9.5. Resource Modeling

Whenever an activity of a certain type is to be performed, various resources may be required for performing it and have to be allocated first. We can distinguish between ‘active’ (or performer) resources, such as human resources, robots or IT systems, which execute activities, and ‘passive’ resources, such as rooms or devices, which are used by performers for executing activities.

Business process models focus on describing the possible sequences of events and activities, based on conditional and parallel branching. But they may also describe resource roles, which associate resources of a certain type with activities of a certain type.

In the field of Business Process Management (BPM), there is a tendency to consider workflow processes as paradigmatic for business processes. However, workflow processes typically only have (human) performer resources, while business processes in general often have one or more passive resources in addition to a performer. Due to this tendency, BPM research often adopts a simplistic view of business processes.

E.g., in (Pufahl et al. 2021), resource allocation is defined as the “assignment of a process task to the most appropriate resource”, while in general, it’s the other way around: resources are assigned to a task. The authors also point out that the vast majority of BPM research studies on resource allocation make the assumption that a task requires just one resource and a resource cannot be used in more than one task at the same time. As opposed to this simplistic view of business processes in BPM, in DES, tasks can have more than one required resource, and resources may be used in more than one task at the same time.

It is important to distinguish between the passive resources required for performing an activity and the inputs required for performing a processing (or transformation) activity. While the passive resources used in an activity survive the activity, and can be reused in the next one, inputs are either processed, or transformed, into outputs of the activity, and cannot be reused for the next activity. In the literature, this conceptual distinction is often not made. Instead, e.g., in (Russell et al. 2004; Goel and Lin 2021), processing activity inputs are called ‘consumable resources’. However, resources cannot be ‘consumed’, but they can be worn out by activities, such that if their degree of wear falls below a certain threshold, they can no longer be used and are depleted.

For various reasons, resources can go out of order: machines may fail or may have to get scheduled maintenance, nurses or doctors may get sick or may be out-of-duty or off-shift. Consequently, both planned and unexpected resource outages may have to be modeled.