Abstract
Planning and scheduling have an interesting relationship. In informal usage, planning and scheduling are frequently treated almost interchangeably. In contrast, formally, their refined and tightly specified descriptions appear quite different: planning is a PSPACE-hard problem, while scheduling is an archetypal NP-hard problem. The research communities that examine these problems seem, to a large extent, to share this dichotomy, with surprisingly distinct research communities having developed around the two problems. Despite this division, there have been many individuals in both communities who have recognised the importance and relevance of the relationships between the two problems. The apparent simplicity of the separation offered by the complexity classes of the different problems belies the difficulty of giving a clean intuition to the differences between planning and scheduling—a difficulty which seems to return to the much less sharply drawn boundaries of the informal usage of the terms. The common characterisation is that planning is the problem of deciding what actions should be performed to solve a problem, while scheduling is about deciding when to execute them and with what resources. For example, this is the characterisation used by Garrido et al. in their paper in this issue. Although this characterisation is convenient, it is a simplification: which action to use to solve a problem frequently depends on how resources are to be allocated, so that plans depend on scheduling choices, while equally, what resources are available to support the scheduling decisions depend on which actions are selected to solve a problem. Thus, the choices made in planning and scheduling are interlocked and interdependent.
Published Version
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