The semantics of real-time database applications are discussed, and the authors show how the usual correctness criteria of concurrency control can be weakened to incorporate the concept of similarity. The proposed new correctness criterion exploits the interchangeability of sufficiently precise input data values. Also proposed in the paper is the idea of physical schedules in which a real-time database scheduler may skip unimportant computation to meet time constraints. The correctness of physical schedules is justified by the notion of similarity. The authors then take a semantic approach based on similarity in order to propose a sufficient condition for scheduling real-time transactions without locking of data.

The paper makes a meaningful contribution to the literature on real-time transaction scheduling. Although it is theoretical, the illustrative examples facilitate understanding. The paper should be accessible to anyone with a general background in real-time scheduling and database concurrency control. A good review of recent work on related issues is provided.