1. A computer-implemented method for instantiating one or more resources of an IT-service, each resource being of one of one or more resource types, each resource being provided by one of one or more resource managers of an IT-service management system, the method comprising: analyzing, by a management engine, a service model of said IT-service, the service model comprising nodes respectively representing one of the resources and respectively being indicative of a resource type of said represented resource, each of the nodes comprising a generic API, the generic API not being specific to any particular one of the resource managers and providing at least one abstract method for instantiating the resource represented by the respective node; and for each of the nodes, executing, by the management engine, at runtime of the IT-service management system: determining the one of the one or more resource types being indicated by said node; evaluating a service provider catalog, the service provider catalog being indicative of one or more of the resource managers respectively being operable to provide a resource of a given resource type, for determining one or more of the resource managers respectively being operable to provide a resource having the determined resource type, and for determining an address of each of said determined one or more resource managers; selecting one of the one or more resource managers; sending a request to the address of the selected resource manager, the request being a request for a description of a resource-manager-specific API of said resource manager; receiving the requested description from the selected resource manager; overriding said at least one abstract method by a resource-manager specific method of the resource-manager-specific API for instantiating said resource, said resource-manager specific method being specified in the received description; and executing the overridden at least one abstract method for instantiating the resource represented by said node.