A core idea of cloud computing is elasticity, i.e., enabling applications to adapt to varying load by dynamically acquiring and releasing cloud resources. One concrete realization is cloud bursting, which is the migration of applications or parts of applications running in a private cloud to a public cloud to cover load spikes. Actually building a cloud bursting enabled application is not trivial. In this paper, the authors introduce a reference model and middleware realization for cloud bursting, thus enabling elastic applications to run across the boundaries of different cloud infrastructures.