What is Byzantine Generals’ Problem?

A thought experiment that is intended to illustrate the difficulty of reaching consensus in a distributed system. In the problem, a group of generals who each command a portion of the army, surround a city. These generals must develop a plan to either attack or to retreat. Every general must reach a collective decision (if a unified decision is not reached, and some generals decide to attack, while others retreat, then the uncoordinated attack or retreat, the army will fail). Generals must coordinate via messengers (whom may lie) and generals themselves may be traitorous.