  Distributed systems are now both very large and highly dynamic. Peer to peer
overlay networks have been proved efficient to cope with this new deal that
traditional approaches can no longer accommodate. While the challenge of
organizing peers in an overlay network has generated a lot of interest leading
to a large number of solutions, maintaining critical data in such a network
remains an open issue. In this paper, we are interested in defining the portion
of nodes and frequency one has to probe, given the churn observed in the
system, in order to achieve a given probability of maintaining the persistence
of some critical data. More specifically, we provide a clear result relating
the size and the frequency of the probing set along with its proof as well as
an analysis of the way of leveraging such an information in a large scale
dynamic distributed system.
