1. A method for invoking a composite network service in a computer system, wherein the composite network service comprises a plurality of network services, each network service comprising operations, and each operation having a plurality of input parameters and a plurality of output parameters, the method comprising determining operations relating to the composite network service to be invoked and provided via a communication network by analyzing a data model having nodes representing the parameters and directed relations, wherein each directed relation represents an operation relating to at least one network service and connects at least one node representing each of said plurality of input parameters of an operation to at least one node representing each of said plurality of output parameters of the operation; determining, for the composite network service, a service invocation path in the data model from at least one node representing at least one given input parameter to at least one node representing at least one given output parameter; invoking operations relating to the composite network service along the determined service invocation path in an order in accordance with the service invocation path and using an output parameter of a previous operation as an input parameter for a next operation; and performing a reachability analysis to determine which of said plurality of output parameters is reachable based on one or more of said plurality of input parameters.