8. A context server for a communication network, said context server configured to provide a context-based service to a terminal of said communication network, comprising: a query processor configured to: a) receive a query from a service application capable of implementing said context based service, said query indicating that said context server should perform an action when a query condition is fulfilled, said query condition referring to one or more attributes of derived context information indicative of a context of said terminal; b) generate a query evaluation trigger indicating that said query condition should be evaluated when said derived context information is updated; c) identify raw context information allowing derivation of said derived context information; and d) generate a calculation trigger indicating that said derived context information should be calculated when an update of said part of said raw context information is received from said terminal; and a trigger processor configured to: e) receive from said terminal an update of said part of said raw context information and, according to said calculation trigger, calculate a new value of said derived context information according to said update; and f) according to said query evaluation trigger, evaluate said query condition by using said new value and, if said query condition is fulfilled, performing said action.
8. A context server for a communication network, said context server configured to provide a context-based service to a terminal of said communication network, comprising: a query processor configured to: a) receive a query from a service application capable of implementing said context based service, said query indicating that said context server should perform an action when a query condition is fulfilled, said query condition referring to one or more attributes of derived context information indicative of a context of said terminal; b) generate a query evaluation trigger indicating that said query condition should be evaluated when said derived context information is updated; c) identify raw context information allowing derivation of said derived context information; and d) generate a calculation trigger indicating that said derived context information should be calculated when an update of said part of said raw context information is received from said terminal; and a trigger processor configured to: e) receive from said terminal an update of said part of said raw context information and, according to said calculation trigger, calculate a new value of said derived context information according to said update; and f) according to said query evaluation trigger, evaluate said query condition by using said new value and, if said query condition is fulfilled, performing said action. 9. The context server according to claim 8 , wherein said query processor is capable of being configured to identify said raw context information as raw context information, the variation of which may determine a variation of said one or more attributes referred to by said query condition.