Weak consistency vs. access control: Distributed path maintenance

The proliferation of connected electronic devices, along with their possible states, rises new challenges for data maintenance in terms of efficiency, scalability and security. In the context of data replication at large, weak consistency has been adopted as a standard in uniformly trusted systems. Recently, a distributed framework has been proposed to handle untrusted systems, by adding access control at the file level. Yet, if no particular care is paid to the effective connectivity of the overlay, with respect to dynamically evolving access rights, updates of replicas may not propagate correctly anymore. This paper proposes the design and simulation of a middleware application in charge of maintaining the connectivity of the overlay used for update propagation. We propose two techniques improving the resilience of any overlay that may be disconnected as access right policies are modified or as critical nodes crash: (i) the first one detects problems due to a node blocking the propagation of updates, and propagates the alert to the application at each node; (ii) the second one, allowed to modify the overlay, reacts to a blocking node by rewiring the overlay in order to preserve at least one path between any pairs of replicas, while conserving the structural characteristics of the overlay. We illustrate the applicability of our middleware through simulations; they show that at a tunable overhead, overlay connectivity is maintained, despite local decisions that would have otherwise disrupted the replication service.