Bounding the effects of compensation under relaxed multi-level serializability

The multi-level transaction concept provides a powerful tool for structuring activities in multidatabase systems. However, even multi-level serializability is sometimes too restrictive as a correctness criterion, either because of very high concurrency requirements, or because of the practical difficulties of implementing a scheduler in actual production environments. The extended multi-level transaction model presented in this paper supports higher concurrency in cases where higher level operations commute in one direction, but not in the other-i.e., when it is valid to interchange them when they occur in one order in a history, but not when they occur in the other order. We introduce a relaxed correctness criterion based on allowing a bounded number of out of order conflicts at each level in the multi-level framework, where the bound can be different for different levels. Finally we discuss the properties of compensation in this framework, developing a theory of compensation which depends only on the semantics of the operations and not on the particular state of the database. We illustrate the use of these concepts in the context of a particular class of practical applications.