Using XML-schema to model data from present and future astronomical databases

XML (eXtensible Markup Language) has now become an almost inescapable tool for the exchange of formatted data over the World Wide Web and is now becoming extensively used in astronomical data bases to interweave them, therefore making it an important tool for the building of the Virtual Observatory. Document Type Definitions (DTD) have been initially used to describe XML documents formatting; they offer very limited possibility about data typing, constraints or bounds for the handling of data-oriented documents. These limitations can now be overcome thanks to the XML Schema recommendation. The potential benefit of XML Schema for astronomical databases is illustrated through an ongoing implementation of XML Schema for formatted data output in the Besancon Double and Multiple Star database.