  Virtual clusters are widely used computing platforms than can be deployed in
multiple cloud platforms. The ability to dynamically grow and shrink the number
of nodes has paved the way for customised elastic computing both for High
Performance Computing and High Throughput Computing workloads. However,
elasticity is typically restricted to a single cloud site, thus hindering the
ability to provision computational resources from multiple geographically
distributed cloud sites. To this aim, this paper introduces an architecture of
open-source components that coherently deploy a virtual elastic cluster across
multiple cloud sites to perform large-scale computing. These hybrid virtual
elastic clusters are automatically deployed and configured using an
Infrastructure as Code (IaC) approach on a distributed hybrid testbed that
spans different organizations, including on-premises and public clouds,
supporting automated tunneling of communications across the cluster nodes with
advanced VPN topologies. The results indicate that cluster-based computing of
embarrassingly parallel jobs can benefit from hybrid virtual clusters that
aggregate computing resources from multiple cloud back-ends and bring them
together into a dedicated, albeit virtual network.
  The work presented in this article has been partially funded by the European
Union's (EU) Horizon 2020 research project DEEP Hybrid-DataCloud (grant
agreement No 777435).
