Autonomous robot fleets are complex systems that require the interaction and communication between heterogeneous hardware and software. Despite many years of work in robotics, there is still a lack of established software architecture and middleware, in particular for large scale multi-robots systems. Many research teams are still writing specific hardware orientated software that is very tied to a robot. This vision makes sharing modules or extending existing code difficult. A robotic middleware should be designed to abstract the low-level hardware architecture, facilitate communication and integration of new software. In this paper, we present and compare seven existing middlewares capable of being used in multi-robot systems. We also present two dedicated cloud based multi-robots platforms. After this analysis, we discuss why a cloud of robots and not a cloud for robots is more suitable in a fleet context.