Reaching Agreement Among $k$ out of $n$ Processes

Abstract

In agreement problems, each process has an input value and must choose a decision (output) value. Given $n\geq 2$ processes and $m \geq 2$ possible different input values, we want to design an agreement algorithm that enables as many processes as possible to decide on the (same) input value of one of the processes, in the presence of $t$ crash failures. Without communication, when each process simply decides on its input value, at least $\lceil (n-t)/m \rceil$ of the processes are guaranteed to always decide on the same value. Can we do better with communication? For some cases, for example when $m=2$, even in the presence of a single crash failure, the answer is negative in a deterministic asynchronous system where communication is either by using atomic read/write registers or by sending and receiving messages. The answer is positive in other cases.