Consistency Enforcement in Databases

摘要

Consistency enforcement aims at systematically modifying a database program such that the result is consistent with respect to a specified set of integrity constraints. This modification may be done at compile-time or at run-time. The commonly known run-time approach uses rule triggering systems (RTSs). It has been shown that these systems cannot solve the problem in general. As an alternative greatest consistent specializations (GCSs) have been studied. This approach requires the modified program specification to be a maximal consistent diminution of the original one with respect to some partial order. The chosen order is operational specialization. On this basis it is possible to derive a commutativity result and a composi-tionality result. The first one enables step-by-step enforcement for sets of constraints. The second one reduces the problem to providing the GCSs just for basic operations, whereas for complex programs the GCS can be easily determined. The approach turns out to be well-founded since the GCS for such complex programs is effectively computable if we require loops to be bounded. Despite its theoretical merits the GCS approach is still too coarse. This leads to the problem of modifying the chosen specialization order and to relax the requirement that the result should be unique. One idea is to exploit the fact that operational specialization is equivalent to the preservation of a set of transition invariants. In this case a reasonable order arises from a slight modification of this set, in which case we talk of a maximal consistent effect preserver (MCE). However, a strict theory of MCEs is still outstanding.