The Incompleteness of Planning with Volatile External Information

In many real-world planning environments, some of the information about the world is both external (the planner must request it from external information sources) and volatile (it changes before the planning process completes). In such environments, a planner faces two challenges: how to generate plans despite changes in the external information during planning, and how to guarantee that a plan returned by the planner will remain valid for some period of time after the planning ends. Previous works on planning with volatile information have addressed the first challenge, but not the second one. This paper provides a general model for planning with volatile external information in which the planner offers a guarantee of how long the solution will remain valid after it is returned, and an incompleteness theorem showing that there is no planner that can succeed in solving all solvable planning problems in which there is volatile external information.