  Youtopia is a platform for collaborative management and integration of
relational data. At the heart of Youtopia is an update exchange abstraction:
changes to the data propagate through the system to satisfy user-specified
mappings. We present a novel change propagation model that combines a
deterministic chase with human intervention. The process is fundamentally
cooperative and gives users significant control over how mappings are repaired.
An additional advantage of our model is that mapping cycles can be permitted
without compromising correctness.
  We investigate potential harmful interference between updates in our model;
we introduce two appropriate notions of serializability that avoid such
interference if enforced. The first is very general and related to classical
final-state serializability; the second is more restrictive but highly
practical and related to conflict-serializability. We present an algorithm to
enforce the latter notion. Our algorithm is an optimistic one, and as such may
sometimes require updates to be aborted. We develop techniques for reducing the
number of aborts and we test these experimentally.
