Ken Currie

O-Plan: the open planning architecture

Abstract O-Plan is an AI planner based on previous experience with the Nonlin planner and its derivatives. Nonlin and other similar planning systems had limited control architectures and were only partially successful at limiting their search spaces. O-Plan is a design and implementation of a more flexible system aimed at supporting planning research and development, opening up new planning methods and supporting strong search control heuristics. O-Plan takes an engineering approach to the construction of an efficient domain-independent planning system which includes a mixture of AI and numerical techniques from operations research. The main contributions of the work are centred around the control of search within the O-Plan planning framework, and this paper outlines the search control heuristics employed within the planner. These involve the use of condition typing, time and resource constraints and domain constraints to allow knowledge about an application domain to be used to prune the search for a solution. The paper also describes aspects of the O-Plan user interface, domain description language (Task Formalism or TF) and the domains to which O-Plan has been applied.

Exploiting temporal coherence in nonlinear plan construction

Correct conventional nonlinear planners operate in accordance with Chapmans modal truth criterion (MTC). The MTC characterizes the conditions under which an assertion will be true at a point in a nonlinear plan. However, the MTC is not all one requires in order to build a realistic planning system: it merely sanctions the use of a number of plan modifications in order to achieve each assertion in a developing plan. The number of modifications that can be made is usually very large. To avoid breadth‐first search a planner must have some idea of which plan modification to consider. We describe a domain‐independent search heuristic called temporal coherence, which helps guide the search through the space of partial plans defined by the MTC. Temporal coherence works by suggesting certain orderings of goal achievement as more appealing than others, and thus by finding bindings for plan variables consistent with the planners overall goals. Our experience with a real nonlinear planner has highlighted the need for such a heuristic. In this paper, we give an example planning problem and use it to illustrate how temporal coherence can speed the search for an acceptable plan. We also prove that if a solution exists in the partial plan search space defined by the MTC, then there exists a path to that solution which is sanctioned by temporal coherence.

THE APPLICATION OF PARALLELISM IN COMMERCIAL DYNAMIC INFORMATION SYSTEMS: THE FORTH-CLYDE PROJECT

Abstract The last two decades have seen the development of a structured and methodical approach to systems analysis. Despite new approaches to this task, the development of computer systems continues to take too long for the purposes of supporting changes in business markets. This paper suggests that the problem is likely to be successfully addressed by three new design objectives: provision for prompt recognition of the need for change, provision for change itself, and parallel rather than sequential process architecture. Systems with these design characteristics are said to be Dynamic Information Systems, and a definition is proposed. Dynamic Information Systems predicate maximised concurrency. In order to ensure that this is incorporated in the design, a new viewpoint called parallelism is described. Parallelism recognises the natural concurrency of events, and analyses their interrelations by means of the concept of episodes. Episodes are defined and their application and uses introduced. The addition of parallelism and the concept of episodes to existing systems analysis methodologies allows the analysis of business organisations according to a threefold view: persistent, cyclic, and dynamic. This provides the structure and the means to design an information system with provision for the implementation of continuous change. The cost of computer system re-development cycles is expected to be progressively eliminated.