Data Exchange over Web-based Applications with DXL

With large volumes of data being exchanged on the Internet, query languages are needed to bridge the gap between databases and the web. Furthermore, the differentiation in data types used by webbased applications is ever growing, despite all standardization efforts. The Data eXchange Language (DXL) provides an extensible base language designed to exchange data from heterogeneous sources into a single target. One application of DXL, the focus in this article, is to retrieve data from databases, and yield the result in an XML document. However, the real application area of DXL is much broader since DXL provides a framework which allows data of a particular source to be queried and/or constructed by its original query language. This is achieved by DXL's extensibility mechanism which allows other query languages to be embedded into a DXL query. The scope of this article is to compare DXL to other related query languages, discuss DXL's features and architecture, and present the base language denition of DXL. Furthermore we will discuss two extensions of DXL which allows us to query and construct databases and XML documents. Finally we will use these extensions in a newsgroup example, to illustrate DXL's main features, with respect to querying heterogeneous sources, and its recursive behavior.