System of systems (SoS) is the viewing of multiple, dispersed, independent systems in context as part of a larger, more complex system. A system is a group of interacting, interrelated and interdependent components that form a complex and unified whole.
The goal of an SoS architecture is to get maximum value out of a large system by understanding of how each of the smaller systems work, interface and are used. Such designs require systems thinking -- a holistic approach to analysis that focuses on the way constituent parts interoperate, work over time and function within the context of a larger, evolving system.
In the data center, independent constituent parts of a large system are connected through SoS-defined software interfaces called middleware. Such programs ensure that constituents do not compete for subtasks within the larger system and provide messaging services so that constituent systems can communicate.