XML-enable their database products.

An XML-enabled database supports an automatic mapping from its internal model (relational, object-relational or object-oriented) to XML. These mappings may be simple or complex. A simple mapping might assign an element to every row of a table, and make each column in that row either an attribute or a subelement of the row’s element. Such a mapping is straightforward to generate automatically. A more complicated mapping would allow nested structures to be created. Extensions of SQL with nested queries in the select clause have been developed to allow easy creation of nested XML output. Some database products also allow XML queries to access relational data by treating the XML form of relational data as a virtual XML document.

Data Mediation

Comparison shopping is an example of a mediation application, in which data about items, inventory, pricing, and shipping costs are extracted from a variety of Web sites offering a particular item for sale. The resulting aggregated information is significantly more valuable than the individual information offered by a single site.

A personal financial manager is a similar application in the context of banking. Consider a consumer with a variety of accounts to manage, such as bank accounts, savings accounts, and retirement accounts. Suppose that these accounts may be held at different institutions. Providing centralized management for all accounts of a customer is a major challenge. XML-based mediation addresses the problem by extracting an XML representation of account information from the respective Web sites of the financial institutions where the individual holds accounts. This information may be extracted easily if the institution exports it in a standard XML format, and undoubtedly some will. For those that do not, wrapper software is used to generate XML data from HTML Web pages returned by the Web site. Wrapper applications need constant maintenance, since they depend on formatting details of Web pages, which change often. Nevertheless, the value provided by mediation often justifies the effort required to develop and maintain wrappers.

Once the basic tools are available to extract information from each source, a mediator application is used to combine the extracted information under a single schema.

This may require further transformation of the XML data from each site, since different sites may structure the same information differently. For instance, one of the banks may export information in the format in Figure 10.1, while another may use the nested format in Figure 10.3. They may also use different names for the same information (for instance, acct-number and account-id), or may even use the same name for different information. The mediator must decide on a single schema that represents all required information, and must provide code to transform data between different representations. Such issues are discussed in more detail in Section 19.8, in the con- text of distributed databases. XML query languages such as XSLT and XQuery play an important role in the task of transformation between different XML representations.