The eXtensible Markup Language (XML) is a machine readable format for structured information representation on the Web and data exchange across networks and between heterogeneous information system. An XML document primarily consists of a strictly nested hierarchy of tagged elements with a single root. Each tagged element has a sequence of zero or more attribute/value pairs, and a sequence of zero or more subelements. Standard vocabularies are summarised in Document Type Definitions (DTDs). A DTD is essentially a formal grammar which declares tags and their structural relations and restricts the structure of valid documents. For example, a simplified document type description for a PC-Modem card is sketched in Figure .
Figure: XML Document Type Definition
Access to semi-structured XML Web data is realised through a number of
query languages as XML-QL (cf. [DFF98]) or Lore (cf. [GMW99]).
The XML-QL example in Figure selects in the document at
www.modem_company.com all modems that support "Video-Conferencing" and generates a price list.
Figure: XML-QL Query
The facilitator knowledge base encompasses descriptions of all registered service providers, which are consistent with a given set of DTDs and found at specific URLs. This knowledge is stored as facts about DTDs and associated lists of the locations (URLs, Uniform resource locators) of individual services which are consistent with the type definitions.