Method and network node (2) for a communication network (1) providing a service in the communication network. The network node (2) is arranged to communicate with 5 context sources (12, 15, 16) providing input values for a triggered expression evaluation, and to provide a network service to communication devices (12, 15, 16) based on the result of the triggered expression evaluation. An evaluation result of the triggered expression evaluation is obtained upon a change to an input value, and the triggered expression evaluation comprises the evaluation of nodes in an expression 10 graph. Each internal node is assigned a priority based on the level of the node in the expression graph. Actual reception of a trigger by an internal node is scheduled based on the priority of the internal node, such that the internal node will only receive a trigger after internal nodes with a higher priority have received their associated triggers.