A Retrospective on the Reactive Event Calculus and Commitment Modeling Language

Social commitments in time: Satisfied or compensated was the title of a presentation given at the 7th DALT workshop edition [34] in which we proposed a layered architecture for modeling and reasoning about social commitments. We gave emphasis to modularity and to the need of accommodating certain temporal aspects in order for a commitment modeling framework to be flexible enough to adapt to diverse commitment theories, and expressive enough to model realistic scenarios. We grounded the framework on two formalisms: the Reactive Event Calculus (REC) and the Commitment Modeling Language (CML). In this retrospective, we review recent developments of this line of work, and discuss our contribution in a broader context of related research. 1 A Short Introduction to REC and CML Social commitments are a well-known concept in Multi-Agent Systems (MAS) research [8, 31]. They are commitments made from an agent to another agent to bring about a certain property. In broad terms, a social commitment represents the commitment that an agent, called debtor, has towards another agent, called creditor, to bring about some property or state of affairs, which is the subject of the commitment. In some instantiations of this idea, such as [18, 37], the subject of a commitment is a temporal logic formula. Representing the commitments that the agents have to one another and specifying constraints on their interactions in terms of commitments provides a principled basis for agent interactions [35]. From a MAS modelling perspective, a role can be modelled by a set of commitments. For example, a seller in an online market may be understood as committing to its price quotes and a buyer may be understood as committing to paying for goods received. Commitments also serve as a natural tool to resolve design ambiguities. The formal semantics enables verification of conformance and reasoning about the MAS specifications [17] to define core interaction patterns and build on them by reuse, refinement, and composition. Central to the whole approach is the idea of manipulation of commitments: their creation, discharge, delegation, assignment, cancellation, and release, since commitments are stateful objects that change in time as events occur. Time C. Sakama et al. (Eds.): DALT 2011, LNAI 7169, pp. 120–127, 2012. Springer-Verlag Berlin Heidelberg 2012 A Retrospective on the REC and CML 121 and events are, therefore, essential elements. Literature distinguishes between base-level commitments, written C(x, y, p), and conditional commitments, written CC(x, y, p, q) (x is the debtor, y is the creditor, and p/q are properties). CC(x, y, p, q) signifies that if p is brought out, x will be committed towards y to bring about q. In our DALT 2009 paper Social commitments in time: Satisfied or compensated [34], we drew inspiration from work by Mallya et al. [24] and gave emphasis to temporal aspects of commitments. We wanted to propose an expressive enough notation, to be able to model commitment properties that have to be satisfied at specific time points or along specific intervals, and introduce a notion of compensation, with a mind on some scenarios in which social commitments may realistically be used. To this end, we identified a number of desiderata for social commitment frameworks. We then defined a new notation for commitments and commitment specification programs: the Commitment Modeling Language (CML). Finally, we proposed an abstract commitment framework architecture and a concrete instance of it that supports CML. In such an instance, temporal reasoning with commitments is operationalized using the Reactive Event Calculus (REC), and various verification tasks can be accomplished thanks to an underlying declarative, computational logic-based framework. The architecture proposed in [34] consists of four layers: a user application layer, a commitment modeling layer, a temporal representation and reasoning layer, and a reasoning and verification layer. On the top layer, the user can define contracts or social interaction rules using commitments. Such definitions are based on a language provided by the layer below. The commitment modeling language is implemented using a temporal representation and reasoning framework, which is in turn built on top of a more general reasoning and verification framework, which lies at the bottom layer. It is important to rely on a formal framework that accommodates various forms of verification, because in this way commitments can be operationalized and the user can formally analyze commitment-based contracts, reason on the state of commitments, plan for actions needed to reach states of fulfillment, and track the evolution of commitments at run-time. Indeed, the underlying reasoning and verification layer must be powerful enough to accommodate temporal representation and reasoning. Our proposal also included a concrete instance of such an architecture. We report it here (see Fig. 1). At the bottom of the stack lay a number of Prolog+CLP modules, which implement the SCIFF family of proof-procedures and provide the SCIFF language to the layer above [1]. The SCIFF framework is based on abductive logic programming and it consists of a declarative specification language and a family of proof-procedures for reasoning from SCIFF specifications. Some kinds of reasoning are: deduction, hypothetical reasoning, static verification of properties, compliance checking and run-time monitoring. In general, SCIFF comes in handy for a number of useful tasks in the context of agent interaction. Its main metaphor is that of expectation about events. A simple introduction to SCIFF and its usage is given in [35], where expectations are 122 P. Torroni et al. User and Domain Knowledge Base Commitment Modeling Language Reactive Event Calculus SCIFF Framework (CML Program)