Reverting the Effects of XQuery Update Expressions

The need of reverting the effects of updates on the affected documents arises in many contexts, ranging from undos in transactional applications to versioning systems. In this paper, we investigate this issue for XQuery Update expressions, relying on the Pending Update List (PUL) obtained from the evaluation of an expression on a document. Specifically, we introduce an inversion operator, that, given a PUL to be applied on a document, allows to determine a corresponding inverted PUL that, applied on the modified document, produces the original document. Moreover, an alternative approach for enriching a PUL with additional information, so that it can be inversely applied, is proposed and the two approaches are experimentally compared.