HDI is available in both appliance and software only configurations. The Hardware based appliance models include a High Availability Cluster configuration as well as a single node, fully integrated configuration. The software only configurations are virtual machine implementaitons and currently are qualified for and supported by VMware and Hyper-V.
A low end HDI configuration is available but will be transitioning into an End Of Sale status in April 2017. For information on this End of Sale see this FAQ: