A symbiotic or online simulation is defined as one that interacts with the physical system in a mutually beneficial way. The simulation is driven by real time data collected from a physical system under control and needs to meet the real-time requirements of the physical system. In turn, the results from the "what-if" experiments performed by the simulator can be used to control the dynamic behaviour of the physical system. Such a simulation tool is for real-time planning and is to foresee and advise on real time problems. It aims to improve performance, to adapt to sudden and unexpected events and to improve aspects of safety and security of the physical system.