Consider the : N processes, each with an input value, must agree on a common output value, and that output must be one of the inputs. In an asynchronous system where one process may crash, the Fischer-Lynch-Paterson (FLP) impossibility result tells us no deterministic algorithm exists.
is impossible in asynchronous systems because the input complex is "connected" but the output complex is not. Model Fault Tolerance: distributed computing through combinatorial topology pdf