Peer data exchange

• Authors: Ariel Fuxman;Phokion G. Kolaitis;Renée J. Miller;Wang-Chiew Tan
• Venue: Proceedings of the twenty-fourth ACM SIGMOD-SIGACT-SIGART symposium on Principles of database systems
• Year: 2005

In this paper, we introduce and study a framework, calledpeer data exchange, for sharing and exchanging data betweenpeers. This framework is a special case of a full-fledged peer datamanagement system and a generalization of data exchange between asource schema and a target schema. The motivation behind peer dataexchange is to model authority relationships between peers, where asource peer may contribute data to a target peer, specified usingsource-to-target constraints, and a target peer may usetarget-to-source constraints to restrict the data it is willing toreceive, but cannot modify the data of the source peer.A fundamental algorithmic problem in this framework is that ofdeciding the existence of a solution: given a source instance and atarget instance for a fixed peer data exchange setting, can thetarget instance be augmented in such a way that the source instanceand the augmented target instance satisfy all constraints of thesetting? We investigate the computational complexity of the problemfor peer data exchange settings in which the constraints are givenby tuple generating dependencies. We show that this problem isalways in NP, and that it can be NP-complete even for "acyclic"peer data exchange settings. We also show that the data complexityof the certain answers of target conjunctive queries is in coNP,and that it can be coNP-complete even for "acyclic" peer dataexchange settings.After this, we explore the boundary between tractability andintractability for the problem of deciding the existence of asolution. To this effect, we identify broad syntactic conditions onthe constraints between the peers under which testing for solutionsis solvable in polynomial time. These syntactic conditions includethe important special case of peer data exchange in which thesource-to-target constraints are arbitrary tuple generatingdependencies, but the target-to-source constraints arelocal-as-view dependencies. Finally, we show that the syntacticconditions we identified are tight, in the sense that minimalrelaxations of them lead to intractability.