1. A system having physical resources for data processing, for data storage resources and for network communications to couple the data processing and data storage resources, the system also having two or more virtual overlay infrastructures mapped onto the physical resources, each virtual overlay infrastructure comprising one or more virtual entities for data processing, one or more virtual data storage entities and one or more virtual network entities arranged to couple others of the virtual entities, the overlay infrastructures being isolated from each other, and having an inter-infrastructure interface for providing a controlled interaction between different ones of the isolated virtual overlay infrastructures.