  Federations among sets of Cloud Providers (CPs), whereby a set of CPs agree
to mutually use their own resources to run the VMs of other CPs, are considered
a promising solution to the problem of reducing the energy cost. In this paper,
we address the problem of federation formation for a set of CPs, whose solution
is necessary to exploit the potential of cloud federations for the reduction of
the energy bill. We devise a distributed algorithm, based on cooperative game
theory, that allows a set of CPs to cooperatively set up their federations in
such a way that their individual profit is increased with respect to the case
in which they work in isolation, and we show that, by using our algorithm and
the proposed CPs' utility function, they are able to self-organize into
Nash-stable federations and, by means of iterated executions, to adapt
themselves to environmental changes. Numerical results are presented to
demonstrate the effectiveness of the proposed algorithm.
