The model comprises a set of Entities, a set of Resources that can be stored, consumed and produced by the Entities, and a set of Markets that mediate resource exchanges among Entities. The connectivity of Entities to markets may reflect the topology of physical networks if such networks are required for resource exchange. The primary state variables and processes that define Entities, including both the internal consumption and production processes and interactions with other Entities through exchange processes, are summarized in the causal loop diagram above