On the Life-Cycle of BDI Agent Goals

Introduction. Deliberation over courses of action to pursue is fundamental to agent systems. Agents designed to work in dynamic environments, such as a rescue robot or an online travel agent, must be able to reason about what actions they should take, incorporating deliberation into their execution cycle, reviewing decisions and taking corrective action with appropriate focus and frequency. Not only must agents reason about the effects of their courses of action, they must also consider the semantics of these corrective actions. Systems based on the well-known Belief-Desire-Intention (BDI) framework most often ascribe a set of goals to the agent, which is equipped with various techniques to deliberate over and manage this set. The centrality of reasoning over goals is seen in the techniques investigated in the literature, which include subgoaling and plan selection, detection and resolution of conflicts or opportunities for cooperation [9], checking goal properties to specification [10, 5], failure recovery and planning [2], and dropping, aborting, or suspending and resuming goals [7]. A variety of goals are described in the literature, including goals of performance of a task, achievement of a state, querying truth of a statement, testing veracity of beliefs, and maintenance of a condition [1, 11]. An agent must manage such a variety of goals, while incorporating pertinent sources of information into its decisions over them, such as preferences, quality goals, motivational goals, and advice [10]. The complexity of agent goal management stems from this combination of the variety of goals and the breadth of deliberation considerations. It is furthered because each goal can be dropped, aborted, suspended, or resumed (as illustrated in Figure 1) at arbitrary times. While goals themselves are static (i.e., they are specified at design time, and do not change during execution), their behaviour is dynamic: a goal may undergo a variety of changes of state during its execution cycle [5]. This evolution may include its initial adoption by the agent, being actively pursued, being suspended and then later resumed, and eventually succeeding (or failing). (Maintenance goals have a subtle life-cycle: the goal is retained even when the desired property is true; it is possible that such goals are never dropped.) Our work analyzes the behaviour of the above types of goals, including the behaviour when goals are aborted or suspended. We consider the complete life-cycle of goals, from their initial adoption by the agent to the time when they are no longer of interest, and all stages in between; we account for the dynamics of plan execution and sub-goaling. We develop a generic framework for goal states and transitions that captures the life-cycle of goals—shown in summary in Figure 1; the Active and Suspended states decomposed further [8]—