Views as first-class citizens in object-oriented databases

摘要

Extensibility and dynamic schema evolution are among the attractive features that lead to the wide acceptance of the object-oriented paradigm. Not knowing all class hierarchy details should not prevent a user from introducing new classes when necessary. Naive or professional users may define new classes either by using class definition constructs or as views. However, improper placement of such classes leads to a flat hierarchy with many things duplicated. To overcome this problem, we automated the process in order to help the user find the most appropriate position with respect to her class in the hierarchy regardless of her knowledge of the hierarchy. The system must be responsible for the proper placement of new classes because only the system has complete knowledge of the details of the class hierarchy, especially in a dynamic environment where changes are very frequent. In other published work, we proved that to define a view it is enough to have the set of objects that qualify to be in a view in addition to having message expressions (possible paths) that lead to desired values within those objects. Here, we go further to map a view that is intended to be persistent into a class. Then we investigate the proper position of that class in the hierarchy. To achieve this, we consider current characteristics of a new class in order to derive its relationship with other existing classes in the hierarchy. Another advantage of the presented model is that views that generate new objects are still updatable simply because we based the creation of new objects on existing identities. In other words, an object participates inside view objects by its identity regardless of which particular values from that object are of interest to the view. Values are reachable via message expressions, not violating encapsulation. This way, actual values are present in only one place and can be updated.