PlanWorks: A Debugging Environment for Constraint Based Planning Systems

In recent years, model-based planning has moved from reasoning about models largely described with propositions (e.g. STRIPS) to reasoning about time, resources, and constraints on complex numeric quantities (Smith, Frank, & Jónsson 2000), (Fox & Long 2003). Numerous planning and scheduling systems employ underlying constraint reasoning systems to handle the richness and diversity of such constraints (Frank & Jónsson 2003). Debugging such systems involves the search for errors in model rules, constraint reasoning algorithms, search heuristics and the problem instance (initial state and goals). In order to effectively find such problems, users must see why each state or action is in a plan by tracking causal chains back to part of the initial problem instance. They must be able to visualize complex relationships among many different entities and distinguish between those entities easily. For example, a variable can be shared between several constraints, as well as part of a state or activity in a plan; the activity can arise as a consequence of another activity and a model rule. Finally, they must be able to track each logical inference made during planning. We have developed PlanWorks, a browse-based system for debugging constraint-based planning and scheduling systems. PlanWorks assumes a strong transaction model of the entire planning process, including adding and removing parts of the constraint network, variable assignment, and constraint propagation. The planner logs all transactions to a relational database that is tailored to support queries for a variety of components. Visualization components consist of specialized views to display different forms of data (e.g. constraints, activities, resources, and causal links). Each view allows user customization in order to display only the most relevant information. Inter-view navigation features allow users to rapidly switch views to examine the trace of the process from different perspectives. Transaction query mechanisms allow users access to the logged transactions to visualize activities across the entire planning process. While originally developed for debugging, PlanWorks has the potential to serve as a knowledge capture tool and an end-user