Instantiating a resource of an IT-service includes analyzing a service model of the IT-service where the service model includes a node representing a resource for providing the IT-service. The node has a generic API providing an abstract method for instantiating the resource. A determination is made whether the node is a multi-instance node representing a resource being instantiable multiple times. If so, a determination is made whether a first instance of the resource represented by the multi-instance node already exists in a runtime environment providing the IT-service. If so, a selection criteria is computed to identify and select a resource manager configured to instantiate a second instance of the resource based on collocation or anti-collocation parameters in the runtime environment. The abstract method for instantiating the second instance of the resource is overridden with a resource-manager specific method of a resource-manager-specific API specified by the selected resource manager.