Topological treatment of early-deciding set-agreement

This paper considers the k-set-agreement problem in a synchronous message passing distributed system where up to t processes can fail by crashing. We determine the number of communication rounds needed for all correct processes to reach a decision in a given run, as a function of k, the degree of coordination, and f <= t the number of processes that actually fail in the run. We prove a lower bound of min([f/k] + 2, [t/k] + 1) rounds. Our proof uses simple topological tools to reason about runs of a full information set-agreement protocol. In particular, we introduce a topological operator, which we call the early deciding operator, to capture rounds where k processes fail but correct processes see only k - 1 failures.