Concurrency control for design objects with versions in CAD databases

摘要

CAD databases must support versions of design objects. New versions of an object are usually derived from older ones. The set of versions and the successor relation form a version graph, which is modified by dedicated operations. When such modifications are invoked by parallel transactions anomalies can occur. Moreover, there may be other reasons to forbid certain concurrent changes of the version graph during a transaction, e.g. the derivation of new successors of an existing version.This paper presents a flexible technique based on locking which solves these problems. Individual versions are treated like objects of their own. The successor structure is modelled by new virtual objects. Changes of the successor structure can be prevented by new lock modes, so-called prevention modes. In some cases additional virtual (prevention) objects have to be introduced which serve as a basis to install prevention locks. In contrast to normal locks, prevention locks do not grant any access rights to their holders. Two ways to realize prevention locks are proposed. They differ in the complexity of their basic compatibility tests.Finally, the approach presented here is compared with previous work and connections with issues in protection and authorization are discussed.