  Knowledge bases (KBs) are not static entities: new information constantly
appears and some of the previous knowledge becomes obsolete. In order to
reflect this evolution of knowledge, KBs should be expanded with the new
knowledge and contracted from the obsolete one. This problem is well-studied
for propositional but much less for first-order KBs. In this work we
investigate knowledge expansion and contraction for KBs expressed in DL-Lite, a
family of description logics (DLs) that underlie the tractable fragment OWL 2
QL of the Web Ontology Language OWL 2. We start with a novel knowledge
evolution framework and natural postulates that evolution should respect, and
compare our postulates to the well-established AGM postulates. We then review
well-known model and formula-based approaches for expansion and contraction for
propositional theories and show how they can be adapted to the case of DL-Lite.
In particular, we show intrinsic limitations of model-based approaches: besides
the fact that some of them do not respect the postulates we have established,
they ignore the structural properties of KBs. This leads to undesired
properties of evolution results: evolution of DL-Lite KBs cannot be captured in
DL-Lite. Moreover, we show that well-known formula-based approaches are also
not appropriate for DL-Lite expansion and contraction: they either have a high
complexity of computation, or they produce logical theories that cannot be
expressed in DL-Lite. Thus, we propose a novel formula-based approach that
respects our principles and for which evolution is expressible in DL-Lite. For
this approach we also propose
  polynomial time deterministic algorithms to compute evolution of DL-Lite KBs
when evolution affects only factual data.
