Efficient Fork-Linearizable Access to Untrusted Shared Memory

When data is stored on a faulty server that is accessed concurrently by multiple clients, the server may present inconsistent data to different clients. For example, the server might complete a write operation of one client, but respond with stale data to another client. Mazi\`{e}res and Shasha (PODC 2002) introduced the notion of {fork-consistency, also called fork-linearizability, which ensures that the operations seen by every client are linearizable and guarantees that if the server causes the views of two clients to differ in a single operation, they may never again see each other's updates after that without the server being exposed as faulty. In this paper, we improve the communication complexity of their fork-linearizable storage access protocol with $n$ clients from $\Omega(n^2)$ to $O(n)$. We also prove that in every such protocol, a reader must wait for a concurrent writer. This explains a seeming limitation of their and of our improved protocol. Furthermore, we give novel characterizations of fork-linearizability and prove that it is neither stronger nor weaker than sequential consistency.