Tight Bounds on Channel Reliability via Generalized Quorum Systems

Communication channel failures are a major concern for the developers of modern fault-tolerant systems. However, while tight bounds for process failures are well-established, extending them to include channel failures has remained an open problem. We introduce generalized quorum systems -- a framework that characterizes the necessary and sufficient conditions for implementing atomic registers, atomic snapshots, lattice agreement and consensus under arbitrary patterns of process-channel failures. Generalized quorum systems relax the connectivity constraints of classical quorum systems: instead of requiring bidirectional reachability for every pair of write and read quorums, they only require some write quorum to be unidirectionally reachable from some read quorum. This weak connectivity makes implementing registers particularly challenging, because it precludes the traditional request/response pattern of quorum access, making classical solutions like ABD inapplicable. To address this, we introduce novel logical clocks that allow write and read quorums to reliably track state updates without relying on bidirectional connectivity.