Once Upon a Time a DTD Evolved into Another DTD...

XML has become an emerging standard for data representation and data exchange over the web. In many applications a schema is associated with an XML document to specify and enforce the structure of the document. The schema may change over time to reflect a change in the real-world, a change in the user’s requirements, mistakes or missing information in the initial design. In this paper, we consider DTDs as XML schema mechanism, and present an approach to manage DTD evolution. We build a set of DTD changes. We identify invariants which must be preserved across DTD changes. We define the semantics of each DTD change such that the new DTD is valid, existing documents conform to the new DTD, and data is not lost if possible. We illustrate our approach with a scenario.