Three Layer Evolution Model for XML Stored in Relational Databases

XML-relational systems with well defined XML and relational schemas are widely used in industry. In the presence of rapidly changing requirements both schemas of such a model need continuous evolution, which can be performed by a formal framework that describes the evolution of the mutually dependent schema pairs of the system as sequences of elementary schema transformations. There are a number of common tasks, like relational schema performance tuning, XML syntax clean-up, etc that stand apart from changes of semantics of the schemas. We discuss a framework with three layers of operations: the changes in XML schema only, the semantic changes that affect both schemas, and the changes that affect relational schema only. We show how the layered framework allows to formalize and address the above mentioned tasks. We consider the formal quality of the solutions of the tasks that may be achieved inside the framework.