Adding Schema Evolution to the Persistent Development Environment Oberon-D

In this report we show how schema evolution was added to Oberon-D. Oberon-D is an ongoing research project which adds database functionality to the Oberon system. In Oberon-D persistence is obtained by a persistent heap on the disk. Persistent objects, which are not referenced by other persistent objects are reclaimed by a Stop & Copy garbage collector. Schema evolution is done during a garbage collector run. Objects are not physically restructured, but a new object (conforming to the definition of the modified type) is created instead. A transformation function reads the value in the old object and assigns it - with possible modifications - to the new object. All references to the object are updated to point to the newly created object. This technique does not need an additional process nor does it produce additional delays. We show the use of schema evolution as well as its implementation.