t-Resilient Immediate Snapshot Is Impossible

An immediate snapshot object is a high level communication object, built on top of a read/write distributed system in which all except one processes may crash. It allows each process to write a value and obtains a set of pairs (process id, value) such that, despite process crashes and asynchrony, the sets obtained by the processes satisfy noteworthy inclusion properties.

Considering an n-process model in which up to t processes are allowed to crash (t-crash system model), this paper is on the construction of t-resilient immediate snapshot objects. In the t-crash system model, a process can obtain values from at least \((n-t)\) processes, and, consequently, t-immediate snapshot is assumed to have the properties of the basic \((n-1)\)-resilient immediate snapshot plus the additional property stating that each process obtains values from at least \((n-t)\) processes. The main result of the paper is the following. While there is a (deterministic) \((n-1)\)-resilient algorithm implementing the basic \((n-1)\)-immediate snapshot in an \((n-1)\)-crash read/write system, there is no t-resilient algorithm in a t-crash read/write model when \(t\in [1\ldots (n-2)]\). This means that, when \(t<n-1\), the notion of t-resilience is inoperative when one has to implement t-immediate snapshot for these values of t: the model assumption “at most \(t<n-1\) processes may crash” does not provide us with additional computational power allowing for the design of a genuine t-resilient algorithm (genuine meaning that such an algorithm would work in the t-crash model, but not in the \((t+1)\)-crash model). To show these results, the paper relies on well-known distributed computing agreement problems such as consensus and k-set agreement.