1. A controller device for configuring one or more network elements of a software-defined network, the controller device comprising: a data interface that receives relation data items and action policy rules, each relation data item expressing a pair of entities of the network and a relationship between the entities, and each action policy rule expressing a network-related event and one or more network-related actions to be carried out in response to the event; a network interface that transmits and receives data to and from the network; and a processor that generates configuration data for the one or more network elements, wherein the processor is configured to: derive and store implicit relation data items expressing implicit relations between pairs of the entities on the basis of the received relation data items, whereby a first one of the implicit relations is derived using a combination of first and second ones of the relation data items, the first one of the relation data items expressing a relationship between first and second ones of the entities, the second one of the relation data items expressing a relationship between the second one and a third one of the entities, and the first one of the implicit relations being derived such to express a relationship between the first and third ones of the entities, generate and store the configuration data for the one or more network elements on the basis of the relation data items, the action policy rules, and the derived implicit relations, and cause the network interface to transmit, as network data receivable and operable by the one or more network elements, the configuration data to the one or more network elements.