1. A method for providing a service in a communication network, in which the service comprises evaluating a request comprising an expression, the expression being a function of a plurality of elements, each element relating to data originating from one or more context sources available in the communication network, each of the one or more context sources having an associated context source weight for a query by the service, the method comprising: determining a context source from the one or more context sources for which evaluation of the expression has a lowest expression evaluation weight, based on the associated context source weights, and interrogating the context source determined to have the lowest expression evaluation weight.