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An operational semantics for the goal life-cycle in BDI agents

Abstract

A fundamental feature of intelligent agents is their ability to deliberate over their goals. Operating in an environment that may change in unpredictable ways, an agent needs to regularly evaluate whether its current set of goals is the most appropriate set to pursue. The management of goals is thus a key aspect of an agent’s architecture. Focusing on BDI agents, we consider the various types of goals studied in the literature, including both achievement and maintenance goals. We develop a detailed description of goal states (such as whether goals have been suspended or not), and a comprehensive suite of operations that may be applied to goals (including dropping, aborting, suspending and resuming them). We provide an operational semantics corresponding to this detailed description in an abstract agent language (CAN), and demonstrate on a detailed real-life scenario. The three key contributions of our generic framework for goal states and transitions are (1) to encompass both goals of accomplishment and rich goals of monitoring, (2) to provide the first specification of abort and suspend for all the common goal types, and (3) to account for plan execution as well as the dynamics of subgoaling. Our semantics clarifies how an agent can manage its goals, based on the decisions that it chooses to make, and further provides a foundation for correctness verification of agent behaviour.

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Notes

  1. Some minor points of clarification can be added to that workshop paper.

  2. In our formal semantics, the goals technically commence in Pending but transition immediately to Active due to their activation conditions.

  3. In the former case, the goal can have a plan or subgoal attached. Therefore a goal \(g\) suspended from Active should have suspend methods run for \(g\) and its subgoals, if any. This ideal is not reflected in our formal semantics, since it is a substate detail described in [40].

  4. That is, resume is a top-level command. Hence, the reconsideration condition is a ‘note’ from the agent to itself to guide its deliberation over the suspended goal: a sufficient but not necessary condition for when the agent should next look at the goal.

  5. An alternative would be to have it in the Pending state initially, with its activation condition being the maintenance condition.

  6. http://www.cs.rmit.edu.au/~jah/orpheus.

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Acknowledgments

We are grateful to an anonymous reviewer for this point. We thank Lin Padgham, Sebastian Sardiña, and the participants of the DALT’10 workshop for discussions. We thank the reviewers of the earlier versions of this article for thoughtful and detailed comments which have improved the work. JT acknowledges the support of the Australian Research Council and Agent Oriented Software Pty. Ltd. under Grant LP0453486. NYS acknowledges the support of the University Research Board of the American University of Beirut, and thanks the Operations group at the Judge Business School and the fellowship at St Edmund’s College, Cambridge. The work of DNM and NYS through SRI International was supported in part by the Defense Advanced Research Projects Agency (DARPA) under Contract No. FA8750-07-D-0185/0004. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of DARPA, or the Air Force Research Laboratory.

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Correspondence to Neil Yorke-Smith.

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Harland, J., Morley, D.N., Thangarajah, J. et al. An operational semantics for the goal life-cycle in BDI agents. Auton Agent Multi-Agent Syst 28, 682–719 (2014). https://doi.org/10.1007/s10458-013-9238-9

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Keywords

  • BDI agents
  • Goal management
  • Operational semantics