Scheduling with State Resources: the Trolley Problem

A state resource is one whose condition can vary over time. Each activity that uses a state resource may, throughout its execution, require the resource to be in a specific state or in any of a given set of states. Two activities may not overlap if they require incompatible states of the same state resource during their execution. A state resource does not constrain the number of activities that can be executed in parallel, provided that the overlapping activities require compatible states of the resource.

Typical examples of state resources are the color of a traffic light--the activity "going through the intersection" necessitates the state "green" of the traffic light--or the temperature of an oven--the activity "melting a piece of metal" requires the oven to be at a given temperature; several pieces of metal can simultaneously use the oven provided they need the same temperature.

The state resource concept includes a large range of resources. As we see in the two examples above, state transitions of a state resource can be imposed by the environment (the traffic light) or controlled by a schedule (the oven temperature). A state transition can be instantaneous or not--the oven needs time to warm or to cool. In this and other chapters, we illustrate how Scheduler allows us to represent complex state resources.

This chapter introduces a basic example of a state resource: a trolley in a production shop, used to carry items to the various machines. The position of the trolley represents its state.