Abstract
The foundation of cloud computing is to leverage virtualised physical computing resources by offering them on demand as elastic infrastructure services. Processing, storage and networking resources can thus be perceived and traded comparable to existing utility services. A restriction of conventional infrastructure services is the missing flexibility for scenarios in which coarse-grained provisioning from a single provider is not sufficient anymore. Cloud federation and multiplexing are recent techniques to overcome the provider binding and lock-in limitation. In this article, we give a brief overview about this research direction and then present a new variety, nested clouds, which helps overcoming the coarse-grained virtualisation, billing and trading limitations. In particular, we contribute a design for the Nested Cloud virtual machine and the corresponding spot market Highly-Virtualising Cloud Resource Broker and report on our experience with these systems. We argue that nested clouds help consumers in decreasing their costs and increasing their flexibility for infrastructure services by recycling and repurposing unused capacities.